VLF stability in Williamson-type amplifiers
Engineer
Hi, Patrick,
I've looked at your "standard" VLF stabilization coupling network for
this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on
the o/p tubes.) It appears to provide a forward gain roll-off below
about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB)
below about 3.2 Hz. Assuming my calculations are correct, what is the
purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll-
off continue at 6 dB/octave below 15 Hz would scotch any VLF
oscillation. Is this a phase shift issue? The simple -6dB /octave
roll-off leave a 90 degree phase advance in place below, say, 10 Hz
whereas the shelf would appear to avoid that (I'm not certain as,
regrettably, no pspice to hand at present!) I stabilized my
Williamson clone with just 0.05 uF coupling caps to the 6L6's with an
"aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the
100K a bit....) Do I really need a VLF shelf below that?
Thanks and cheers,
Roger
John L Stewart
'Engineer[_2_ Wrote:
> ;933456']This one's for Patrick in Oz.
One needs to be a bit careful with these cct values since they may be in
order for Patrick T's OPTs. Your Hammond will probably benifit from a
shelf at a higher f since Pat Ts transformers are sure to have a much
larger primary inductance. The loudspeaker resonance will complicate
things since it is in the same region you are trying to fix.
I've successfully used the parallel RC with differential amp driver to
avoid the additional phase shift at LF. But with the split load phase
inverter is a problem.
Patrick, your thoughts?
Cheers, John
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--
John L Stewart
Patrick Turner
The Wiiliamson has two RC coupled stages and OPT in the loop
surrounded by NFB so it is very prone to LF oscillations, especially
if you have a preamp powered off the pwr amp PSU and then you try to
boost the bass with a tone control network.
Williamson stipulated that the OPT have at least 100H at Vaa = 5Vrms
to get LF stability.
Nearly all OPTs made since 1949 have been by ppl who thought
Williamson was a wanker who made the amp cost of production too high.
These ppl didn't use enough P turns or big enough core size so lots of
Williamsons actually do oscillate at LF but it went un-noticed by the
dumb DIYers of yesteryear because they had no oscilloscopes to see the
small oscillations at below 10Hz. OPT core permeability rises with
applied voltage and so does Lp so the amplitude of LF oscillations may
be kept low if Lp rises enough to prevent any increase in amplitude.
Many people used CR coupilng values which encouraged phase shift
caused oscillations.
This is the background theory of how the Williamson and many other
tube amps become a phase shift oscillator at LF.
In a williamson with V1 direct coupled to V2 concertina and CR coupled
balanced amp with CR coupled OP tubes, the best place to put a
shelving network is between concertina and balanced amp, and there
must be TWO networks, one from concertina anode and one from cathode.
OK, the 0.47 + 1M strapped with 0.047 + 220k acts like this :-
At 1 kHz, all C have low Z so the phase shift is low and the driver,
ie concertina or whatever you have "sees" a load of 220k. As F is
reduced, there is a pole between 0.047 and 220 at 15.4Hz which is
usually above the F at which the amp may want to oscillate. Sometimes
I have used 0.022uF, so pole is at 32Hz. phase shift caused is less
than 90 degrees. As F is further reduced, the network response tends
to flatten to a shelf formed by 1M and 220k, ie, signal flattens at
roughly -15db, 0.18 times the 1kHz level. The 0.047 has gone to an
open circuit by 2Hz with little effect. But the 0.47 the rolls the
response off at the pole between 0.47uF and 1.22M ohms, ie at 0.27Hz,
below which there is an ultimate phase shift of 90d, but at such a low
F as to not cause bothers because its well below the poles of all
other sages.
The Williamson amp published in August 1949 has CR before balanced amp
with 0.05uF + 470k and before KT66 0.25uF + 100k. So poles are at
6.76Hz an 6.36Hz and so by 3Hz phase shift because of the two networks
would have been about 120d and with another 60d from OPT, maybe it
oscillated if the OPT didn't have enough Lp, and open loop gain > 1
where 180d phase shift exists. Ppl tend to try using larger C values
only to find oscillation frequency Fo just goes lower, or smaller C
and Fo rises. But the GAIN at where Fo is likely to occur must be
lowered. The only good way is with a shelving network. This means the
open loop gain without GNFB, OLG, seems to roll off at say -3dB at
30Hz and ppl panic and say "OMG, I've lost the bass." But when they
connect the GNFB its nice and stable and -3db occurs at 7Hz and there
is no peak in the response below 30Hz.
What is happening with GNFB applied is that there is 20dB NFB applied
to all F down to about 35Hz below which the amount of NFB is reduced.
Maybe only 10dB at 10Hz, and hardly any at 5Hz, because OLG has been
reduced so much by the shelving network. But there is ENOUGH NFB being
applied at 10Hz to still get amp Rout quite low enough, and reduce THD
etc. Nobody has much of anything below 20Hz in music so signals below
20Hz are tiny so they don't create much THD/IMD so lots of NFB at 10Hz
is NOT required. This is especially valid where the OP stage is a
triode type which has Rout < RL even with no GNFB. Williamson's
original KT66 triodes had Ra-a of 3k2, plus maybe Rw = 400 ohms, so DF
without NFB = 10k/3k6 = 2.77, and not bad, needing only 10dB NFB to
get DF to 10.
On page 346, RDH4, there is a Wlliamson with 807 in beam tetrode mode
which is VERY LIKELY to oscillate with the higher tetrode gain. CR
networks have 0.05 + 470k and 0.05 + 500k before 807. Looks like an
oscillator to me!
The Williamson rule was that there should be 100H for a load of 10k,
even at low Va-a levels. This meant RLa-a = Lp reactance at 15.9Hz. In
otherwords the amp will show less than 1 dB of response roll off due
to load reduction at 15.9Hz. Everyone mostly ROTFL at Willy, and they
made OPTs where ZLp = RL at 40Hz at low Va-a, and RLa-a was raised to
say 5k and tubes pushed into class AB with little class A, LF fidelity
dissappeared, power doubled though, bean conters were happy, and the
brandname sold yet another fraud to the public. One can build an amp
with only 20H of Lp, and the NFB will desperately try to compensate
the response towards flatness but don't expect the best bass
performance at loud levels and without core saturation effects. Unless
shelving networks are placed in such amps they will oscillate for
sure. Some makers avoided the issue by using only two amp stages, ie,
like Quad-II. This removed one of the 3 places where there could be an
ultimate 90d phase shift at F which is too high, ie, where OLG > 1.0
and where total OLG phase shift > 180d.
So Quad-II has a Tiny Toy OPT with 3,180 turns around a core Afe =
25mm x 25mm, while the Williamson original had 4,400t around 44 x 32.
I'd suggest the Quad-II has a lot less Lp than Williamson does, and I
know that the Quad-II saturates at 49Hz at 420Va-a while the
Williamson saturates at 15Hz at 420Va-a. The ratio of Lp to RL in the
original Williamson is much BETTER than in the Quad-II.
But Quad-II have 0.1uF + 680k between EF86 drivers and KT66, so as
years pass Eg1 goes positive without the 100k needed to stop the Eg1
rise. The Rout of EF86 is rather high, determined by the 180k and 680k
and EF86 Ra all in parallel, about 140k, so the pole in CR is at
1.9Hz, and low enough. Walker couldn't use less tha 680k for KT66 Rg
because that'd load the EF86 down an prevent the gain he needed. So
QUad-II has a really bad bodgie designed network in place. The triode
driver of the williamson allows the Rg = 100k without gain loss, and
the low Rg makes the KT66 last a long time. Leak also had bad design
problems. None of those old British brands got everything right.
Most of what I have said is incomprehensible. I bet you are entirely
baffled. You'll never learn until you build and measure everything
while asking questions and while sitting with a dual trace
oscilloscope to SEE the phase shifts and gain changes in networks. Its
real basic stuff and to build good amps you must know all about it.
After building 10 amps, maybe the penny will drop. Most Diyers build
one amp, and then forget the way they muddled through the process.
Read this page carefully, http://www.turneraudio.com.au/basic-tube-%283%29.htm
Its about time I renovated the page. I dunno how the "-%283%29" got
into the title line, years go by, I change ISP, and shit happens to
titles and text and formatting. I am re-editing pages now, lots to do.
Patrick Turner.
Patrick Turner
> On Sat, 18 Jun 2011 00:19:29 +0000, John L Stewart
> shelf? Why not just let it continue since we don't intend to amplify
> anything that low anyway?
Roger does not appear to deeply understand the cause of problems due
to phase shift. Its easier for me to explain my circuit techiniques
and ideas rather than to labour away and say why someone is wrong.
Once ppl learn **at least** what I know, they'll ALWAYS have
completely stable stable amps with fabulous bass sound.
Even if an amp is stable without the shelving network, shelving should
be used especially where there is some peaking in the region where "we
don't intend to amplify anything that low anyway" as you say. The
shelving improves overload charactecter and saturation effect
behaviour.
I will never ever say you too need to learn a lot more. There, I
didn't say it now did I?
Patrick Turner.
Patrick Turner
841a...@audiobanter.com> wrote:
> 'Engineer[_2_ Wrote:
>
>
>
>
>
> > ;933456']This one's for Patrick in Oz.
> > Hi, Patrick,
> > I've looked at your "standard" VLF stabilization coupling network for
> > this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on
> > the o/p tubes.) It appears to provide a forward gain roll-off below
> > about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB)
> > below about 3.2 Hz. Assuming my calculations are correct, what is the
> > purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll-
> > off continue at 6 dB/octave below 15 Hz would scotch any VLF
> > oscillation. Is this a phase shift issue? The simple -6dB /octave
> > roll-off leave a 90 degree phase advance in place below, say, 10 Hz
> > whereas the shelf would appear to avoid that (I'm not certain as,
> > regrettably, no pspice to hand at present!) I stabilized my
> > Williamson clone with just 0.05 uF coupling caps to the 6L6's with an
> > "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the
> > 100K a bit....) Do I really need a VLF shelf below that?
> > Thanks and cheers,
> > Roger
>
> One needs to be a bit careful with these cct values since they may be in
> order for Patrick T's OPTs.
Usually the R&C values I choose, ie, 0.47, 1M, 0.047, 220k are a great
starting point for 95% of amps which I find do oscillate at LF
especially when no load is on the output, and when the amp was
designed by accountants.
> Your Hammond will probably benifit from a
> shelf at a higher f since Pat Ts transformers are sure to have a much
> larger primary inductance. The loudspeaker resonance will complicate
> things since it is in the same region you are trying to fix.
Once the amp is made unconditionally stable by methods I have
indicated in 101 posts to this group, and which by now should be
engraved into everyon'e brains, then the use of any possible LCR
configured load including any type of LS whatsoever should be used
with NO PROBLEMS. Most dynamic speakers with a ported box have two
peaks in Z of up to 10x nominal Z either side of the box resonant F
which typically may be 40Hz when Z usually is just below nominal Z for
the speaker. The high Z peaks don't cause any problem anyone has to
worry about, even if they were as low as 6 Hz, which is never the
case, and when a speaker Z = voice coil resistance + filter coil
resistance.
>
> I've successfully used the parallel RC with differential amp driver to
> avoid the additional phase shift at LF. But with the split load phase
> inverter is a problem.
Its no problem. after concertina you need TWO networks one for each
phase, and then everything IS OK. The GEC book from 1957 with 17 amp
circuits from 5W to 1,100W backs up everything I say here, and
provides schematics.
> Patrick, your thoughts?
You have them now. I suggest ppl building new amps or farnarkling
around with old bits of junk need to remember that science needs to be
respected at all times. For that to happen, they must stop imagining
all sorts of invalid things, and consider themselves dumb unless
proven otherwise. No loss of face in that. It should lead to 101
questions which can be gradually answered in time and with practice.
They should learn to quickly draw a response graph in an exercise book
with NO GNFB then plot response with 6, 12, 18 dB of GNFB to "see wot
'appens" and often they will find that they can't even apply 6dB of
NFB without their little consolderation becoming an oscillator. Then
they are lost because they have no natural idea about Nyquist theory
which deals with stability with NFB around a bandwidth limited amp
with a high amount of phase shift at each end of the band.
Patrick Turner.
>
> Cheers, John
>
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> --
> John L Stewart- Hide quoted text -
>
> - Show quoted text -
Alex Pogossov
"flipper" <fli...@fish.net> wrote in message
-------------------
>>Even if an amp is stable without the shelving network, shelving should
>>be used especially where there is some peaking in the region where "we
>>don't intend to amplify anything that low anyway" as you say. The
>>shelving improves overload charactecter and saturation effect
>>behaviour.
>
> at frequencies that will never be presented to the amplifier.
Do you want rumble (from say a pick-up playing a warped record) driving your
amp to saturation, if your amp is close to LF oscillation?
Alex Pogossov
"Engineer" <junk...@rogers.com> wrote in message
news:2c765374-a2bf-4be4...@c41g2000yqm.googlegroups.com...
Your agressive LF frequency compensation might cause undesirable effects.
1. 100K bias is bit too low -- loading preceeding stage (driver, phase
splitter) unnecessarily. Perhaps you can achieve the same result by 220K and
0.022uF coupling.
2. By agressively cutting LF from 32Hz you reduce loop gain in the working
range (20Hz) thus increasing output impedance, reducing speaker damping,
increasing intermodulation and distortion (if it matters at 20Hz?) This is
aggravated by the fact that the driver stage has to labour 3dB harder at
20Hz because of the attenuation in your agressive circuit.. So I bet you
will get overall results twice worse than Mr Turner would have done with his
smart shelving.
3. Though your amp might appear stable, most likely it will be peaking close
to oscillation at 8Hz or so where the +90deg lead from your agressive
circuit will combine with +90deg lead from your OPT at the 0dB loop gain
crossing. Any LF rumble might drive your amp into overload.
So I need to admit, shelving is wiser, because it does not take out precious
dBs from the loop gain in the whole audio range and gives a better phase
margin, and no peaking. Mr Turner makes a deep 12dB/octave nose dive below
15...20Hz by combining OPT effect with 220K/0.05uF effect, but then, closer
to 0dB crossing, he gently goes out of the deep nose dive, shelves the beast
out and happily crossing 0dB at 8dB/octave. Past that he can dive deep
again -- does not carem as no oscillation will occur below 0dB.
However, both shelving and "agressive" compensation have one drawback in
common. NFB gain is constant at all frequencies since NFB is simply a
resistive divider. Thus the NFB tries to have the amp's responce flat (at
low level of course) down to possibly 3Hz or so. This is crasy and
unnecessary.
For that reason, instead of shelving interstage coupling I shelf the NFB.
Instead of a resisive divider I would put a resistor (typically 100R) in
series with a capacitor (order of 47...100 uF). Interstage coupling is kept
a simple semi-agressive RC circuit (say 0.022uF and 330K). Overall loop gain
shelving effect is the same as in Mr Turner desihns, but because of the
deepening feedback at VLF the amp becomes a rumble filter itself. A drawback
of that method is that you need an electrolytic in the feedback, Some people
do not "trust" electrolytics as a frequency shaping components.
Regards,
Alex
Patrick Turner
> "flipper" <flip...@fish.net> wrote in message
And its not uncommon for there to be 10% difference in Ia at idle for
each output tube and then any very low frequencies can easily push the
core into saturation and distortion. So what is wanted is a reduction
of gain at such very low F below 15Hz, and the NFB cannot become a
real pest at such LF.
I'll always get blasted by those who think shelving networks are a
bodgie way of building tube amps, they have no other better solutions
though. Make the OPT better they scream. OK, then you want it to weigh
tonne? OK, the screamers have crummy OPTs pulled from something
designed by a bean counter and unless they understand critical damping
and shelving networks they'll always have an inferior amp. Often it
takes me 1/2 a day to get a given amp channel optimised
experimentally, with my mind driven by the metal picture, not
guesswork. If the whingers are so pissed off about me saying you gotta
do this, you gotta do that, and they just can't understand, they can
always take the contraption they have assembled to someone who does
know about these things.
And the HF shelving networks and Zobel networks are also very
important to get right. Its not uncommon for OP tubes to give bursts
of RF when the F goes real low ans saturation is approached. AND
nobody wants the amp to break into strong RF oscillations during a
bias failure event. Shit DOES happen.
Patrick Turner.
Patrick Turner
> "Engineer" <junk2...@rogers.com> wrote in message
>
> news:2c765374-a2bf-4be4...@c41g2000yqm.googlegroups.com...
>
>
>
>
>
> > This one's for Patrick in Oz.
> > Hi, Patrick,
> > I've looked at your "standard" VLF stabilization coupling network for
> > this amplifier class (0.05 parallel 1Mohm, with 220Kohm grid leak on
> > the o/p tubes.) It appears to provide a forward gain roll-off below
> > about 15 Hz with a shelf at about 1/6 of the normal gain (-15.5 dB)
> > below about 3.2 Hz. Assuming my calculations are correct, what is the
> > purpose of the 1 Mohm across the 0.05 uF? Surely just letting the roll-
> > off continue at 6 dB/octave below 15 Hz would scotch any VLF
> > oscillation. Is this a phase shift issue? The simple -6dB /octave
> > roll-off leave a 90 degree phase advance in place below, say, 10 Hz
> > whereas the shelf would appear to avoid that (I'm not certain as,
> > regrettably, no pspice to hand at present!) I stabilized my
> > Williamson clone with just 0.05 uF coupling caps to the 6L6's with an
> > "aggressive" 100K grid leak (- 3dB at 32 Hz, but I may increase the
> > 100K a bit....) Do I really need a VLF shelf below that?
> > Thanks and cheers,
> > Roger
>
> Your agressive LF frequency compensation might cause undesirable effects.
>
> 1. 100K bias is bit too low -- loading preceeding stage (driver, phase
> splitter) unnecessarily. Perhaps you can achieve the same result by 220K and
> 0.022uF coupling.
But with 6SN7, and dc carrying RL = say 39k then total RL = 28k, and
this is about 3 x Ra of the tube which is fine. Bias R up to 150k in
OP stage is about right. I've seen too many tubes with several volts
dc across the high value bias resistance, so the tube is being turned
on by this positive bias and things only get worse if the tube heats
up more - there is a positive bias effect.
Just using 220k and 0.022uF from driver anode to output grid gives
pole at 32Hz, and 88d phase shift at say
8 Hz, maybe it oscillates.
>
> 2. By agressively cutting LF from 32Hz you reduce loop gain in the working
> range (20Hz) thus increasing output impedance, reducing speaker damping,
> increasing intermodulation and distortion (if it matters at 20Hz?) This is
> aggravated by the fact that the driver stage has to labour 3dB harder at
> 20Hz because of the attenuation in your agressive circuit.. So I bet you
> will get overall results twice worse than Mr Turner would have done with his
> smart shelving.
The shelving leaves some gain available and reduces phase shift. So
usually there is enough FB operative at 20Hz, and with lower phase
shift at 20Hz its more effective than otherwise.
>
> 3. Though your amp might appear stable, most likely it will be peaking close
> to oscillation at 8Hz or so where the +90deg lead from your agressive
> circuit will combine with +90deg lead from your OPT at the 0dB loop gain
> crossing. Any LF rumble might drive your amp into overload.
That's why I have referred him to some typical response graphs at my
website.
>
> So I need to admit, shelving is wiser, because it does not take out precious
> dBs from the loop gain in the whole audio range and gives a better phase
> margin, and no peaking. Mr Turner makes a deep 12dB/octave nose dive below
> 15...20Hz by combining OPT effect with 220K/0.05uF effect, but then, closer
> to 0dB crossing, he gently goes out of the deep nose dive, shelves the beast
> out and happily crossing 0dB at 8dB/octave. Past that he can dive deep
> again -- does not carem as no oscillation will occur below 0dB.
>
> However, both shelving and "agressive" compensation have one drawback in
> common. NFB gain is constant at all frequencies since NFB is simply a
> resistive divider. Thus the NFB tries to have the amp's responce flat (at
> low level of course) down to possibly 3Hz or so. This is crasy and
> unnecessary.
Indeed. The trick is to get the shelving right, neither over done or
uderdone.
Same goes for HF shelving.
The other way to do LF shelving is to reduce the amount of NFB at LF
so you have a parallel network of C&R in series with the FB resistance
in the NFB divider network. I've never needed to use this method. Once
a shelving network is connected and a plotted F response shoes no
peaking outside the 20Hz to 20kHz band, it is wise to plot the
response at the anode output of V1 just before any shelving networks
in the input/driver line up between 1Hz and 1MHz. The signal from V1
output to OP tube grids is called the ERROR SIGNAL, because it
contains a pure version of the music signal PLUS a fraction of a phase
inverted version of the THD/IMD phase shift and any other artifact
generated by the amp. The Error Signal will never be a flat response
except in the middle of the AF band. The peaks in this signal at the
ends of the band and beyond the band should ideally not exceed 3dB
above the level in the centre of the band. Such peaking is inevitable
in most amps as the NFB causes more signal to be applied at band ends
to maintain the output level as flat.
But never should the peaking ever cause any input tube or driver tubes
to become overloaded, work into grid current, become cut off, etc.
Hence driver amps should always be able to make TWICE the voltage one
needs at the OP grids. When testing with a 5kHz square wave, then you
may see some huge peaks appear in the error signal. And especially at
the anodes of OP tubes. The amp is having troubles dealing with HF.
The shelving tends to prevent the amp from bothering to fix the HF
part if square waves above 20kHz, which is fine, lots of NFB above
20kHz, say at 60kHz, a typical HF oscillation frequency, is entirely
pointless.
>
> For that reason, instead of shelving interstage coupling I shelf the NFB.
You have read my mind a bit.
> Instead of a resisive divider I would put a resistor (typically 100R) in
> series with a capacitor (order of 47...100 uF).
But what of the phase shift of that C? Isn't it better to have R&C in
parallel inserted from FB take off at OPT sec to the feedback R? Say
you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT
signal is applied to V1 cathode. Say one adds 3k3 so you then have
3k3, 1k0, then 100r at k to 0V at V1. Then ß becomes 0.022, much less
than 0.09, and at very low F there is no phase shift, so with less NFB
its probably going to be stable. But there isn't enough FB at higher F
so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms
reactance and 3k3 is well shunted.
Peaking still has to be checked in output and in error signal.
>Interstage coupling is kept
> a simple semi-agressive RC circuit (say 0.022uF and 330K). Overall loop gain
> shelving effect is the same as in Mr Turner desihns, but because of the
> deepening feedback at VLF the amp becomes a rumble filter itself. A drawback
> of that method is that you need an electrolytic in the feedback, Some people
> do not "trust" electrolytics as a frequency shaping components.
You can use 63V rated plastic caps OK. I doubt you need 47uF caps.
Switched networks in FB loops were often used for tone controls, not
my scene in power amps though.
Patrick Turner.
>
> Regards,
> Alex- Hide quoted text -
Alex Pogossov
"Patrick Turner" <in...@turneraudio.com.au> wrote in message
news:4339858a-099e-4d81...@x38g2000pri.googlegroups.com...
Patrick:
But what of the phase shift of that C? Isn't it better to have R&C in
parallel inserted from FB take off at OPT sec to the feedback R? Say
you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT
signal is applied to V1 cathode. Say one adds 3k3 so you then have
3k3, 1k0, then 100r at k to 0V at V1. Then � becomes 0.022, much less
than 0.09, and at very low F there is no phase shift, so with less NFB
its probably going to be stable. But there isn't enough FB at higher F
so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms
reactance and 3k3 is well shunted.
Peaking still has to be checked in output and in error signal.
Alex:
I suggested something quite opposite. The NFB divider looks like:
- 1K from the speaker terminal to cathode of the driver stage;
- 100R from the cathode to 100uF capacitor;
- the other end of the 100uF capacitor is tied to GND.
Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in
this case), giving -90deg phase lag in the loop below 10Hz. This is in
effect turning the amp into a anti-rumble filter.
From the first glance it might sound crazy to increase the loop gain at VLF
where we want an overal reduction of yje loop gain, but consider this:
- OPT typically gives +90deg lead below 15...20Hz;
- interstage coupling (simple RC with no shelving, 220K and 0.033uF) is
calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead
below;
- but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz!
And this lag maintains down until 1Hz!
At LF one lag subtracts from two leads and in combination we have only
+90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
crossed safely at only +90deg!
Killing many birds with one stone:
- perfect transient with no peaking;
- natural low cut off in the whole amp (sort of built-in anti-rumble
filtering);
- reduced error signal at VLF as more feedback is applied;
- no need to use two identical shelving circuits in push-pull amps -- just
one extra electrolytic.
A drawback - a despised electrolytic as a shaping component.
Patrick Turner
> "Patrick Turner" <i...@turneraudio.com.au> wrote in message
>
> news:4339858a-099e-4d81...@x38g2000pri.googlegroups.com...
>
> Patrick:
> But what of the phase shift of that C? Isn't it better to have R&C in
> parallel inserted from FB take off at OPT sec to the feedback R? Say
> you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT
> signal is applied to V1 cathode. Say one adds 3k3 so you then have
> than 0.09, and at very low F there is no phase shift, so with less NFB
> its probably going to be stable. But there isn't enough FB at higher F
> so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms
> reactance and 3k3 is well shunted.
>
> Peaking still has to be checked in output and in error signal.
>
> Alex:
> I suggested something quite opposite. The NFB divider looks like:
> - 1K from the speaker terminal to cathode of the driver stage;
> - 100R from the cathode to 100uF capacitor;
> - the other end of the 100uF capacitor is tied to GND.
My apologies, I didn't understand you. And the reason I didn't
understand was because the technique you are suggesting is completely
unknown in the range of traditional means of applying NFB in tube
amps.
Your method here is normal procedure in solid state amps which are all
dc coupled and without any large phase shifts at very low frequencies
down to DC.
>
> Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in
> this case), giving -90deg phase lag in the loop below 10Hz. This is in
> effect turning the amp into a anti-rumble filter.
Yes, I see you are against rumble. But in any tube amp with 2 CR
couplings and an OPT the gain below 10Hz quickly falls to zero at DC
and there is no need to roll off LF any more than it naturally is
rolled off. So if a signal at 1Hz enters the amp, the FB fed back is
extremely small because the output signal is so small.
So I cannot see your method would improve LF behaviour. I've read most
of the texts about NFB applications in tube amps and I don't recall a
single instance where your idea has been applied by any commercial
manufacturer nor have I read of anyone supporting it.
>
> From the first glance it might sound crazy to increase the loop gain at VLF
> where we want an overal reduction of yje loop gain, but consider this:
> - OPT typically gives +90deg lead below 15...20Hz;
> - interstage coupling (simple RC with no shelving, 220K and 0.033uF) is
> calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead
> below;
> - but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz!
> And this lag maintains down until 1Hz!
Well, your NFB method does not increase OLG. OLG remains what it is
regardless of the NFB network. But th closed loop gain, CLG, could be
boosted with positive FB. 3 lots of reactive phase advances in the amp
will add to a rapid phase turnover per octave below 10Hz. So maximum
phase shift can be more than 180d then because the Lp inductance falls
to near zero near dc, you have only dc winding resistance in the OP
stage so phase shift lessens a bit towards DC. But exactly what anyone
measures depends on voltage applied and fiddling around with trying to
increase NFB as one gets close to DC makes no sense to me at all.
When you have measured and demonstrated your technique and analysised
it all with maybe 10 detailed pages on a website with photos of the
amps, then ppl might say you have something to offer. People here are
very difficult people. We cannot agree with anything anyone says
unless they offer the truth, the whole truth, and nothing but the
truth, so help them in the eyes of the God Of Triodes :-)
> At LF one lag subtracts from two leads and in combination we have only
> +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
> crossed safely at only +90deg!
I'll believe works when I see it.
>
> Killing many birds with one stone:
> - perfect transient with no peaking;
> - natural low cut off in the whole amp (sort of built-in anti-rumble
> filtering);
> - reduced error signal at VLF as more feedback is applied;
> - no need to use two identical shelving circuits in push-pull amps -- just
> one extra electrolytic.
>
> A drawback - a despised electrolytic as a shaping component.
Perhaps there are other drawbacks you have not thought about.
I suggest you embark on a course of soldering in your laboratory to
prove your idea works. We all look forward to results.
Patrick Turner.
Engineer
Thanks for all the valuable comment. Here's what I've gleaned from
this so far...
A VLF shelf has merit in that it leaves some Global NFB at VLF and
mitigates the -90 deg phase shift at critical instability frequencies
that adds to the -90 deg in the OPT and the -90 in the other RC
coupler... easy to get to 180 ! This suggest use large C's (and
normal Rg's) inside the NFB loop except for ONE shelf network.
My Fisher OPT is presumably "medium quality" (is that more or less
than the Hammond 1620, 1650F, 1645, etc. series?) in that it came from
a claimed 30 watt RMS Fisher amp (model KX-200PP, not mine... OPT
bought on eBay.)
My 100K grid resistor on the 6L6's is too low... I'll up it to 220
Kohms and recalculate the C for the shelf I need.
Rumble will be cut off in the pre-amp so none hits the PA.
BTW, I do know about phase shift... also gain and phase margins,
Nyquist diagrams, PID control, feedback controller tuning and
stability, open and closed loop Bode plots... and the whole ball of
wax of stochastic sampled data control (off topic!), as I was a
practicing control systems engineer for some 30 years. But the last
18 years were spent in engineering management (pays better!), so you
get a bit rusty, but I still knew when any staffers tried to BS me...
fortunately, not often, as I never forgot the basics! Now retired.
However, tube audio is pretty straightforward in principle but, of
course, needs close design attention or it will bite you.
Again, my thanks to all, particularly Patrick. We shall overcome!
Cheers,
Roger
Alex Pogossov
When you have measured and demonstrated your technique and analysised
it all with maybe 10 detailed pages on a website with photos of the
amps, then ppl might say you have something to offer. People here are
very difficult people. We cannot agree with anything anyone says
unless they offer the truth, the whole truth, and nothing but the
truth, so help them in the eyes of the God Of Triodes :-)
> At LF one lag subtracts from two leads and in combination we have only
> +90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
> crossed safely at only +90deg!
I'll believe works when I see it.
>
> Killing many birds with one stone:
> - perfect transient with no peaking;
> - natural low cut off in the whole amp (sort of built-in anti-rumble
> filtering);
> - reduced error signal at VLF as more feedback is applied;
> - no need to use two identical shelving circuits in push-pull amps -- just
> one extra electrolytic.
>
> A drawback - a despised electrolytic as a shaping component.
Perhaps there are other drawbacks you have not thought about.
I suggest you embark on a course of soldering in your laboratory to
prove your idea works. We all look forward to results.
Alex:
I am not into building audio tube amps, because it is crazy to do while SS
works (or can potentially work) much better in all respects, apart from
creating a warm fuzzy feeling. But I restore and improve radios, and use
this feedback increase at LF technique. Of course there is no rumble in an
AM receiver, but fadings, beating of two stations on close but not
synchronised carriers and simply skimming the band makes lots of VLF.
As you know to minimise AF load on the AM detector, it is not uncommon to
have grid leak in the first audio stage of 10M with 0.05uF of coupling cap.
This huge time constant makes the first tube (6SJ7, e.g.) virtually open to
DC. VLF undulations result in 30V swings on the plate of the 1-st stage
bringing it close to saturation. Part of this reaches the grid of the power
stage (say, 6V6) unnecessarily swinging its bias and plate current. The
later in turn cause undulations on the lightly filtered +B rail, and even
might fall into resonance with the supply CLC filter.
I am not very impressed with the shelving approach, because it is only an
attenuator. While it would cut off 6V6 grid excursions, it will not prevent
from 6SJ7 front overloading.
Instead I use a circuit which helps reduce VLF voltage applied between grid
and cathode of the first stage 6SJ7.
You would say: Use a low cut-off filter. But it is a complication. Much
better is to turn the amp into an active high pass filter by means of the
NFB.
Patrick Turner
>
> > Perhaps there are other drawbacks you have not thought about.
>
> > I suggest you embark on a course of soldering in your laboratory to
> > prove your idea works. We all look forward to results.
>
> > Patrick Turner.
>
> Thanks for all the valuable comment. Here's what I've gleaned from
> this so far...
> A VLF shelf has merit in that it leaves some Global NFB at VLF and
> mitigates the -90 deg phase shift at critical instability frequencies
> that adds to the -90 deg in the OPT and the -90 in the other RC
> coupler... easy to get to 180 ! This suggest use large C's (and
> normal Rg's) inside the NFB loop except for ONE shelf network.
Basically, you have it right. measurements of response with and
without GNFB and networks should confirm the ideas I have presented.
> My Fisher OPT is presumably "medium quality" (is that more or less
> than the Hammond 1620, 1650F, 1645, etc. series?) in that it came from
> a claimed 30 watt RMS Fisher amp (model KX-200PP, not mine... OPT
> bought on eBay.)
Most OPTs in generic mass made stuff is not state of the art. Its
usually well short of that, like Quad-II which saturates at too high a
frequency and which has very high winding losses. Nice effort by Quad
though, real nice, just marred by using wire too thin, and not having
enough iron; thus the amp retail price could compete with all the
other mass made muck in the market place. To expect any brand name to
make real good stuff is expecting far too much unless the brands
happen to be ARC, Conrad Johnson, Manley Labs and a few others, and
you will never find spare non fucked up OPTs on E-bay taken from some
old ARC amp which someone canobolised to sell the parts.
Hammond PP 1650F for 60W for 6k6:4,8&16 ohms have about 50mH of
leakage inductance. They are barely any better than the horror OPTs
fitted to Jolida amps. But I have fitted Hammonds into Jolidas which
had shorted turns due to utterly attrocious Chinese winding methods.
The critical damping networks and shelving networks are 100% necessary
to get the Jolidas to be where they should be. Sound is OK, despite
quality shortcomings. i
> My 100K grid resistor on the 6L6's is too low... I'll up it to 220
> Kohms and recalculate the C for the shelf I need.
Once you have the shelving networks and a response that is say -3dB at
7Hz and its stable and no peaking in bass response below 30Hz, AND you
put in a passive HP filter using 0.22uF and 100k at the amp input,
rumble should never be a problem with vinyl.
> Rumble will be cut off in the pre-amp so none hits the PA.
> BTW, I do know about phase shift... also gain and phase margins,
> Nyquist diagrams, PID control, feedback controller tuning and
> stability, open and closed loop Bode plots... and the whole ball of
> wax of stochastic sampled data control (off topic!), as I was a
> practicing control systems engineer for some 30 years. But the last
> 18 years were spent in engineering management (pays better!), so you
> get a bit rusty, but I still knew when any staffers tried to BS me...
> fortunately, not often, as I never forgot the basics! Now retired.
> However, tube audio is pretty straightforward in principle but, of
> course, needs close design attention or it will bite you.
> Again, my thanks to all, particularly Patrick. We shall overcome!
> Cheers,
> Roger-
The tube amp must be considered to be an active pass band filter which
includes a loop of GNFB which extends the open loop band width. All
sorts of traps exist for the un-wary who have never studied phase
shift.
I'm no real expert and dealing with reactance equations with J factor
and square root of -1 and vectors. All the high-end maths is all
incomprehensible to me. I've never studied at a university course
covering such stuff. But I have enough hands on experience to know
what BS behaviour to expect from amps and how to get maximum stable
bandwidth with any sort of load at the highest possible power. There
are limits to what may be had, mostly defined by the OPT.
In my new amps the open loop bandwidth at full power without GNFB is
designed to be 14Hz to 65kHz. When I use GNFB without any shelving
networks the darn things will often oscillate at very low F and at
very high F, depending on the load. But with 65kHz of BW to begin with
these oscillations are easy to tame and get a stable closed loop BW =
65kHz, same as the open loop, and with unusually high "magin of
stability".
With the most inferior OPTs I have used in re-engineering efforts,
maybe I get full PO BW from 50Hz to 25kHz, and the F at which these
amps try to oscillate at are close to or within the 20 to 20kHz band.
At 1/4 full PO, ie, 1/2 Vo, bandwidth then becomes about 25Hz to 30kHz
and as long as low average levels are used the sound is passable.
Patrick Turner.
Engineer
(snip)
> ... But I restore and improve radios, and use
> this feedback increase at LF technique. Of course there is no rumble in an
> AM receiver, but fadings, beating of two stations on close but not
> synchronised carriers and simply skimming the band makes lots of VLF.
>
> As you know to minimise AF load on the AM detector, it is not uncommon to
> have grid leak in the first audio stage of 10M with 0.05uF of coupling cap.
> This huge time constant makes the first tube (6SJ7, e.g.) virtually open to
> DC. VLF undulations result in 30V swings on the plate of the 1-st stage
> bringing it close to saturation. Part of this reaches the grid of the power
> stage (say, 6V6) unnecessarily swinging its bias and plate current. The
> later in turn cause undulations on the lightly filtered +B rail, and even
> might fall into resonance with the supply CLC filter.
>
> I am not very impressed with the shelving approach, because it is only an
> attenuator. While it would cut off 6V6 grid excursions, it will not prevent
> from 6SJ7 front overloading.
>
> Instead I use a circuit which helps reduce VLF voltage applied between grid
> and cathode of the first stage 6SJ7.
>
> You would say: Use a low cut-off filter. But it is a complication. Much
> better is to turn the amp into an active high pass filter by means of the
> NFB.
Alex, this interests me as I'm an avid AM receiver "improver", too,
but it's not clear to me just what you are referring to.
Yes, the common 0.05 uF and 10 meg grid leak (or even the more common
4.7 megs) gives the triode audio stage "near DC amp" behavior (to as
low as 0.3 Hz -3dB) but the DC comes from the detector diode,
proportional to RF signal level, and feeds back via the AVC line (to
the IF and converter tubes) to alleviate this. Also, I don't see this
VLF getting to the o/p tube unless someone has used a huge coupling
cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.
Perhaps there's a good reason for this little 0.005 uF... it also
stops the tiny speakers from rattling too much!
Could you please describe the cct fix you allude to? Sorry, I can't
access binaries so it has to be words or .jpg pics to Flickr, or
somewhere!
Thanks and cheers,
Roger
Patrick Turner
> On Jun 19, 7:47 am, "Alex Pogossov" <apogo...@tpg.com.au> wrote:
>
> (snip)
>
>
>
>
>
> > ... But I restore and improve radios, and use
> > this feedback increase at LF technique. Of course there is no rumble in an
> > AM receiver, but fadings, beating of two stations on close but not
> > synchronised carriers and simply skimming the band makes lots of VLF.
I also am always working on AM radio restorations. Even in old radios
with working original circuits there is very little trouble with
anything you are saying about VLF. Makers normally manage to make
their radios stable at VLF and fading and variations in applied AVC
voltage cause no audible problems.
>
> > As you know to minimise AF load on the AM detector, it is not uncommon to
> > have grid leak in the first audio stage of 10M with 0.05uF of coupling cap.
> > This huge time constant makes the first tube (6SJ7, e.g.) virtually open to
> > DC.
The 10M and 0.05 is used to bias a triode-diode or pentode-diode tube
with cathode taken to 0V. Its grid leak bias. Bias will wobble a bit
when tuning but once tuned I've never encountered VLF bothers. SW
signals dafe all over the place but generate very small carrier levels
so the AVC hardly varies at all.
> VLF undulations result in 30V swings on the plate of the 1-st stage
> > bringing it close to saturation.
Not always, and it usually does not matter. But I never just repair a
damn awful bean counter designed radio where the part list has been
minimised to maximise the profits. I always dump the tube rectifier,
put in large PSU caps, reduce hum by 30dB at least, then dump 6V6 and
put in EL34 in triode, add a couple of tubes to make a decent detector
and generally make the sound quality far better than the day they made
the radio 50 years ago.
> Part of this reaches the grid of the power
> > stage (say, 6V6) unnecessarily swinging its bias and plate current. The
> > later in turn cause undulations on the lightly filtered +B rail, and even
> > might fall into resonance with the supply CLC filter.
After fixing maybe 50 AM radios, I've never encountered your problems,
and I'm one of the most observant techs around.
>
> > I am not very impressed with the shelving approach, because it is only an
> > attenuator. While it would cut off 6V6 grid excursions, it will not prevent
> > from 6SJ7 front overloading.
Not if the whole amp has been designed right!
>
> > Instead I use a circuit which helps reduce VLF voltage applied between grid
> > and cathode of the first stage 6SJ7.
>
> > You would say: Use a low cut-off filter. But it is a complication. Much
> > better is to turn the amp into an active high pass filter by means of the
> > NFB.
Good luck, you might need it.
>
> Alex, this interests me as I'm an avid AM receiver "improver", too,
> but it's not clear to me just what you are referring to.
> Yes, the common 0.05 uF and 10 meg grid leak (or even the more common
> 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as
> low as 0.3 Hz -3dB) but the DC comes from the detector diode,
> proportional to RF signal level, and feeds back via the AVC line (to
> the IF and converter tubes) to alleviate this. Also, I don't see this
> VLF getting to the o/p tube unless someone has used a huge coupling
> cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.
> Perhaps there's a good reason for this little 0.005 uF... it also
> stops the tiny speakers from rattling too much!
> Could you please describe the cct fix you allude to? Sorry, I can't
> access binaries so it has to be words or .jpg pics to Flickr, or
> somewhere!
> Thanks and cheers,
> Roger- Hide quoted text -
There is often AF signal down to about 20Hz in transmitted AM signals.
Old radio OPTs often saturate at 80Hz, full power, and speakers don't
go low so depending on the radio one has to ensure the CR couplings
have poles which are a lot higher than what's used in a hi-fi amp. The
typical AM radio might have a 6AV6 triode-diode first AF tube coupled
to the 6V6 output and NO GNFB whatsoever. Use of any FB at all often
makes oscillations happen because of very poor OL bandwidth. And
because AF is limited by the sideband cutting, ie, the AF BW is
defined by the total response of 5 tuned circuits then the rising
speaker AF output without GNFB gives a wanted boost to F between 1kHz
and 3 kHz before the AF drops away at 20dB per octave. But if the IFTs
are well made and some other tricks are used to reduce selectivity
while maitainining skirt selectivity over 30kHz away from some other
station then audio BW can be 8kHz and then a flat amp with NFB makes
good sense. If proper rules are followed as one does for a hi-fi amp
you can get AM that is excellent sound and none of the VLF
instabilities being cited here.
I can listen to the news broadcast on AM radio and then switch to the
FM tuner which is also broadcasting the same news from the same feed
and I hear better sound via the AM radio I have. The tuner is an old
Marantz chip based thing from late 1970s, not bad in its own way, but
it don't way very much :-)
Patrick Turner.
Alex Pogossov
Alex, this interests me as I'm an avid AM receiver "improver", too,
but it's not clear to me just what you are referring to.
Yes, the common 0.05 uF and 10 meg grid leak (or even the more common
4.7 megs) gives the triode audio stage "near DC amp" behavior (to as
low as 0.3 Hz -3dB) but the DC comes from the detector diode,
proportional to RF signal level, and feeds back via the AVC line (to
the IF and converter tubes) to alleviate this. Also, I don't see this
VLF getting to the o/p tube unless someone has used a huge coupling
cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.
Perhaps there's a good reason for this little 0.005 uF... it also
stops the tiny speakers from rattling too much!
Could you please describe the cct fix you allude to? Sorry, I can't
access binaries so it has to be words or .jpg pics to Flickr, or
somewhere!
Thanks and cheers,
Roger
Alex:
Nice to meet another radio man on the tube amp list. Of course the easiest
improvement of a tube radio would be getting rid of the tubes and replacing
evething, in the first place, an audio amp with an IC, source follower after
the detector, op-amp based tone control, etc. A simple op-amp based audio
amp powered from ~6.3V winding would work far better than a boring 6AV6 +
6AQ5. However, it is not the way. The trick is to improve a radio keeping
the tubes and approximately the same topology, not adding much.
There are several issues, e.g.:
1. How to make AM detector to handle 95% of modulation, not 60...70% as in
most of the boring radios;
2. Reduce distortion and booming in audio.
Part 1 would cover optimisation of the AM detector, including unveiling of
the "three-germanium-diodes-in-series" witchcraft of Mr.Turner. Which
detector is the best? Vacuum diode? Silicon? Germanium? And why.
Part 2 would cover adding NFB to the audio amp for distortion reduction and
speaker damping, at the same time turning the amp in a high-pass filter (not
to overload a lousy OPT and speaker with bass and DC level fluctuations).
I will try to write up something on these issues from both theoretical and
practical experience.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jun 18, 9:56�pm, "Alex Pogossov" <apogo...@tpg.com.au> wrote:
> > "Patrick Turner" <i...@turneraudio.com.au> wrote in message
> >
> > news:4339858a-099e-4d81...@x38g2000pri.googlegroups.com...
> >
> > Patrick:
> > But what of the phase shift of that C? Isn't it better to have R&C in
> > parallel inserted from FB take off at OPT sec to the feedback R? Say
> > you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT
> > signal is applied to V1 cathode. Say one adds 3k3 so you then have
> > 3k3, 1k0, then 100r at k to 0V at V1. Then � becomes 0.022, much less
Hi Patrick,
I like Alex's idea. As you point out, this feedback scheme was widely used in
solid state amps, although as you allude to, it didn't have anything to do with
feedback stability, my take was that the reason for its use was to increase the
amount of feedback at DC in order to minimize the DC offset at the output of the
amp. The thing I always liked about these solid state amps is the built in
rumble filter, which Alex also seems to like.
So this discussion gets me thinking, how could I apply Alex's idea to a tube amp
without using the despised electrolytic capacitor.
My solution, multiply the impedance of the feedback network by 220 X, making
Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor
0.47uF. Of course this network will no longer drive the cathode of V1 because
its impedance is too high, so I propose to add an additional triode operating as
a cathode follower driven by the high impedance feedback network and direct
coupled to the cathode of V1.
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/
John Byrns
"Alex Pogossov" <apog...@tpg.com.au> wrote:
There is nothing wrong with discussing radio's on this "tube amp list". If you
lookup the charter for rec.audio.tubes, you will find that the group is not
restricted to amps, "Radio Circuits" are explicitly listed as an allowed topic
of discussion!
I too noticed Patrick's mention of "three-germanium-diodes-in-series" and
wondered what this was all about? The last time Patrick discussed his AM
detector designs, IIRC the Turner standard AM detector was an RF cathode
follower driving a single germanium diode.
John L Stewart
Alex, this interests me as I'm an avid AM receiver "improver", too,
but it's not clear to me just what you are referring to.
Yes, the common 0.05 uF and 10 meg grid leak (or even the more common
4.7 megs) gives the triode audio stage "near DC amp" behavior (to as
low as 0.3 Hz -3dB) but the DC comes from the detector diode,
proportional to RF signal level, and feeds back via the AVC line (to
the IF and converter tubes) to alleviate this. Also, I don't see this
VLF getting to the o/p tube unless someone has used a huge coupling
cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.
Perhaps there's a good reason for this little 0.005 uF... it also
stops the tiny speakers from rattling too much!
Could you please describe the cct fix you allude to? Sorry, I can't
access binaries so it has to be words or .jpg pics to Flickr, or
somewhere!
Thanks and cheers,
Roger
Has anyone ever tried the attached cct? Shows up in some form in the front
of all the RCA Tube Handbooks. It avoids the AC coupling between the diode
detector & 1st AF amp.
Earlier apps used the low mu 85 triode but more recently the 6SR7 or 6BF6
have been recommended.
Cheers, John
+-------------------------------------------------------------------+
|Filename: AM Detector & Biased DC Amp A.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=220|
Engineer
> "Engineer" <junk2...@rogers.com> wrote in message
Alex, you too! I regularly replace the tube diode detector with a
1N34A and use NFB to the cathode of the audio triode from the OPT
secondary (47 to 100 ohms to GND and whatever FB resistor I need to
get about 10 dB loss in sensitivity in the audio stages), but I don't
try to steer the audio frequency response - it just extends a bit
under NFB with less distortion and I haven't had a speaker rattle
problem from too much bass extension.
Looking forward to your schematics (NOT on binaries, please!)
Cheers,
Roger
John Byrns
John L Stewart <John.L.Stew...@audiobanter.com> wrote:
> Alex, this interests me as I'm an avid AM receiver "improver", too,
> but it's not clear to me just what you are referring to.
> Yes, the common 0.05 uF and 10 meg grid leak (or even the more common
> 4.7 megs) gives the triode audio stage "near DC amp" behavior (to as
> low as 0.3 Hz -3dB) but the DC comes from the detector diode,
> proportional to RF signal level, and feeds back via the AVC line (to
> the IF and converter tubes) to alleviate this. Also, I don't see this
> VLF getting to the o/p tube unless someone has used a huge coupling
> cap to it. The typical 0.005 uF and 470K start to roll off at 67 Hz.
> Perhaps there's a good reason for this little 0.005 uF... it also
> stops the tiny speakers from rattling too much!
> Could you please describe the cct fix you allude to? Sorry, I can't
> access binaries so it has to be words or .jpg pics to Flickr, or
> somewhere!
> Thanks and cheers,
> Roger
>
> Has anyone ever tried the attached cct? Shows up in some form in the front
> of all the RCA Tube Handbooks. It avoids the AC coupling between the diode
> detector & 1st AF amp.
>
> Earlier apps used the low mu 85 triode but more recently the 6SR7 or 6BF6
> have been recommended.
>
> +-------------------------------------------------------------------+
> |Filename: AM Detector & Biased DC Amp A.jpg |
> |Download: http://www.audiobanter.com/attachment.php?attachmentid=220|
> +-------------------------------------------------------------------+
Hi John,
I haven't used this circuit myself, but I believe it was used as part of one
modification to update the legendary Western Electric 10-A Radio Receiver. I
think the goals of the update were to eliminate overloading in the RF amplifier
stages that resulted from the early design pentode tubes that were used, to
replace the anode bend audio detector with a more linear diode detector, and to
fix the AGC detector/amplifier to eliminate the sensitivity of the AGC voltage
to modulation level. The receiver has 5 tubes and I think the objective was to
do the modification by simply replacing the existing tube sockets with octal
sockets, without adding more tubes.
The RF amplifier fix was simple, the original RF amplifier pentodes were
replaced with the modern metal octal 6K7 tubes. I don't remember exactly how
the other two tube sockets were used. The problem with the AGC was that it was
sensitive to heavy modulation, this was said to be because the AGC detector was
a "square law detector", but I suspect that the problem was more likely that the
AGC detector/amplifier was responsive to the modulation peaks rather than the
average carrier level. At any rate IIRC I believe that the circuit you are
referring to was used as the new detector and AGC amplifier. I don't remember
what the 5th tube was used for after the modification, probably an audio
amplifier stage to drive the line out transformer that was driven by the anode
bend detector in the original circuit.
Patrick Turner
Alex said...> > > A drawback - a despised electrolytic as a shaping
component.
>
I said.....> > Perhaps there are other drawbacks you have not thought
about.
>
> > I suggest you embark on a course of soldering in your laboratory to
> > prove your idea works. We all look forward to results.
>
> Hi Patrick,
>
> I like Alex's idea. As you point out, this feedback scheme was widely used in
> solid state amps, although as you allude to, it didn't have anything to do with
> feedback stability, my take was that the reason for its use was to increase the
> amount of feedback at DC in order to minimize the DC offset at the output of the
> amp. The thing I always liked about these solid state amps is the built in
> rumble filter, which Alex also seems to like.
With SS and total direct coupling there is good LF stability and LF BW
is limited by the rail caps.
But one sure doesn't want any DC entering the differential input pair
of any SS amp because it will
appear at the output. DC gain = unity.
I cannot think of any reason to apply the idea in any tube amp.
>
> So this discussion gets me thinking, how could I apply Alex's idea to a tube amp
> without using the despised electrolytic capacitor.
>
> My solution, multiply the impedance of the feedback network by 220 X, making
> Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor
> 0.47uF. Of course this network will no longer drive the cathode of V1 because
> its impedance is too high, so I propose to add an additional triode operating as
> a cathode follower driven by the high impedance feedback network and direct
> coupled to the cathode of V1.
Its very easy to have a differential pair at the input of any PP OR SE
amp, and the two high impedance grid inputs are used as ports for the
input and the FB signal. So the FB network can all be high impedance
as you say, but why you'd have a series cap in there is really unknown
to me. An adequate rumble filter can be made with a single passive C&R
HPF at input, or two in series so although it's -3dB at say 10Hz,
ultimately roll off becomes -12dB/octave below 3Hz.
Patrick Turner.
Patrick Turner
> In article <4dff537...@dnews.tpgi.com.au>,
> "Alex Pogossov" <apogo...@tpg.com.au> wrote:
>
>
>
>
>
> > "Engineer" <junk2...@rogers.com> wrote in message
I tried 3 germanium diodes in series after an IFT secondary with a
recent radio job with following C = 200k and a small cap, maybe 68pF I
recall. following this there was an emitter follower comprising 2
generic darlington connected signal bjts operating from a +50V supply
The 200k biases the bjt base from +35V. Anyway, I found the 3 series
Gw diodes gave a much higher reverse current leakage because their
backward resistance is in series. Forward voltage drop is still low,
but at very low level signals the THD due to diode turn on was still
low, and low level short wave audio was OK, as good as with tube
diodes.
Ge diodes I used were unknown types in my parts bins; maybe those with
highest reverse voltage ratings have least reverse leakage, but you
guys must try lots and lots of things to make things work they way you
want, like I do during nearly every day of my life.
The emitter follower convert the output pf the diode + RC circuit to
low Z for driving a volume control and it buffers the detector output
to prevent high THD when modulation exceeds 70% I get very low
detector distortion. Its necessary to have a low distortion detector
because radio stations use high percentage mod for efficiency, wheras
in old days mod % was quite low.
I usually connect last IFT winding directly to a grid of cathode
follower with IFT winding biased at about +30Vdc. From CF low Z output
I then use Ge diode to charge 220pF with say 500k taken to 0V, so Ge
diode is slightly biased with 0.07mA, ie, its "on" a bit, and the
leakage doesn't cause trouble in such a low Z drive circuit. See the
AM section of the AM/FM radio at....
http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.htm
The CF shows 6AU7, but should be 6AU6,- typo - but could also be 1/2
12AU7, then detector circuit, and then another 1/2 12AU7 CF to buffer
detector output. The 330k and 100k in my schematic keep the AM radio
output about level with what comes from the FM circuit.
Patrick Turner.
Alex Pogossov
"John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
news:John.L.Stew...@audiobanter.com...
It looks like this circuit serves as an AGC amplifier rather than a buffer
between the AM detector and the volume control.
Two factors would limit the depth of modulation it can handle:
1. Volume control is AC coupled (via C6) to the AM detector load;
2. AGC filter R4C2 is also coupled to the AM detector load.
Thus this circuit is a poor performer.
Another problem is that it requires careful adjustment (of the two
potentiometers, or rather tapped resistors) to have a desired initial AGC
voltage (about -3V I guess). I saw a better version of this circuit where
one diode of this say 6Q7 tube is used as an AM detector, and the other
diode anode serves as an AGC output, at least not allowing the AGC voltage
to go positive.
Regards,
Alex
Alex Pogossov
OK, the 0.47 + 1M strapped with 0.047 + 220k acts like this :-
At 1 kHz, all C have low Z so the phase shift is low and the driver,
ie concertina or whatever you have "sees" a load of 220k. As F is
reduced, there is a pole between 0.047 and 220 at 15.4Hz which is
usually above the F at which the amp may want to oscillate. Sometimes
I have used 0.022uF, so pole is at 32Hz. phase shift caused is less
than 90 degrees. As F is further reduced, the network response tends
to flatten to a shelf formed by 1M and 220k, ie, signal flattens at
roughly -15db, 0.18 times the 1kHz level. The 0.047 has gone to an
open circuit by 2Hz with little effect. But the 0.47 the rolls the
response off at the pole between 0.47uF and 1.22M ohms, ie at 0.27Hz,
below which there is an ultimate phase shift of 90d, but at such a low
F as to not cause bothers because its well below the poles of all
other sages.
Alex:
Shelving network does a good job, but it has a drawback. It is an attenuator
for low frequencies. If low frequency signal or DC step gets applied to the
input of the amp, the first stage will or might overload, while the shelving
circuit will protect the next stage from the overloading.
To prevent the front stage from overloading at VLF it is better to apply the
shelving as a local feedback, i.e. in the cathode circuit of the 1-st stage.
Imagine the cathode of the 1-st stage is not directly connected to a
feedback divider (say 1K/100R) but through a RC circuit of paralleled 10K
and 5uF (approx.) This cathode degeneration at LF will act similar to the
attenuator shelving, but will prevent overloading of the first stage by VLF.
Then you do not need 1M||0.05uF interstage attenuator.
Of course in this cathode shelving the grid DC bias voltage has to be
elevated, so the grid can not be directly connected to the volume control.
However it is not a big deal. there are three known ways of elevating grid
bias voltage:
- fixed divider from upply voltage;
- divider cathode-grid-ground;
- split cathode resistor and from the tap thus formed throw 1M to the grid.
Regards,
Alex
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> > So this discussion gets me thinking, how could I apply Alex's idea to a
> > tube amp
> > without using the despised electrolytic capacitor.
> >
> > My solution, multiply the impedance of the feedback network by 220 X,
> > making
> > Alex's 1K resistor 220k, his 100 Ohm resistor 22k, and his 100uF capacitor
> > 0.47uF. �Of course this network will no longer drive the cathode of V1
> > because
> > its impedance is too high, so I propose to add an additional triode
> > operating as
> > a cathode follower driven by the high impedance feedback network and direct
> > coupled to the cathode of V1.
>
> Its very easy to have a differential pair at the input of any PP OR SE
> amp, and the two high impedance grid inputs are used as ports for the
> input and the FB signal. So the FB network can all be high impedance
> as you say, but why you'd have a series cap in there is really unknown
> to me. An adequate rumble filter can be made with a single passive C&R
> HPF at input, or two in series so although it's -3dB at say 10Hz,
> ultimately roll off becomes -12dB/octave below 3Hz.
The reason for the "series cap" is because it provides the same function as your
low frequency gain stepping network, but does the job more effectively with
fewer drawbacks, as Alex points out. I suppose that Alex's scheme has the
drawback of affecting the amplifiers frequency response if the gain change must
start in the audible range to achieve stability. The rumble filter effect is
not the point, it is just a bonus that comes with the circuit.
Patrick Turner
> > Its very easy to have a differential pair at the input of any PP OR SE
> > amp, and the two high impedance grid inputs are used as ports for the
> > input and the FB signal. So the FB network can all be high impedance
> > as you say, but why you'd have a series cap in there is really unknown
> > to me. An adequate rumble filter can be made with a single passive C&R
> > HPF at input, or two in series so although it's -3dB at say 10Hz,
> > ultimately roll off becomes -12dB/octave below 3Hz.
>
> The reason for the "series cap" is because it provides the same function as your
> low frequency gain stepping network, but does the job more effectively with
> fewer drawbacks, as Alex points out. I suppose that Alex's scheme has the
> drawback of affecting the amplifiers frequency response if the gain change must
> start in the audible range to achieve stability. The rumble filter effect is
> not the point, it is just a bonus that comes with the circuit.
>
> --
> Regards,
>
> John Byrns
I've made a considerable number of phono amps and I've never had to
deliberately include a rumble filter in any of them nor in any power
amp. I've seen speaker cones wobbling around especially with bass
boost on a tone control turned up on some amps made by others but not
much of that happens in my amps.
The shelving idea is not mine though; it was invented and used long
ago by all the brightest engineers and they published and I agreed
they were right.
The worst effects of having bass response going too low in tube amps
is best witnessed if one uses pink noise which has a flat bandwidth
from say 1Hz to 20kHz, and as one cranks the volume you'll hear
knocking noise in the OPT from core saturation before clipping occurs.
Just put in first order HPF with pole at 14Hz and the knocking noise
vanishes, and level can be raised to clipping OK. Usually there is no
music or vinyl artifacts worse than VLF in full range pink noise.
I get fabulous bass from my phono amps but the VLF don't make it past
the amp. No global FB is used anywhere. No peaks in VLF response.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> >
> > > Its very easy to have a differential pair at the input of any PP OR SE
> > > amp, and the two high impedance grid inputs are used as ports for the
> > > input and the FB signal. �So the FB network can all be high impedance
> > > as you say, but why you'd have a series cap in there is really unknown
> > > to me. An adequate rumble filter can be made with a single passive C&R
> > > HPF at input, or two in series so although it's -3dB at say 10Hz,
> > > ultimately roll off �becomes -12dB/octave below 3Hz.
> >
> > The reason for the "series cap" is because it provides the same function as
> > your
> > low frequency gain stepping network, but does the job more effectively with
> > fewer drawbacks, as Alex points out. �I suppose that Alex's scheme has the
> > drawback of affecting the amplifiers frequency response if the gain change
> > must
> > start in the audible range to achieve stability. �The rumble filter effect
> > is
> > not the point, it is just a bonus that comes with the circuit.
>
> I've made a considerable number of phono amps and I've never had to
> deliberately include a rumble filter in any of them nor in any power
> amp. I've seen speaker cones wobbling around especially with bass
> boost on a tone control turned up on some amps made by others but not
> much of that happens in my amps.
You are fixated on the "rumble filter" aspect, which isn't the point of Alex's
circuit, it is just an extra feature that comes for free. And when we are
talking about phono reproduction, rumble isn't really the issue even there, it
is the VLF signals caused by record warp. The record warp signals can be
largely mitigated by the proper choice of arm mass and cartridge compliance.
> The shelving idea is not mine though; it was invented and used long
> ago by all the brightest engineers and they published and I agreed
> they were right.
Yes, we know the shelving idea isn't yours, it was used in many amps, but wasn't
commonly used in the typical USA built amplifiers which usually had only two low
frequency poles, and so didn't require a low frequency shelving network for
stability, high frequency shelving networks were another mater though, and were
commonly used.
Here is a simplified schematic of an AM broadcast transmitter from the 1940s
that includes a low frequency shelving network
http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg
Notice that the audio section includes no fewer than 10 low frequency poles, 7
of them within the negative feedback loop, no wonder a low frequency gain
stepping network was required!
> The worst effects of having bass response going too low in tube amps
> is best witnessed if one uses pink noise which has a flat bandwidth
> from say 1Hz to 20kHz, and as one cranks the volume you'll hear
> knocking noise in the OPT from core saturation before clipping occurs.
> Just put in first order HPF with pole at 14Hz and the knocking noise
> vanishes, and level can be raised to clipping OK.
I would expect that Alex's feedback circuit also helps with your "knocking noise
in the OPT", for real world signals although not to quite the same extent as
your first order HPF does. If that isn't enough, your "first order HPF" can
always be used in combination with Alex's circuit. But this "knocking noise in
the OPT" is really beside the point of Alex's circuit which is to stablize the
feedback loop at low frequencies.
> Usually there is no
> music or vinyl artifacts worse than VLF in full range pink noise.
> I get fabulous bass from my phono amps but the VLF don't make it past
> the amp. No global FB is used anywhere. No peaks in VLF response.
I take it your are referring to the phono preamp only in this last paragraph?
TerentijP
������ ��� - �������� � �����.
+-------------------------------------------------------------------+
+-------------------------------------------------------------------+
--
TerentijP
Patrick Turner
> You are fixated on the "rumble filter" aspect, which isn't the point of Alex's
> circuit, it is just an extra feature that comes for free. And when we are
> talking about phono reproduction, rumble isn't really the issue even there, it
> is the VLF signals caused by record warp. The record warp signals can be
> largely mitigated by the proper choice of arm mass and cartridge compliance.
I doubt I am fixated about rumble because I never have any trouble
with it. I question if there is any benefit in Alex's circuit. Has
anyone tried it? where are measured results?
>
> > The shelving idea is not mine though; it was invented and used long
> > ago by all the brightest engineers and they published and I agreed
> > they were right.
>
> Yes, we know the shelving idea isn't yours, it was used in many amps, but wasn't
> commonly used in the typical USA built amplifiers which usually had only two low
> frequency poles, and so didn't require a low frequency shelving network for
> stability, high frequency shelving networks were another mater though, and were
> commonly used.
>
> Here is a simplified schematic of an AM broadcast transmitter from the 1940s
> that includes a low frequency shelving network
>
> http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg
>
> Notice that the audio section includes no fewer than 10 low frequency poles, 7
> of them within the negative feedback loop, no wonder a low frequency gain
> stepping network was required!
Sometimes two lots of shelving are needed.
But notice that the two loops of balanced GNFB come from a pair of
dividers at the output but there's a cap in series with the FB, so
there **cannot be dc FB**. FB reduces as F goes low because of the
cap.
> > The worst effects of having bass response going too low in tube amps
> > is best witnessed if one uses pink noise which has a flat bandwidth
> > from say 1Hz to 20kHz, and as one cranks the volume you'll hear
> > knocking noise in the OPT from core saturation before clipping occurs.
> > Just put in first order HPF with pole at 14Hz and the knocking noise
> > vanishes, and level can be raised to clipping OK.
>
> I would expect that Alex's feedback circuit also helps with your "knocking noise
> in the OPT", for real world signals although not to quite the same extent as
> your first order HPF does. If that isn't enough, your "first order HPF" can
> always be used in combination with Alex's circuit. But this "knocking noise in
> the OPT" is really beside the point of Alex's circuit which is to stablize the
> feedback loop at low frequencies.
I am having trouble seeing how DC FB around a tube amp could make
anything better at all. Where is the evidence? While an OPT is making
knocking noises, the core inductance is intermittently becoming
negligible and the RL becomes = to Rw, thus causing gross distortions
to all signal F. Any kind of GNFB makes it worse.
I'd like to see a schematic of what Alex proposes before making up my
mind finally about it.
>
> > Usually there is no
> > music or vinyl artifacts worse than VLF in full range pink noise.
> > I get fabulous bass from my phono amps but the VLF don't make it past
> > the amp. No global FB is used anywhere. No peaks in VLF response.
>
> I take it your are referring to the phono preamp only in this last paragraph?
Well yes. I sure don't need rumble filters in my power amps. Not even
in AF amps in radios, with say 12AX7 driving EL34 in triode with 12dB
FB. I've been doing all this for years and never have had bothers with
VLF.
I guess I know enough about CR time constants to get by.
Patrick Turner.
Patrick Turner
> >+90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
> >crossed safely at only +90deg!
>
> lead/lag analysis because, if I understand your circuit correctly,
> that cap is at the summing junction so shouldn't it introduce poles
> into both the NFB and the input signal?
I'm not the only one questioning Alex's FB network. You are having
trouble accepting Alex's claims.
Where's the evidence? its no fucking good making claims without backup
evidence.
Patrick Turner.
Patrick Turner
> On Wed, 22 Jun 2011 03:50:30 -0700 (PDT), Patrick Turner
>
> <i...@turneraudio.com.au> wrote:
>
> >> You are fixated on the "rumble filter" aspect, which isn't the point of Alex's
> >> circuit, it is just an extra feature that comes for free. And when we are
> >> talking about phono reproduction, rumble isn't really the issue even there, it
> >> is the VLF signals caused by record warp. The record warp signals can be
> >> largely mitigated by the proper choice of arm mass and cartridge compliance.
>
> >I doubt I am fixated about rumble because I never have any trouble
> >with it.
>
> ever talk about.
I talked about other issues, although perhaps you didn't notice.
Rumble filters are not needed unless you have an extremely poor TT,
recording etc, and I ain't got either, so I don't have rumble
filters.
>
> > I question if there is any benefit in Alex's circuit.
>
> He provided a list.
Maybe all totally invalid. I've asked for EVIDENCE that the claims he
makes are valid.
Deafening silence ever since.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> >
> > You are fixated on the "rumble filter" aspect, which isn't the point of
> > Alex's
> > circuit, it is just an extra feature that comes for free. �And when we are
> > talking about phono reproduction, rumble isn't really the issue even there,
> > it
> > is the VLF signals caused by record warp. �The record warp signals can be
> > largely mitigated by the proper choice of arm mass and cartridge
> > compliance.
>
> I doubt I am fixated about rumble because I never have any trouble
> with it. I question if there is any benefit in Alex's circuit. Has
> anyone tried it? where are measured results?
The why do you keep on about the rumble issue? "Rumble Filter" was an
unfortunate choice of words by Alex, and I have unfortunately helped perpetuate
it. A better description of the effect would be a HPF with a VLF cutoff. It is
really not correct to call it a rumble filter because rumble is usually in the
audible band and hence a "Rumble Filter" has a much higher cutoff frequency. As
I said in an earlier post the effect of the filter is more to eliminate subsonic
signals resulting from record warp. So lets just call it a "VLF HPF that has
useful properties, in addition to its primary purpose of helping to stabilize
the feedback.
> > > The shelving idea is not mine though; it was invented and used long
> > > ago by all the brightest engineers and they published and I agreed
> > > they were right.
> >
> > Yes, we know the shelving idea isn't yours, it was used in many amps, but
> > wasn't
> > commonly used in the typical USA built amplifiers which usually had only
> > two low
> > frequency poles, and so didn't require a low frequency shelving network for
> > stability, high frequency shelving networks were another mater though, and
> > were
> > commonly used.
> >
> > Here is a simplified schematic of an AM broadcast transmitter from the
> > 1940s
> > that includes a low frequency shelving network
> >
> > http://www.qsl.net/wa2whv/XT1A/xt1schem.jpg
> >
> > Notice that the audio section includes no fewer than 10 low frequency
> > poles, 7
> > of them within the negative feedback loop, no wonder a low frequency gain
> > stepping network was required!
>
> Sometimes two lots of shelving are needed.
>
> But notice that the two loops of balanced GNFB come from a pair of
> dividers at the output but there's a cap in series with the FB, so
> there **cannot be dc FB**. FB reduces as F goes low because of the
> cap.
Yes, that would appear to result in several undesirable consequences.
> > > The worst effects of having bass response going too low in tube amps
> > > is best witnessed if one uses pink noise which has a flat bandwidth
> > > from say 1Hz to 20kHz, and as one cranks the volume you'll hear
> > > knocking noise in the OPT from core saturation before clipping occurs.
> > > Just put in first order HPF with pole at 14Hz and the knocking noise
> > > vanishes, and level can be raised to clipping OK.
> >
> > I would expect that Alex's feedback circuit also helps with your "knocking
> > noise
> > in the OPT", for real world signals although not to quite the same extent
> > as
> > your first order HPF does. �If that isn't enough, your "first order HPF"
> > can
> > always be used in combination with Alex's circuit. �But this "knocking
> > noise in
> > the OPT" is really beside the point of Alex's circuit which is to stablize
> > the
> > feedback loop at low frequencies.
>
> I am having trouble seeing how DC FB around a tube amp could make
> anything better at all.
It's not the "DC feedback", it is compensating phase shift introduced by Alex's
network. I believe a similar compensation scheme is widely used at high
frequencies in the form of a capacitor across the part or all of the feedback
resistor.
> Where is the evidence? While an OPT is making
> knocking noises, the core inductance is intermittently becoming
> negligible and the RL becomes = to Rw, thus causing gross distortions
> to all signal F. Any kind of GNFB makes it worse.
How many people have a need to reproduce "pink noise" that extends down to DC,
if such a signal even exists?
> I'd like to see a schematic of what Alex proposes before making up my
> mind finally about it.
Alex described the circuit, and I improved the idea by adding a cathode follower.
> > > Usually there is no
> > > music or vinyl artifacts worse than VLF in full range pink noise.
> > > I get fabulous bass from my phono amps but the VLF don't make it past
> > > the amp. No global FB is used anywhere. No peaks in VLF response.
> >
> > I take it your are referring to the phono preamp only in this last
> > paragraph?
>
> Well yes. I sure don't need rumble filters in my power amps. Not even
> in AF amps in radios, with say 12AX7 driving EL34 in triode with 12dB
> FB. I've been doing all this for years and never have had bothers with
> VLF.
>
> I guess I know enough about CR time constants to get by.
--
John Byrns
flipper <fli...@fish.net> wrote:
> On Sat, 18 Jun 2011 21:56:00 +1000, "Alex Pogossov"
> <apog...@tpg.com.au> wrote:
>
> >
> >"Patrick Turner" <in...@turneraudio.com.au> wrote in message
> >news:4339858a-099e-4d81...@x38g2000pri.googlegroups.com...
> >
> >Patrick:
> >But what of the phase shift of that C? Isn't it better to have R&C in
> >parallel inserted from FB take off at OPT sec to the feedback R? Say
> >you have 1k0 and 100r as the normal FB divider so that 1/11 of the OPT
> >signal is applied to V1 cathode. Say one adds 3k3 so you then have
> >3k3, 1k0, then 100r at k to 0V at V1. Then � becomes 0.022, much less
> >than 0.09, and at very low F there is no phase shift, so with less NFB
> >its probably going to be stable. But there isn't enough FB at higher F
> >so you shunt the 3k3 with say 6.8uF. So at 100Hz the 6.8uF = 233 ohms
> >reactance and 3k3 is well shunted.
> >
> >Peaking still has to be checked in output and in error signal.
> >
> >Alex:
> >I suggested something quite opposite. The NFB divider looks like:
> >- 1K from the speaker terminal to cathode of the driver stage;
> >- 100R from the cathode to 100uF capacitor;
> >- the other end of the 100uF capacitor is tied to GND.
> >
> >Thus the feedback "beta" increases (!) at low frequencies (below 10Hz in
> >this case), giving -90deg phase lag in the loop below 10Hz. This is in
> >effect turning the amp into a anti-rumble filter.
> >
> >From the first glance it might sound crazy to increase the loop gain at VLF
> >where we want an overal reduction of yje loop gain, but consider this:
> >- OPT typically gives +90deg lead below 15...20Hz;
> >- interstage coupling (simple RC with no shelving, 220K and 0.033uF) is
> >calculated to give -3dB corner at say 15...20Hz and also gives +90 deg lead
> >below;
> >- but this "funny" NFB with a 100R and 100uF gives -90deg LAG below 10Hz!
> >And this lag maintains down until 1Hz!
> >
> >At LF one lag subtracts from two leads and in combination we have only
> >+90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
> >crossed safely at only +90deg!
>
> Interesting idea but I'm having trouble wrapping my head around the
> lead/lag analysis because, if I understand your circuit correctly,
> that cap is at the summing junction so shouldn't it introduce poles
> into both the NFB and the input signal?
I'm having some trouble wrapping my mind around this summing junction issue.
What is the problem if it does introduce a pole into the input signal? That
would not affect the feedback signal, and at worst would only enhance the
"Rumble Filter" effect, if it even does anything additional to the input signal
at all. I will have to write out the transfer function and see if there is any
additional affect on the input signal. If it does have some additional effect
on the input signal, which is considered undesirable, my cathode follower
modification described in an earlier post should eliminate the unwanted effect.
Patrick Turner
> On Wed, 22 Jun 2011 16:57:27 -0700 (PDT), Patrick Turner
>
> <i...@turneraudio.com.au> wrote:
> >> >At LF one lag subtracts from two leads and in combination we have only
> >> >+90deg below 10Hz. Therefore, 0dB line at about 3Hz (typically) will be
> >> >crossed safely at only +90deg!
>
> >> Interesting idea but I'm having trouble wrapping my head around the
> >> lead/lag analysis because, if I understand your circuit correctly,
> >> that cap is at the summing junction so shouldn't it introduce poles
> >> into both the NFB and the input signal?
>
> >I'm not the only one questioning Alex's FB network.
>
>
> > You are having
> >trouble accepting Alex's claims.
>
> prevents peaking, the POINT of the suggestion, is self evident.
>
> >Where's the evidence? its no fucking good making claims without backup
> >evidence.
>
What sort of fish gave you the idea of calling yourself FLIPPER?
Doncha feel embarassed most days to be seen by others as a half
brained fish? Where is the evidence you can think about anything?
NOBODY HAS POSTED EVIDENCE THAT THE NFB IDEA AS DESCRIBED BY ALEX WILL
WORK TO TO ANYTHING WORTHWHILE.
I do not have my hands across my eyes, hands across my ears, or hands
across my mouth like the three unwise monkeys who would not see the
truth, hear the truth, or speak the truth.
SO WHERE IS THE TRUTH?
Truth needs to be proven to be true.
So Flipper, perhaps you can spend a few days to prepare a lecture on
the issue and explore all possibilities but I won't accept
simulations. You shall build a circuit, you shall apply a healthy self
critical attitude, and shall carefully measure all voltages and
currents and waveforms and distortions and phase shifts. You shall not
dither about on news groups throwing mud and accusing ppl of crapping
when they don't.
When that's all done we all might learn and beneft.
Patrick Turner
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> Truth needs to be proven to be true.
>
> So Flipper, perhaps you can spend a few days to prepare a lecture on
> the issue and explore all possibilities but I won't accept
> simulations. You shall build a circuit, you shall apply a healthy self
> critical attitude, and shall carefully measure all voltages and
> currents and waveforms and distortions and phase shifts. You shall not
> dither about on news groups throwing mud and accusing ppl of crapping
> when they don't.
>
> When that's all done we all might learn and beneft.
I am not an NFB expert, you are the reining expert on that topic, however I do
have some knowledge of the design of equalizer networks and the poles and zeros
involved. Before I get into that and what I think is its relationship to the
negative feedback issue. Aside from the feedback stability issues we are mainly
concerned with, it appears obvious by inspection that in the infrasonic region
below the pole frequency of Alex's network, the network reduces the potential
for overload in the first stage, while your network increases the potential for
first stage overload.
Now as far as how Alex's feedback network might help improve amplifier
stability, from my perspective it is just a tool in your toolbox that requires
some sophistication to apply usefully. The first and most obvious thing Alex's
network does is increase the loop gain, I think that is the correct term correct
me if I have the wrong term. Normally increasing the loop gain increases
stability problems so that at first blush Alex's network might appear to be
counter productive.
However the network introduces both a zero and pole into the response, with the
zero at a higher frequency than the pole. Remember this network is just another
tool in your toolbox; it is not a cure all and requires some sophistication in
its application. Now the one thing I know about stabilizing the low frequency
response of a feedback system is that it is all about correctly placing the
poles, and zeros if there are any. If the only constraint on the pole
frequencies is achieving stability, then there is no problem in the first place;
we can simply choose the pole frequencies to insure stability. Unfortunately in
the real world there are other constrains and requirements on the choice of pole
frequencies, so we can run into problems trying to meet all the requirements at
the same time. The zero in Alex's network can be used to cancel one of the
original low frequency poles, and replace it with a new pole at a lower
frequency. If you are a clever designer you may be able to use this to affect a
better compromise in the choice of pole frequencies.
Your network works in a similar way except that the pole frequency is higher
than the frequency of the zero, and that it decreases the loop gain rather than
increasing it.
Patrick Turner
providing any truth about a NFB idea **which may,** or **may not** be
of any use, I'll leave the brain dead fish response to my questions...
>
> I don't give a tinker's damn what you deign to 'accept' and "shall"
> not waste my time with your shalls.
I didn't expect anything different from you. You are allergic to any
truth you don't believe in.
You just don't understand what Alex is proposing either.
And you cannot convince a living soul that Alex's method works to
provide any benefit to a tube power amp.
Patrick Turner.
Patrick Turner
> In article <fd99cd9c-0593-4262-985d-150c1028e...@q14g2000prh.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
> > Truth needs to be proven to be true.
>
> > So Flipper, perhaps you can spend a few days to prepare a lecture on
> > the issue and explore all possibilities but I won't accept
> > simulations. You shall build a circuit, you shall apply a healthy self
> > critical attitude, and shall carefully measure all voltages and
> > currents and waveforms and distortions and phase shifts. You shall not
> > dither about on news groups throwing mud and accusing ppl of crapping
> > when they don't.
>
> > When that's all done we all might learn and beneft.
>
> I am not an NFB expert, you are the reining expert on that topic, however I do
> have some knowledge of the design of equalizer networks and the poles and zeros
> involved.
I am NOT an expert. I'm just a fucken bloke who does a bit with a
soldrin' iron. In my work shed where I spend days and months and years
fixing up the shortcomings of one tragic amp after another, ideas I
may addopt and employ are good because they are found to be and can be
proven to be, or they are better left well alone.
If I was the expert, I would be up there with Peter Baxandal an
Williamson and 1/2 a dozen others who would be able to mathematically
calculate whether or not The Alex NFB loop is of any benefit or not.
I've realised that asking or persuading anyone here to give some proof
of their ideas and circuit techniques doesn't work. Ppl make claims,
then won't prove them to be true.
I've an open mind. But I question every damn thing I come into contact
with. I try to assume nothing.
> Before I get into that and what I think is its relationship to the
> negative feedback issue. Aside from the feedback stability issues we are mainly
> concerned with, it appears obvious by inspection that in the infrasonic region
> below the pole frequency of Alex's network, the network reduces the potential
> for overload in the first stage, while your network increases the potential for
> first stage overload.
I know for a fact that in my amps the input tube has an unused dynamic
headroom of perhaps +20dB. Ie, the input tube could produce 50Vrms
easily, but in my circuits maybe only has to make 5Vrms at CLIPPING!
The gain shelving network I use may indeed result in V1 having to
procuce a peaked response in order to keep the V0 at a constant
voltage level. The peak in response is below 20Hz, and usually less
than a few dB,
and so the input tube NEVER EVER overloads, ie, is forced into cut off
ot grid current - unless the amp is taken to clipping - perhaps,
because usually the middle driver stage overloads before the input
tube. The output stage usually overloads first at clipping.
>
> Now as far as how Alex's feedback network might help improve amplifier
> stability, from my perspective it is just a tool in your toolbox that requires
> some sophistication to apply usefully. The first and most obvious thing Alex's
> network does is increase the loop gain, I think that is the correct term correct
> me if I have the wrong term.
I welcome any new ideas, but some ideas turn out to not be very good.
I need to ss the proposer of any new ideas is ready to prove the worth
of any new idea. Not too much to expect, now is it?
Ppl hate proving claims. They are so often allergic to spending a day
in their workshop soldering and measuring something then recording it
all and writing up the experiments complete with nicely drawn circuit
diagram which all of us can try to verify their claims.
I do not see how NFB "increases loop gain". Which gain do you mean?
There is Open Loop Gain, OLG, ie, the gain without a stitch of any
GNFB, and there is the "Closed Loop Gain," CLG, ie the gain when GNFB
has been connected.
> Normally increasing the loop gain increases
> stability problems so that at first blush Alex's network might appear to be
> counter productive.
The OLG is whatever it is and cannot be boosted by GNFB. The CLG at LF
*can be* boosted at LF if the FB network is designed to reduce applied
NFB.
I want to see a schematic with all test results before I make up my
mind on Alex's FB "trick." It could be a clever trick, or a swindle.
> However the network introduces both a zero and pole into the response, with the
> zero at a higher frequency than the pole. Remember this network is just another
> tool in your toolbox; it is not a cure all and requires some sophistication in
> its application. Now the one thing I know about stabilizing the low frequency
> response of a feedback system is that it is all about correctly placing the
> poles, and zeros if there are any.
In any amp where there are say 2 CR coupled stages and a final stage
with LR then you have a recipe for LF instability and a poor margin of
stability at LF.
If there were ways of putting LF phase shift tailoring and/or shelving
networks in GNFB networks to better control LF behaviour in tube amps,
then one would think thay have all been invented and would have been
widely addopted bt brand names. But I ain't seen anything different to
what I know and to what the many amp designers of the past have done.
> If the only constraint on the pole
> frequencies is achieving stability, then there is no problem in the first place;
> we can simply choose the pole frequencies to insure stability.
All of what you say is jargon. In practice, getting good amp behaviour
at LF is more complex, and a few sentences here land well short of
providing anyone with a "how-to-do-it" better method they may follow.
> Unfortunately in
> the real world there are other constrains and requirements on the choice of pole
> frequencies, so we can run into problems trying to meet all the requirements at
> the same time. The zero in Alex's network can be used to cancel one of the
> original low frequency poles, and replace it with a new pole at a lower
> frequency. If you are a clever designer you may be able to use this to affect a
> better compromise in the choice of pole frequencies.
I'll let Alex or anyone else proove that the idea works as well as
forecast. Why should I waste my time doing their R&D experiments?
>
> Your network works in a similar way except that the pole frequency is higher
> than the frequency of the zero, and that it decreases the loop gain rather than
> increasing it.
When ppl say such a sentence, I remain unable to gain meaning.
"frequency of the zero?" Who on earth knows what that means?
What this discussion needs but has not got is a means of being able to
post schematics, then Alex or anyone else can post up a scanned
schematic with ***ALL*** test results. REAL progress is made, rather
than clouds of meaningless hot air.
Patrick Turner.
John Byrns
flipper <fli...@fish.net> wrote:
> I don't know why. I wrote it at the exact spot in the message where it
> matters, the lead/lag analysis, and said "lead/lag analysis."
If you don't know why, then please explain exactly where the summing junction is
located in this circuit? In inverting amplifiers the summing junction is
usually explicit and easy to see in the schematic diagram. Non inverting
amplifiers like this one are a different matter, and the summing junction is
buried somewhere inside the active devices, be they tubes, transistors, or ICs.
What I was trying to say is that I was trying to figure out where the "summing
junction" is actually located in this circuit.
> >What is the problem if it does introduce a pole into the input signal?
>
> The whole point of that paragraph is adding the lead/lags and if
> there's *another* lead/lag it changes the phase analysis.
Does, it? If there is a pole added to the input circuit, it doesn't affect the
"phase analysis" around the feedback loop, or the feedback stability issue,
although it would affect the closed loop gain of the entire amplifier circuit,
and hence what we see on the CRO after we solder the circuit together and apply
a test signal to the input.
> > That
> >would not affect the feedback signal, and at worst would only enhance the
> >"Rumble Filter" effect,
>
> First Patrick was obsessed with the "Rumble Filter" and now you are. I
> *specifically* said my question was about "the lead/lag analysis."
And now it seems to be you that is obsessed with the "Rumble Filter"! In any
case it's not a "Rumble Filter", it's a high pass filter with an infrasonic
cutoff, it wouldn't do a thing for rumble!
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jun 24, 3:34嚙緘m, John Byrns <byr...@sbcglobal.net> wrote:
> > In article
> > <fd99cd9c-0593-4262-985d-150c1028e...@q14g2000prh.googlegroups.com>,
> > 嚙瞑atrick Turner <i...@turneraudio.com.au> wrote:
>
> > Before I get into that and what I think is its relationship to the
> > negative feedback issue. 嚙璀side from the feedback stability issues we are
> > mainly
> > concerned with, it appears obvious by inspection that in the infrasonic
> > region
> > below the pole frequency of Alex's network, the network reduces the
> > potential
> > for overload in the first stage, while your network increases the potential
> > for
> > first stage overload.
>
> I know for a fact that in my amps the input tube has an unused dynamic
> headroom of perhaps +20dB. Ie, the input tube could produce 50Vrms
> easily, but in my circuits maybe only has to make 5Vrms at CLIPPING!
> The gain shelving network I use may indeed result in V1 having to
> procuce a peaked response in order to keep the V0 at a constant
> voltage level. The peak in response is below 20Hz, and usually less
> than a few dB,
> and so the input tube NEVER EVER overloads, ie, is forced into cut off
> ot grid current - unless the amp is taken to clipping - perhaps,
> because usually the middle driver stage overloads before the input
> tube. The output stage usually overloads first at clipping.
I can believe that the first stage of your amps never overloads at infrasonic
frequencies when feed with a signal at the level needed to produce a full power
output at midband frequencies, as a result of the headroom provided in the first
stage, however it doesn't seem credible that the "peaked response" in the first
stage is "less than a few dB". How much is "a few dB"? Have you ever measured
this and made note of it in your notebook along with the relevant parameters of
the amp in question, such as the amount of negative feedback applied and the
characteristics of your gain stepping network?
> > Now as far as how Alex's feedback network might help improve amplifier
> > stability, from my perspective it is just a tool in your toolbox that
> > requires
> > some sophistication to apply usefully. 嚙確he first and most obvious thing
> > Alex's
> > network does is increase the loop gain, I think that is the correct term
> > correct
> > me if I have the wrong term. 嚙�>
> I welcome any new ideas, but some ideas turn out to not be very good.
> I need to ss the proposer of any new ideas is ready to prove the worth
> of any new idea. Not too much to expect, now is it?
>
> Ppl hate proving claims. They are so often allergic to spending a day
> in their workshop soldering and measuring something then recording it
> all and writing up the experiments complete with nicely drawn circuit
> diagram which all of us can try to verify their claims.
You yourself appear to be guilty of this very sin as far as documenting the
effects of your gain stepping network on first stage overloading!
> I do not see how NFB "increases loop gain".
I mean the sum of the "open loop gain" of the amplifier and the gain, actually
negative, of the feedback network.
> Which gain do you mean?
> There is Open Loop Gain, OLG, ie, the gain without a stitch of any
> GNFB, and there is the "Closed Loop Gain," CLG, ie the gain when GNFB
> has been connected.
And there is the Loop Gain, the gain around the feedback loop.
> > Normally increasing the loop gain increases
> > stability problems so that at first blush Alex's network might appear to be
> > counter productive.
>
> The OLG is whatever it is and cannot be boosted by GNFB. The CLG at LF
> *can be* boosted at LF if the FB network is designed to reduce applied
> NFB.
And of course the "CLG" at LF can be reduced if the FB network is designed to
increase the applied NFB. Increasing the NFB at LF is the same thing as
increasing the Loop Gain at LF, and decreasing the NFB at LF is the same thing
as decreasing the Loop Gain at LF.
> I want to see a schematic with all test results before I make up my
> mind on Alex's FB "trick." It could be a clever trick, or a swindle.
I am having trouble believing that you can't visualize Alex's circuit!! It is
simply a series RC network connected between ground and the cathode of the input
stage where the feedback resistor from the oPT is commonly connected, the value
of the feedback resistor being adjusted to maintain the same degree of feedback
at midband.
> > However the network introduces both a zero and pole into the response, with
> > the
> > zero at a higher frequency than the pole. 嚙磋emember this network is just
> > another
> > tool in your toolbox; it is not a cure all and requires some sophistication
> > in
> > its application. 嚙瞇ow the one thing I know about stabilizing the low
> > frequency
> > response of a feedback system is that it is all about correctly placing the
> > poles, and zeros if there are any.
>
> In any amp where there are say 2 CR coupled stages and a final stage
> with LR then you have a recipe for LF instability and a poor margin of
> stability at LF.
>
> If there were ways of putting LF phase shift tailoring and/or shelving
> networks in GNFB networks to better control LF behaviour in tube amps,
> then one would think thay have all been invented and would have been
> widely addopted bt brand names. But I ain't seen anything different to
> what I know and to what the many amp designers of the past have done.
Remember that Alex's circuit has one drawback that displeases both bean counters
and audiophiles alike, it requires adding an electrolytic capacitor to the
circuit. That could explain why it was not widely used except in SS amps.
> > If the only constraint on the pole
> > frequencies is achieving stability, then there is no problem in the first
> > place;
> > we can simply choose the pole frequencies to insure stability.
>
> All of what you say is jargon.
One might say the same thing about your discussions of OPTs here. Jargon is the
price you pay for discussing specialized esoteric subject area.
> In practice, getting good amp behaviour
> at LF is more complex, and a few sentences here land well short of
> providing anyone with a "how-to-do-it" better method they may follow.
I think I made the very same point in an earlier post and pointed out that
Alex's network is only another tool in the designers toolbox that can be used to
help assure stability, it doesn't eliminate the need for a designer know what he
is doing.
> > Unfortunately in
> > the real world there are other constrains and requirements on the choice of
> > pole
> > frequencies, so we can run into problems trying to meet all the
> > requirements at
> > the same time. 嚙確he zero in Alex's network can be used to cancel one of the
> > original low frequency poles, and replace it with a new pole at a lower
> > frequency. 嚙瘢f you are a clever designer you may be able to use this to
> > affect a
> > better compromise in the choice of pole frequencies.
>
> I'll let Alex or anyone else proove that the idea works as well as
> forecast. Why should I waste my time doing their R&D experiments?
Who has asked you to do any R&D experiments? Your attention has been called to
the idea, there is no need to put this tool in your toolbox, and indeed you have
chosen not to include it, hence no need for you to do any R&D.
> > Your network works in a similar way except that the pole frequency is
> > higher
> > than the frequency of the zero, and that it decreases the loop gain rather
> > than
> > increasing it.
>
> When ppl say such a sentence, I remain unable to gain meaning.
> "frequency of the zero?" Who on earth knows what that means?
As a wise sage once said "Google is your friend". Without taking my own advice,
if you write out the transfer function of a network, IIRC the poles are
basically the factors in the denominator and the zeros are the factors in the
numerator, Google could doubtless give you the precise definition.
> What this discussion needs but has not got is a means of being able to
> post schematics, then Alex or anyone else can post up a scanned
> schematic with ***ALL*** test results. REAL progress is made, rather
> than clouds of meaningless hot air.
I believe that there are numerous places that one could post such information.
If anyone has information on this subject but doesn't want to use the photo
posting services that are out there, if they send the images to me I will be
happy to creat a page at my web site to display the submissions. I won't
suggest that you simply post them in the relevant USENT binary group, because I
know that you are using Google Groups. Many people go so far as to block posts
like yours that come through Google Groups.
Patrick Turner
> On Thu, 23 Jun 2011 23:11:11 -0700 (PDT), Patrick Turner
>
> <i...@turneraudio.com.au> wrote:
> >After deleting Flipper's evasions and denials and weaseling out of
> >providing any truth about a NFB idea **which may,** or **may not** be
> >of any use, I'll leave the brain dead fish response to my questions...
>
>
> >> I don't give a tinker's damn what you deign to 'accept' and "shall"
> >> not waste my time with your shalls.
>
> >I didn't expect anything different from you. You are allergic to any
> >truth you don't believe in.
>
> people around.
>
> >You just don't understand what Alex is proposing either.
>
>
> >And you cannot convince a living soul that Alex's method works to
> >provide any benefit to a tube power amp.
>
Then you have very little to contribute to this thread, no?
Patrick Turner
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jun 24, 3:34�pm, John Byrns <byr...@sbcglobal.net> wrote:
>
> I want to see a schematic with all test results before I make up my
> mind on Alex's FB "trick." It could be a clever trick, or a swindle.
>
> > However the network introduces both a zero and pole into the response, with
> > the
> > zero at a higher frequency than the pole. �Remember this network is just
> > another
> > tool in your toolbox; it is not a cure all and requires some sophistication
> > in
> > its application. �Now the one thing I know about stabilizing the low
> > frequency
> > response of a feedback system is that it is all about correctly placing the
> > poles, and zeros if there are any.
>
> In any amp where there are say 2 CR coupled stages and a final stage
> with LR then you have a recipe for LF instability and a poor margin of
> stability at LF.
OK, I have worked through some of the math and understand more fully what is
going on with Alex's feedback network. As I said the network introduces both a
zero and a pole in the loop gain.
Ignoring the added pole for a moment, the zero can be placed so that it exactly
cancels the effect of one of the three poles in the amplifier you describe
above, with 2 CR coupled stages and a final stage with LR, effectively reducing
the number of low frequency poles by 1, making the LF stability problem easier
to deal with. The zero effectively cancels both the phase shift and amplitude
roll off caused by the pole that is being canceled. Unfortunately it is
impossible, at least so far as I know, to build a network with an isolated zero
such as I have described, so an actual network, such as Alex's, must include a
pole at a lower frequency. Hopefully this new pole won't cause us too much
trouble if we place it at a very low frequency where the loop gain has already
fallen well below 1.0 as a result of the other two remaining poles that weren't
canceled.
Now the obvious question is, why bother with this extra complexity when we could
simply directly move one of the 3 poles to a very low frequency, as would
probably be part of the normal pole staggering process anyway? I will leave
that for others to comment on as I have not personally mucked about in my
workshop with amplifiers that have 3 LF poles. I suspect that one reason may
have to do with LF overload when using a Bean Counter approved OPT.
I can see how Alex's network has the potential to resolve a problem I have
encountered when mucking about with simpler amplifiers having only 2 poles.
When using OPTs designed by Bean Counters, especially SE OPTs, there is a
tendency towards LF overload in the OPT and final tube(s). I have attempted to
mitigate this problem by choosing a relatively high pole frequency for the
interstage coupling network to keep LF signals out of the OPT and final tube(s).
This puts the interstage pole too close to the pole caused by the OPT which then
causes a bump in the �CLG� low frequency response, plus of course it isn't
really a very good solution to the LF overload problem. It occurs to me that
Alex's feedback network might also offer a solution to the OPT saturation
problem in Bean Counter designed OPTs, just as it offers a solution to the input
stage problem.
Patrick Turner
> In article <6fc198cd-6db6-4c43-9872-4df46cf20...@j14g2000prn.googlegroups.com>,
I wish I could be so sure. Just because you have in theory "cancelled
out" one devil it doesn't mean you've done much, especially
considering that the Laex FB path renders the FB amp to be a cathode
follower at DC, when you have no ability at all for the V0 to follow
and input at DC. The circuit needs to be built and tested. But notice
how silent and bone lazy everyone is about the issue. They just don't
want to get out of the their arm chairs. How can I have respect for
laziness?
> The zero effectively cancels both the phase shift and amplitude
> roll off caused by the pole that is being canceled. Unfortunately it is
> impossible, at least so far as I know, to build a network with an isolated zero
> such as I have described, so an actual network, such as Alex's, must include a
> pole at a lower frequency. Hopefully this new pole won't cause us too much
> trouble if we place it at a very low frequency where the loop gain has already
> fallen well below 1.0 as a result of the other two remaining poles that weren't
> canceled.
>
> Now the obvious question is, why bother with this extra complexity when we could
> simply directly move one of the 3 poles to a very low frequency, as would
> probably be part of the normal pole staggering process anyway? I will leave
> that for others to comment on as I have not personally mucked about in my
> workshop with amplifiers that have 3 LF poles. I suspect that one reason may
> have to do with LF overload when using a Bean Counter approved OPT.
Its not always convenient to move poles down, and often it merely
moves the oscillaton F lower. It can be exasperating to find that even
if you increase coupling caps from say 0.22uF to 2.2uF, the trace on
the CRO still slowly rises and falls because Fo has just gone lower.
But you don't need a bean counter designed OPT to give a pole that is
at an F too high. You could have a well designed OPT for an SE amp
which will is designed to saturate at 20Hz at full PO and yet it
oscillates at LF. This is because the permeability of the core has
been much reduced by the air gap from the maximum one might find in a
PP OPT with fully interleaved laminations. So I have found my shelving
network especially effective in SE amps. The Williamson PP amp
required Lp = 100H minimum with triode Ra-a of 3k2 plus RLa-a = 10k in
parallel, ie, the ratio of RAA to Lp = 2k4:100H, giving a pole at
3.8Hz. With say 4 x EL34 in parallel SET, Ra = 310 ohms, RL might be
1k0, so RA = 236 ohms. Lp might be 8H to give XLp = 1k0 at 20Hz, so
the -3dB response pole is at 236 / ( 6.28 x 8 ) = 4.7Hz. The SE amp
is then in theory slightly worse off with regard to phase shift at
LF.
My shelving network is especially effective where the OPT has far less
inductance than it should and usually the Fsat is also way too high,
eg, in most old radios the OPTs are allowed to saturate at 70Hz. This
is why they are so small in size; Afe is 1/4 of the size needed for hi-
fi, for the number of turns used.
In the case of a pentode or beam tetrode SE OP tube, the open loop LF
pole is determined by the RLa and the Lp, and Ra has little effect
because Ra is such a high R.
So say you have 4 x EL34 in parallel pentode then RLa = 1k2, and Lp
needs to be 9.6H and the response is down 3dB at 20Hz. If the speaker
load is high, or there is no speaker connected, then the LF open loop
pole is between Ra and Lp, and if Ra for the 4 x EL34 = 4k0, then pole
rises to 66Hz. One cannot easily move the pole.
So, just how Alexe's brainchild works in all conditions remains to be
established. Unconditional stability is mandatory in all my amp
designs as it is in all other reputable brand names.
> I can see how Alex's network has the potential to resolve a problem I have
> encountered when mucking about with simpler amplifiers having only 2 poles.
> When using OPTs designed by Bean Counters, especially SE OPTs, there is a
> tendency towards LF overload in the OPT and final tube(s).
Nearly all the SE amps from brand names have Fsat too high and Lp too
low. And so do many PP amps. amps with only 2 stages like Quad-II have
better LF stability with shelving networks. They are inherently stable
because of their single CR coupling and OPT LR but the trace wobbles
after going from high level to zero level. It remains to be seen if
they'd enjoy a cap in the FB network.
> I have attempted to
> mitigate this problem by choosing a relatively high pole frequency for the
> interstage coupling network to keep LF signals out of the OPT and final tube(s).
> This puts the interstage pole too close to the pole caused by the OPT which then
> causes a bump in the ³CLG² low frequency response, plus of course it isn't
> really a very good solution to the LF overload problem. It occurs to me that
> Alex's feedback network might also offer a solution to the OPT saturation
> problem in Bean Counter designed OPTs, just as it offers a solution to the input
> stage problem.
Mere postulations John. The only real truth is known when theory is
applied.
Patrick Turner.
Alex Pogossov
"John Byrns" <byr...@sbcglobal.net> wrote in message
news:byrnsj-9CFA17....@news.giganews.com...
You grasped the idea perfectly well John. This frequency compensation is
useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
amp found in those boring AA5 style radios.
Below is the sequence of mods you do to improve the radio.
1. In an AM detector you remove a cap coupling a detector load (470K||100pF)
to the volume control (1M pot). In fact you are making the volume control
the detector load. This is for the detector to be able to handle nearly 100%
modulation. But now you det DC on the input of the audio amp.
2. You decouple this DC from the pot wiper by a 0.02...0.05uF, but you need
10M input impedance (grid leak) so that the AM detector is not loaded and
100% modulation is still handled.
3. On most stations your AM detector delivers 1...4V of audio, while
sensitivity of a typical boring feedbackless two-stage audio amp is
100...200mV. The radio always works with volume control close to minimum.
The speaker is boomy (not damped), distortion is high. You want to trade the
sensitivity excess for distortion and apply NFB, reducing sensitivity to
0.5...1V. You are enjoying tight crispy sound, but THAT IS WHERE PROBLEMS
BEGIN.
4. Now the bandwidth of your amp is in theory goes to 5...10Hz, but because
of the lousy OPT, these low frequencies do not reach your ears, but only
overload the 6AQ5, since the error signal becomes too large at low
frequencies.
5. Note that neither the american aggressive LF cutoff by reducing
interstage cap to 2000pF nor the Patrick's shelving does not work, since
still yjr NFB is pushing to maintaim unmaintainable LF output, the error
signal is large, the first stage is overloading, and (IMPORTANT!) since the
shelving is virtually a differentiator, it accentuates all the harmonics
generated in the overloaded 1-st stage and feeds them to the 6AQ5. Instead
of overloading the 6AQ5 we have emphasized distortion from the first stage.
6. That is where you need this RRC compensation in the feedback to roll the
low-frequency response of the amp in line with the capability of the lousy
OPT. Typically from 80....100Hz down. Applying the RRC divider provides
undistorted output down to 10...25Hz,
because the error signal remains under control.
7. However at the frequencies lower than 10...25Hz, where the NFB RRC
network levels out at 100% beta, the error signal continues to rise. This
VLF content caused by fadings, AGC knocked by atmospheric interference, etc.
is smaller, and some might stop here, but a purist might like to prevent
overloading even at VLF.
8. To do the above one needs to degenerate the first stage gain, rather than
to use a Partick's attenuator after it. To degenerate the 1-st stage gain
you need to place say 22K resistor in series with the 1-st stage cathode,
and shunt this resistor by a 0.22...0.47uF capacitor. Thus for medium
frequencies the 1-st stage will be working as usual, but ay low frequencies
the transconductance will be degenerating with the perfect linearity of the
first stage maintained. It is the same shelving, but implemented in a wise
linear mode. (Of course the grid leak can not be taken to GND any more, it
should be connected to the cathode or a tap in this 22K resistor.) At
infinitely low frequencies the gain of the first stage is to be degenerated
to 10...20 so that the 1-st stage output is just below the negative bias of
6AQ5 and the later is never overloaded.
Usually I apply mods up to #7 and sometimes #8 too if I can find enough free
solder lugs around the 1-st audio stage tube. Of course, all of the above
implies the cathode of the 1-st audio tube has to be free of duo-diode
functionality. That forces to use separate diodes for AGC and the AM
detector. In some cases instead of 6Q7, 6B8, 6AV6, etc. I might use a 6SL7
tube with one triode as the 1-st audio and the other triode as a diode for
the AM detector, and a silicon diode as an AGC detector.
Regards,
Alex
John L Stewart
Alex Pogossov;933499 Wrote:
> "flipper" <fli...@fish.net> wrote in message
> ---------------------
> >>Even if an amp is stable without the shelving network, shelving
> should
> >>be used especially where there is some peaking in the region where
> "we
> >>don't intend to amplify anything that low anyway" as you say. The
> >>shelving improves overload charactecter and saturation effect
> >>behaviour.-
> >
> > Yes, I'm sure the OP will appreciate improved overload
> characteristics
> > at frequencies that will never be presented to the amplifier.-
>
> Do you want rumble (from say a pick-up playing a warped record) driving
> your
> amp to saturation, if your amp is close to LF oscillation?
One solution is the use of a bandwidth limiting network on the front end
of an amp. I too at that time long ago (40 years!!) had similar
concerns. First of all in those days an important program source was the
turntable & still is with many listeners.
These were not always perfect so that rumble was a concern.
Another concern had to do with the possibility of driving the
loudspeaker at frequencies below it�s normal operating range, perhaps by
the above mentioned turntable rumble. These two problems either
separately or together are a good recipe for Intermodulation Distortion
both in the speaker & the amp.
I was also troubled by possible damage to the output tubes or the
speakers caused by switching transients. As a result, I devised a
passive HP filter for the amps front end which is two stage rather than
one.
To do this you need to arrange the impedance of the second section to be
in the range five to ten times that of the first. That�s how I arranged
the gain control in the first section & something of higher impedance in
the second section. I did not believe a low pass filter made sense at
this point.
Here is the response of the high pass network
High Pass Section- 3 db down @ 20 hz
44 db down @ One hz
170 degree phase shift 12 db / octave
+-------------------------------------------------------------------+
|Filename: Front End A.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=224|
+-------------------------------------------------------------------+
--
John L Stewart
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jun 26, 2:25�am, John Byrns <byr...@sbcglobal.net> wrote:
> >
> > Now the obvious question is, why bother with this extra complexity when we
> > could
> > simply directly move one of the 3 poles to a very low frequency, as would
> > probably be part of the normal pole staggering process anyway? �I will
> > leave
> > that for others to comment on as I have not personally mucked about in my
> > workshop with amplifiers that have 3 LF poles. �I suspect that one reason
> > may
> > have to do with LF overload when using a Bean Counter approved OPT.
>
> Its not always convenient to move poles down, and often it merely
> moves the oscillaton F lower. It can be exasperating to find that even
> if you increase coupling caps from say 0.22uF to 2.2uF, the trace on
> the CRO still slowly rises and falls because Fo has just gone lower.
Hi Patrick,
The phrasing of your comment suggests that you are speaking of moving all the
poles down, in which case as you say "Fo has just gone lower". What I was
considering was moving just one pole down in frequency. This leads directly to
the question of how you choose your pole frequencies? You have explained how
your LF shelving network improves LF stability, although with the risk of
pushing the input stage closer to overload, what you haven't explained, at least
that I remember, is how you choose your pole frequencies, especially in an
amplifier with 3 poles like we are discussing?
John L Stewart
[8. To do the above one needs to degenerate the first stage gain, rather
Regards,
Alex
Here it is again with the 1st AF grid leak corrected. The NFB network
should probably be conjugate with the 1st AF cathode network but that gets
this one to a 3 microfarad NFB cap.
But otherwise would be OK.
Cheers, John
+-------------------------------------------------------------------+
|Filename: Alex Pogossov Solution B.JPG |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=227|
Alex Pogossov
"John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
news:John.L.Stew...@audiobanter.com...
Thanks for drawing the circuit, but it needs corrections.
1. The most important correction is the NFB divider circuit:
Where you currently have 1K, place 100R in series with 10uF. Where you
currently have 1K5 + 1K5||1uF, place just one 330R resistor. This makes NFB
to cut low frequencies below 100Hz in a lousy radio.
2. (Less important) +B=250V, interstage coupling = 0.047uF, output tube=6V6
clone..
3. (Desirable, but not very critical is to add more negative bias to 12AX7
to completely avoid clipping on the input by grid current). Add another 2K2
or so in series with the currently shown 15K resistor. Connect the right end
of 10M grid leak to the junction of these 2K2 and 15K resistors.
Thanks.
I would appreciate if you can redraw the circuit as per the above
corrections. (I do not have a web space to publish such things.) Then Mr.
Turner can criticise the circuit as much as he wishes.
Regards,
Alex
Alex Pogossov
"John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
news:John.L.Stew...@audiobanter.com...
>
This passive high-pass filter is not suitable for an audio amp in a radio
where the input impedance should be "infinite", not 250K, not even 470K.
Alex Pogossov
"John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
news:John.L.Stew...@audiobanter.com...
> should probably be conjugate with the 1st AF cathode network but that gets
> this one to a 3 microfarad NFB cap.
>
> But otherwise would be OK.
>
> Cheers, John
>
>
> +-------------------------------------------------------------------+
> |Filename: Alex Pogossov Solution B.JPG |
> |Download: http://www.audiobanter.com/attachment.php?attachmentid=227|
> +-------------------------------------------------------------------+
John,
your idea of "conjugating" of the NFB circuit with the cathode
transconductance degeneration appears useful. I have tried it and it works,
though you can not manuevre all the nulls and poles independently now.
However it simplifies the circuit and allows to get rid of the 10uF
electrolytic.
If you can visualise the circuit to draw it in your CAD, it would be as
follows:
1. NFB divider consists of 2 resistors: 330R from the speaker output
terminal and then 100R to GND;
2. From the junction of the 330R and 100R a capacitor of about 0.15uF goes
to the cathode of 12AX7;
3. A 22K resistor goes from the speaker (!) to the cathode of 12AX7;
4. 10M grid leak -- from cathode to grid.
5. Input signal is coupled to grid via a 0.02uF (not critical).
Other components are pretty much as per your previous circuit diagram.
With 1Vm (amplitude) input signal this amp delivers full power (about 1.5W)
at midrange. As the frequency of the input signal does down from about 100Hz
to virtually DC, with the same 1Vm amplitude, then the output starts to go
down according to the lousiness of the OPT, but the grid drive on the 6AQ5
remains the same -- no NFB inflicted overdriving.
When I tested the amp with a 5Hz rectangular wave, the transient was ripple
free, almost as the exponential spikes from a differentiating circuit. All
over all, the results are good.
Again I need to mention that such LF frequency compensation makes difference
in lousy amps with tiny OPTs. In a decent amp probably there is no need to
go into such tricks.
John Byrns
"Alex Pogossov" <apog...@tpg.com.au> wrote:
> "John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
> news:John.L.Stew...@audiobanter.com...
> > Here it is again with the 1st AF grid leak corrected. The NFB network
> > should probably be conjugate with the 1st AF cathode network but that gets
> > this one to a 3 microfarad NFB cap.
> >
> > But otherwise would be OK.
> >
Hi Alex,
When John said "The NFB network should probably be conjugate with the 1st AF
cathode network" I was left wondering what he might be talking about,
specifically how he was using the word "conjugate". You say John's "idea of
'conjugating' of the NFB circuit with the cathode transconductance degeneration
appears useful", so apparently you understood what he meant, can you explain for
me what he was talking about, and how it relates to your revised circuit? I
understand the qualitative operation of your revised circuit, although I haven't
tried writing the transfer function to get a more detailed quantitative
perspective.
John L Stewart
Cheers, John
Hello Again Alex- I checked this cct again & find the cutoff begins at an
f1 about 10X too high using the CR network you recommended, 0.22 Ck & 22K
Rk. Have a look at Fig. 12.3B, p484 in RDH4 for guidance. The CR time
constant appears to predict a point well down on the curve, not the -3db
point.
Your proposed cct fix still looks quite useful for the application
discussed.
Cheers, John
Cheers, John
+-------------------------------------------------------------------+
|Filename: TA152-dora9_1.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=228|
Alex Pogossov
"John L Stewart" <John.L.Stew...@audiobanter.com> wrote in message
news:John.L.Stew...@audiobanter.com...
> should probably be conjugate with the 1st AF cathode network but that
> gets this one to a 3 microfarad NFB cap.
>
> But otherwise would be OK.
>
> Cheers, John
>
> Hello Again Alex- I checked this cct again & find the cutoff begins at an
> f1 about 10X too high using the CR network you recommended, 0.22 Ck & 22K
> Rk. Have a look at Fig. 12.3B, p484 in RDH4 for guidance. The CR time
> constant appears to predict a point well down on the curve, not the -3db
> point.
>
> Your proposed cct fix still looks quite useful for the application
> discussed.
>
> Cheers, John
>
> Cheers, John
>
>
> +-------------------------------------------------------------------+
> |Filename: TA152-dora9_1.jpg |
> |Download: http://www.audiobanter.com/attachment.php?attachmentid=228|
> +-------------------------------------------------------------------+
What is this aircraft? Does not look like Me-109.
Patrick Turner
> > John Byrns
>
> > Surf my web pages at, http://fmamradios.com/
>
> You grasped the idea perfectly well John. This frequency compensation is
> useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
> amp found in those boring AA5 style radios.
I agree with everything Alex says below except for a few things.
In most old radios with DC flow across the volume control pot track
the adjustment of the volume is often very noisy after the pot has
aged a few years.
So instead of the conventional arrangements put forward by RDH and
most others to avoid parts costs I will have the last IFT coil biased
at say +50V at one end, and the live end goes to a triode grid of 1/2
12AU7 CF buffer to remove any loading effect of diode detection on the
last IFT LC. Then I use Ge diode feeding RC circuit, and this can
directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
any a volume control and while employing time passive poles to give
say -3dB at 30Hz before any power amp which has NFB. I often add in
another 12AU7 gain stage for tone control to boost/cut treble; bass in
AM is usually OK.
To slightly widen AF response the Q of all IFTs may be reduced by
strapping 100k across each coil. It doesn't work in all sets, but may
be tried. There is a simple method explained in RDH4 to add a few
turns of fine wire around the primary of IFT1 and switch it to being
in series with the sec of IFT1 which will give a doble peak to IFT
response which effectively increases IF channel bandwidth from a
typical 4kHz to 8 kHz. Quad used it.
I've used it - finally there is the the treble which most sets refuse
to give me.
I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
audio amp and with 12dB global NFB.
This works far better than anything with 6V6 or EL84 etc.
The Ge diode in the detector is biased on with a low current in the R.
Its works best with high level signals and up to 100% mod, with 10Vrms
available if you want it.
But anyone could use the normal arrangement of IFT plus tube
rectifiers found in IF amp tubes like 6N8 and thus get say -2Vdc
generated by a carrier with 2pk volts of signal. at 100% mod the Vpk-
pk of audio = 4V, so max audio = 1.4V, no?
The audio signal with negative vdc and some 455kHz ripple could be
direct coupled to a CF triode buffer stage but you'd need a -150Vdc
rail for the cathode resistor, while the anode would go to a +100V
supply. The CF buffer and negative rail was viewed as a waste of money
in 1950, so it was never done. But there's no reason why it can't be
done NOW. And the screen voltage applied to mixer and IF amp is best
regulated at 100Vdc, so that's a good point of supply with CF anodes .
I don't much like tubes like the 6AV6 with grounded cathodes and 10M
grid leak biasing.
The very low F signals generated in AM sets don't seem to cause any
problems in anything I have made.
Patrick Turner.
> Alex- Hide quoted text -
>
> - Show quoted text -
Alex Pogossov
>>> |Filename: TA152-dora9_1.jpg |
>>> |Download: http://www.audiobanter.com/attachment.php?attachmentid=228|
>>> +-------------------------------------------------------------------+
>>
>>What is this aircraft? Does not look like Me-109.
>>
>
>
> http://www.livingwarbirds.com/focke-wulf-ta152.php
>
> http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=455
Thanks. Did not know anything about this model, though I am not an expert in
WW2 aircraft. The long "muzzle" of this beast did not look as anything like
FW-190, but on the other hand the engine looked like air cooled... I was
confused.
Alex Pogossov
news:f8060a23-5198-42fa...@17g2000prr.googlegroups.com...
........................
In most old radios with DC flow across the volume control pot track
the adjustment of the volume is often very noisy after the pot has
aged a few years.
So instead of the conventional arrangements put forward by RDH and
most others to avoid parts costs I will have the last IFT coil biased
at say +50V at one end, and the live end goes to a triode grid of 1/2
12AU7 CF buffer to remove any loading effect of diode detection on the
last IFT LC. Then I use Ge diode feeding RC circuit, and this can
directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
any a volume control and while employing time passive poles to give
say -3dB at 30Hz before any power amp which has NFB. I often add in
another 12AU7 gain stage for tone control to boost/cut treble; bass in
AM is usually OK.
Alex:
Is adding two extra tubes a reasonable "mod"? It is the radio redesign. If
one wants to go that far, op-amps would be far better. Take TLE2074. With
10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase
sensitivity, tone control, buffer, AGC integrator, etc.
And after that why do you need to retain a lousy tube amp? Why not to use an
IC? All the radio can be powered from a 6.3V winding with a bipolar voltage
doubler (four caps, four 1N4004 diodes)
...................
I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
audio amp and with 12dB global NFB.
This works far better than anything with 6V6 or EL84 etc.
Alex:
Works better in which respect? EL34 takes as many amps for its heater as the
whole original radio circuit. What about the poor lousy power transformer?
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> snip for brevity.
>
> > > John Byrns
> >
> > > Surf my web pages at, �http://fmamradios.com/
> >
> > You grasped the idea perfectly well John. This frequency compensation is
> > useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
> > amp found in those boring AA5 style radios.
>
> I agree with everything Alex says below except for a few things.
>
> In most old radios with DC flow across the volume control pot track
> the adjustment of the volume is often very noisy after the pot has
> aged a few years.
>
> So instead of the conventional arrangements put forward by RDH and
> most others to avoid parts costs I will have the last IFT coil biased
> at say +50V at one end, and the live end goes to a triode grid of 1/2
> 12AU7 CF buffer to remove any loading effect of diode detection on the
> last IFT LC.
What is the purpose and or advantage of using a cathode follower between the IFT
and detector diode, especially if you are going to add 100k resistors to widen
the IF response as you discuss below?
> Then I use Ge diode feeding RC circuit, and this can
> directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
> any a volume control and while employing time passive poles to give
> say -3dB at 30Hz before any power amp which has NFB. I often add in
> another 12AU7 gain stage for tone control to boost/cut treble; bass in
> AM is usually OK.
>
> To slightly widen AF response the Q of all IFTs may be reduced by
> strapping 100k across each coil. It doesn't work in all sets, but may
> be tried.
The coupling between the two coils in an IFT often needs to be increased
somewhat when adding resistors, that may explain why it doesn't work in all
sets, if the IFTs are over coupled adding resistors may ne counter productive.
Related to this is the fact that the instruction manual for Heathkit's first AM
tuner kit recommended placing a resistor(s) across the first IFT if a narrower
bandwidth was needed, so things don't always work as expected. The reason the
resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband
tuner with an over coupled 1st IFT, so adding the resistor(s) takes the
transformers close to critical coupling, giving a narrower response. I don't
remember if the Heathkit mod for reducing the bandwidth involved adding
resistors on both the primary and secondary, or on only one winding, I will have
to see if I can find the manual, IIRC they also used a resistor or resistors
during alignment to eliminate the over coupling, allowing the tuner to be
aligned by peaking the IFTs.
> There is a simple method explained in RDH4 to add a few
> turns of fine wire around the primary of IFT1 and switch it to being
> in series with the sec of IFT1 which will give a doble peak to IFT
> response which effectively increases IF channel bandwidth from a
> typical 4kHz to 8 kHz. Quad used it.
QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different
scheme to vary the bandwidth. Rather than using an IFT they used two separate
IF coils with low side capacitive coupling, the low side capacitive coupling
could be switched to vary the bandwidth.
> I've used it - finally there is the the treble which most sets refuse
> to give me.
>
> I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
> audio amp and with 12dB global NFB.
> This works far better than anything with 6V6 or EL84 etc.
>
> The Ge diode in the detector is biased on with a low current in the R.
> Its works best with high level signals and up to 100% mod, with 10Vrms
> available if you want it.
>
> But anyone could use the normal arrangement of IFT plus tube
> rectifiers found in IF amp tubes like 6N8 and thus get say -2Vdc
> generated by a carrier with 2pk volts of signal. at 100% mod the Vpk-
> pk of audio = 4V, so max audio = 1.4V, no?
>
> The audio signal with negative vdc and some 455kHz ripple could be
> direct coupled to a CF triode buffer stage but you'd need a -150Vdc
> rail for the cathode resistor, while the anode would go to a +100V
> supply. The CF buffer and negative rail was viewed as a waste of money
> in 1950, so it was never done. But there's no reason why it can't be
> done NOW. And the screen voltage applied to mixer and IF amp is best
> regulated at 100Vdc, so that's a good point of supply with CF anodes .
>
> I don't much like tubes like the 6AV6 with grounded cathodes and 10M
> grid leak biasing.
>
> The very low F signals generated in AM sets don't seem to cause any
> problems in anything I have made.
--
Regards,
Patrick Turner
> "Patrick Turner" <i...@turneraudio.com.au> wrote in message
>
> news:f8060a23-5198-42fa...@17g2000prr.googlegroups.com...
> ........................
> In most old radios with DC flow across the volume control pot track
> the adjustment of the volume is often very noisy after the pot has
> aged a few years.
>
> So instead of the conventional arrangements put forward by RDH and
> most others to avoid parts costs I will have the last IFT coil biased
> at say +50V at one end, and the live end goes to a triode grid of 1/2
> 12AU7 CF buffer to remove any loading effect of diode detection on the
> last IFT LC. Then I use Ge diode feeding RC circuit, and this can
> directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
> any a volume control and while employing time passive poles to give
> say -3dB at 30Hz before any power amp which has NFB. I often add in
> another 12AU7 gain stage for tone control to boost/cut treble; bass in
> AM is usually OK.
>
> Alex:
> Is adding two extra tubes a reasonable "mod"? It is the radio redesign.
I raise my hat to all the designers of tube AM radios. They worked
under duress and had no liberty to produce the best radio which could
be built because the companies all had to compete with each other for
sales so they all agreed to make the lowest common denominator
radios.
But DIYers were always free to do whatever they damned well liked, and
some did, but there's very little record of the
0.000000000000000000001% of radios made by ppl MORE intelligent that
99.99% of staff employed by companies.
If you consider the bulk and inefficiency of using pre WW2 octals and
most tubes, then why would making a radio with a couple of extra tubes
be a hanging offense? 1950's 7pin and 9pin tubes are far smaller and
compact than th octals they replaced, so you could use more tubes.
When FM radio and TV came into being the number of tubes became a
minor concern - you needed plenty of tubes, sure, and a shirt&trouser
load of other "stuff". Prices for such goods wasn't based on the cost
of production + a margin for profit because the industry had learnt
far higher margins over cost were needed to fund the phasing in of
Solid State and a huge increase om manufacturing infrastructure.
> one wants to go that far, op-amps would be far better. Take TLE2074. With
> 10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to increase
> sensitivity, tone control, buffer, AGC integrator, etc.
Sure. Then you have a solid state radio. Fuck that. Gimme a coupla
triodes.
> And after that why do you need to retain a lousy tube amp? Why not to use an
> IC? All the radio can be powered from a 6.3V winding with a bipolar voltage
> doubler (four caps, four 1N4004 diodes)
I build tube radios, OK.
>
> ...................
> I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
> audio amp and with 12dB global NFB.
> This works far better than anything with 6V6 or EL84 etc.
>
> Alex:
> Works better in which respect? EL34 takes as many amps for its heater as the
> whole original radio circuit. What about the poor lousy power transformer?
I replace PT, maybe add another if I have to. Anything goes, but what
goes out my door gives excellent AM and SW if there are coils, plus I
put in a couple of RCA sockets to take CD player L&R or an FM tuner
L&R so you get mono sound and real Hi-Fi. The trioded EL34 makes
better sound than anu single EL84/6V6. 807 and 6L6 in triode are also
excellent, and need less heater power.
I have a pile of PTs from which I can use for old radios where the PT
has become very fragile and poor insulation after running hot for 70
years.
So when someone getds me to fix a big beautiful floor standing radio,
they get the glory of the nice woodwork and they are not compelled to
listen all day to maybe 1 or 2 stations which sometimes broadcast
something worth listening to. They need to be able to play CDs and and
listen to AM and then they discover the glories of SET amps and wide
AF bandwidth. Most people don't need the sound to be loud, they don't
need stereo, and what they want is GOOD sound, and that's what I give
them. And in 3 years time they don't have to find a tube type
originating from pre-WW2 Europe, they will find spares quite easily
for the next 20 years at least.
Some radio ideas never became popular. The 1947 Tucker Synchrodyne was
such a thing. Synchronous detection really had to wait until the solid
state era where you could become more precise with circuits which all
too often were a nightmare for anyone to get running easily with
tubes.
Patrick Turner.
Patrick Turner
> In article <f8060a23-5198-42fa-87cf-9a3241471...@17g2000prr.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
>
>
>
>
> > snip for brevity.
>
> > > > John Byrns
>
> > > > Surf my web pages at, http://fmamradios.com/
>
> > > You grasped the idea perfectly well John. This frequency compensation is
> > > useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
> > > amp found in those boring AA5 style radios.
>
> > I agree with everything Alex says below except for a few things.
>
> > In most old radios with DC flow across the volume control pot track
> > the adjustment of the volume is often very noisy after the pot has
> > aged a few years.
>
> > So instead of the conventional arrangements put forward by RDH and
> > most others to avoid parts costs I will have the last IFT coil biased
> > at say +50V at one end, and the live end goes to a triode grid of 1/2
> > 12AU7 CF buffer to remove any loading effect of diode detection on the
> > last IFT LC.
>
> What is the purpose and or advantage of using a cathode follower between the IFT
> and detector diode, especially if you are going to add 100k resistors to widen
> the IF response as you discuss below?
The last IFT coil is a high impedance tuned circuit signal source. If
one is going to load it slightly to slightly reduce the Q thus
widening the pass band and AF response then using a pure resistance is
benign. The CF converts the high Z source to low Z source and all the
crapological behaviour of the Ge diode dissappears.
One could still use a 6AL5 if one wanted to.
>
> > Then I use Ge diode feeding RC circuit, and this can
> > directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
> > any a volume control and while employing time passive poles to give
> > say -3dB at 30Hz before any power amp which has NFB. I often add in
> > another 12AU7 gain stage for tone control to boost/cut treble; bass in
> > AM is usually OK.
>
> > To slightly widen AF response the Q of all IFTs may be reduced by
> > strapping 100k across each coil. It doesn't work in all sets, but may
> > be tried.
>
> The coupling between the two coils in an IFT often needs to be increased
> somewhat when adding resistors, that may explain why it doesn't work in all
> sets, if the IFTs are over coupled adding resistors may ne counter productive.
Try things. Predjudice don't belong anywhere when you want to make old
junk meet modern expectations.
In my kichen set I have a knob on front which changes IF gain. This is
done by varying the distance between the two coils on IFT1 so they can
be slightly over coupled after the set is tuned with coils apart.
Magically, the AF bandwidth nearly doubles. Its because the slight
over coupling creates a double IF peak in response which complememts
the single peak of the IFT2, and you get a wider pass band but skirt
selectivity is still very good. I recall Halicrafters had a
communications set with all 3 IFTs having mechanically variable
selectivity. This was a boon to radio hams.
>
> Related to this is the fact that the instruction manual for Heathkit's first AM
> tuner kit recommended placing a resistor(s) across the first IFT if a narrower
> bandwidth was needed, so things don't always work as expected.
I've always found R across IFT tanks reduced Q and increased BW, and
tended to make circuits less likely to oscillate.
> The reason the
> resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband
> tuner with an over coupled 1st IFT, so adding the resistor(s) takes the
> transformers close to critical coupling, giving a narrower response.
Most ordinary radios have lots of selectivity but 2kHz of AF BW - if
you are lucky.
> I don't
> remember if the Heathkit mod for reducing the bandwidth involved adding
> resistors on both the primary and secondary, or on only one winding, I will have
> to see if I can find the manual, IIRC they also used a resistor or resistors
> during alignment to eliminate the over coupling, allowing the tuner to be
> aligned by peaking the IFTs.
>
> > There is a simple method explained in RDH4 to add a few
> > turns of fine wire around the primary of IFT1 and switch it to being
> > in series with the sec of IFT1 which will give a doble peak to IFT
> > response which effectively increases IF channel bandwidth from a
> > typical 4kHz to 8 kHz. Quad used it.
>
> QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different
> scheme to vary the bandwidth. Rather than using an IFT they used two separate
> IF coils with low side capacitive coupling, the low side capacitive coupling
> could be switched to vary the bandwidth.
There are a number of things doable to widen IF BW, but doing them is
always easier said than done.
Patrick Turner.
Alex Pogossov
> speak. I had to look it up.
>
> Rushed to production with quirks in place, reliability problems, and
> only a handful made put that one almost in the 'wish list' category.
>
> I'm curious. Do you all get the History Channel over there or some
> other outlet for their shows? Reason I ask is they had a terrific
> series called "Dogfights" where they used CGI to recreate famous air
> battles, sometimes with the original pilot(s) expounding on the
> experience.
>
> Wasn't just glitter, though. They'd give the performance aspects of
> the aircraft and 'stop frame' the animation at critical points, to
> discuss possible tactics there, then continue on showing the effect of
> what was picked.
>
> You learn real quick there's a lot more to it than just 'performance
> specs'.
>
> One of my favorites is a SBD Dauntless dive bomber that gets jumped by
> a Zero, which conventional wisdom would say is near instant death, but
> things get worse because, as he's fending off that one, he gets jumped
> by a second Zero; which would be bad enough were it not for the third
> Zero that joins in.
>
> Talk about 'long odds', which happens to be the name of the episode.
>
> His rear gunner is useless as he's slammed around like a ping pong
> ball from the violent maneuvers but the SBD pilot kills two Zeros with
> his front 50s, 'dogfight style', and disables the third when it
> apparently tries to ram him.
>
> They put him in fighters after that ;)
>
> http://www.youtube.com/watch?v=zEnDjwXnj3Y
>
Thanks. I will pass this Dogfight link to my brother -- he loves this stuff
and will certainly enjoy. There are chain links to other episodes.
I do not have cable TV here in Sydney and can not watch the History Channel.
Alex Pogossov
Patrick Turner.
====================
Alex replies:
I see your approach to radio design. You are prepared to improve a boring
typical aussie radio by adding a cathode follower, another audio stage for
tone control, replace PT, drill/punch a couple of holes for the extra 2
tubes, etc.
What I can not understand is the craziness. Why go through all this bother
if a better improvement can be done with transistors and op-amps, and the
stuff can be hidden on a small board under the chassis. Unfortunately there
is no area where tubes outperform solid state, except for (literaly) warm
feeling.
Sometimes I do redesign as well. For example, the best AM detector is the
emitter detector. It virtually combines your cathode follower (emitter
follower in this case) with a diode detector (emitter junction). It has
rather high input impedance: beta times higher than of a bare diode
detector. And beta is quite high for the modern transistors. One problem
though. Reverse breakdown voltage of an emitter junction is about 8V, so the
carrier shall not exceed 1Vm (with a margin). Thus the radio needs to have a
perfect AGC.
An AGC in a boring radio is bad. On a week station AM detector gives 0.5V,
while on a strong local station can reach 5...10V. What I do, I put an
integrator in the AGC circuit so that no matter how strong is a station, the
carrier level is about 1V, while the AGC can be anywhere from 0V to --30V.
The radio becomes very convenient. Set volume and cruse the dial. All the
stations will have the same volume. The radio does not blast when running on
a strong local station.
Now I ask you: how would you make an integrator on a tube? Possible, but
cumbersome. Why lika communist, create difficulties and then overcome them
with a loud chest-pounding and shouting slogans?
Well, normally I do not do deep redesign, but just improve whatever possible
keeping the same topology. That is where the idea of low cut off in the
feedback came from. Once you begin to apply NFB to a lousy boring amp (to
reduce the speaker boom and make crispier sond), you want not to extend the
low frequencies below what the lousy OPT can handle naturally. And you can
not use RC high-pass filters on the input because the input impedance must
be 10M or more (not to load the detector). And you can not use a Hi-Z RC
filter of say 470pF and 10M, as hum and noise caused by flickering grid
current would be enormous.
Patrick Turner
> Alex replies:
> I see your approach to radio design. You are prepared to improve a boring
> typical aussie radio by adding a cathode follower, another audio stage for
> tone control, replace PT, drill/punch a couple of holes for the extra 2
> tubes, etc.
>
> What I can not understand is the craziness. Why go through all this bother
> if a better improvement can be done with transistors and op-amps, and the
> stuff can be hidden on a small board under the chassis.
I've got a lotta tubes and bits and peices. Most SS AM radios are
quite poor performers. I enjoy working with tubes.
it seems fraudelent to get someone to pay to have their radio restored
by fitting a little board full of chippy crappy thinges.
One has to draw a line some place.
> Unfortunately there
> is no area where tubes outperform solid state, except for (literaly) warm
> feeling.
Try telling that to ppl who prefer tube gear. It just is not true for
them.
I have a Marantz AM-FM chip based tuner sitting on top of my AM radio.
I can switch to FM if I want to, and the FM is not better sounding
than my tube AM set.
When I switch the tuner to its AM section its much inferior to the
tubes, so I use the tubes for AM and audio, and the chip tuner only
for the FM signal.
Something like a Leak Troughline FM tuner is excellent, also Quad FM
tuners. But the tubed stereo decoders often increased noise and
distortion so they were not always good. The add-on MPX made for Leak
and Quad with Ge transistors were awful. OK, so a chip based MPX Does
Work OK. I once instaled a kit made in the UK into an old Kenwood AM-
FM receiver with mono FM - worked very well, and there wasn't a tubed
MPX in there already.
>
> Sometimes I do redesign as well. For example, the best AM detector is the
> emitter detector. It virtually combines your cathode follower (emitter
> follower in this case) with a diode detector (emitter junction). It has
> rather high input impedance: beta times higher than of a bare diode
> detector. And beta is quite high for the modern transistors. One problem
> though. Reverse breakdown voltage of an emitter junction is about 8V, so the
> carrier shall not exceed 1Vm (with a margin). Thus the radio needs to have a
> perfect AGC.
Recently I used a couple of small bjts in darlington pair EF mode to
buffer the signal after Ge diode and RC - worked very well to avoid
cut off distortion because of AC loading. But I prefer 1/2 a 12AU7.
>
> An AGC in a boring radio is bad. On a week station AM detector gives 0.5V,
> while on a strong local station can reach 5...10V. What I do, I put an
> integrator in the AGC circuit so that no matter how strong is a station, the
> carrier level is about 1V, while the AGC can be anywhere from 0V to --30V.
Yes, but today I finally got a re-built 1935 radio working fairly well
with 6AN7 triode-hexode mixer and 6N8 IF remote cut off IF amp. To
avoid the terrible hum when tuned to AM stations ( due to compact
fluorescent lamps) I replace the existing RF input coil designed for
antenna = 3 metres of wire. The lamps tend to modulate the
electrostatic content of the electromagnetivc wave at the rectifier F,
100Hz plus horrid harmonics. The replacement is a ferrite rod antenna
which reacts to the magnetic portion of the wave which seems to have
far less hum. But the voltage level is smaller at the input. But with
only 2 tubes for the AM tuner, 4 out of 7 local AM stations produce
between -5 and -8 Vdc of AGC voltage, and audio level does not change
much for the stations even though I have no DC amp to assist the AGC
function.
On my own AM radio I have IF amp = 6BX6 which is sharp cut off and I
have local current FB to linearise the IF amp. AGC is applied to an RF
input stage and mixer only, sure the AF levels vary more but it sounds
better.
I've done a few sets which have had all octal tubes, 6U7 RF, 6A8
mixer, 6U7 IF amp, and because you have THREE stages and Gm is only
moderate, they work very well indeed, and give better SW than the two
stages with just mixer and IF amp. I've never felt a need for
amplified AVC voltage.
> The radio becomes very convenient. Set volume and cruse the dial. All the
> stations will have the same volume. The radio does not blast when running on
> a strong local station.
Nor does mine, and I am not alergic to changing volume for the mood,
to turn up music and intelligent discussions, and to trun down the
crap, adds and lying politicians. Moderate AGC is fine for me.
>
> Now I ask you: how would you make an integrator on a tube? Possible, but
> cumbersome. Why lika communist, create difficulties and then overcome them
> with a loud chest-pounding and shouting slogans?
What's an integrator? Last time I looked, it was something than
changed a square wave into a saw tooth wave, ie, RC circuit. Unlike
the Chest Pounding Communists, I am no slave to any unruly mob called
'Socialists For Electronic Correctness'. I'll do things my way, or a
better way if I am convinced its yet another intelligent use of a tube
or two, or three.
> Well, normally I do not do deep redesign, but just improve whatever possible
> keeping the same topology.
I find it impossible to retain existing circuits when I know that I
know better. It is a private thing with me. There are gory details.
But usually the music survives better than otherwise.
> That is where the idea of low cut off in the
> feedback came from. Once you begin to apply NFB to a lousy boring amp (to
> reduce the speaker boom and make crispier sond), you want not to extend the
> low frequencies below what the lousy OPT can handle naturally.
But what if the amp is NOT "lousy boring amp" and what if there is NO
speaker boom,
and no need of the extra NFB at VLF?
Feel free to add the cap if you want but I have yet to explore the
idea and make careful measurements to verify to myself the idea is
worth persuit. So far nobody seems to have devoted a website page with
all the details of performance which is too difficult to explain here
in words only.
> And you can
> not use RC high-pass filters on the input because the input impedance must
> be 10M or more (not to load the detector). And you can not use a Hi-Z RC
> filter of say 470pF and 10M, as hum and noise caused by flickering grid
> current would be enormous.
Sure. I get around all the problems you tell me about by abolishing
the use of grid leak biasing, and using something different. Today I
spent yet another 8 hours on a 1935 radio in which all old tubes were
removed and replaced with 1950s tubes. There had been a mix of 3
oddball European tubes wth Queer Sockets plus a couple of octals made
in Oz; someone had already modified the set, and done it quite badly,
and what was there in 1935 isn't clear, but basically just mixer, IF
amp with diodes, AF input, AF output and type 80 rectifier, and a
Magic Eye. 6 tubes. Now it has 7, with Si diodes to replace 80. I have
yet to re-wire the magic eye. The power lost in the 80 is now used in
the EL34. I found that instead of triode on this set I could have a CT
on the OPT primary for 50% SE UL mode and 12dB GNFB works fine. B+ is
+375V at the same Ia as used for a 6F6 which had been used. The EL34
in SE UL or SE triode gives far nicer sound than 6F6, 42, 6V6, EL84,
and other crap in pentode/tetrode mode with very little GNFB. The only
down side is that when Rout is high with pentode mode the small amount
of treble produced in old radios is "naturally boosted" by the rising
signal at the speaker at higher F because the speaker impedance rises
with F. Some makers were clever to marry the output tubes to just the
right speaker to give a boosted treble. If GNFB is used the treble
dissappears. So then one must use a tweeter. But most old sets without
GNFB become tiring to me and where there should be clear HF there are
competing high levels of "artificial HF", ie, the high levels of THD
and IMD - Its OK if all you need to know is the latest news bulletin,
but not so hot for music.
The set I'm working on will have 2 RCA sockets to combine L&R inputs
from an FM tuner or CD player, ipod, or other i-crap.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
You haven't really answered the question, just spouted a bunch of meaningless
gibberish. My question was what is the advantage of using a cathode follower
between the secondary of the IFT and the detector diode? The cathode follower
is going to reduce the loading on the IFT, which is going in exactly the
opposite direction you seem to want to go when you recommend loading the IFTs
with 100k resistors! What do you gain by adding the cathode follower to the
circuit? What is wrong with simply choosing the diode load to reflect the
desired load to the secondary of the IFT, 100k or whatever? Also the coupling
coefficient between the primary and secondary of each IFT should be properly
coordinated with the selected loading on the IFTs to achieve the desired
bandwidth with maximum response flatness. A cathode follower after the diode
can be useful as one way to eliminate negative peak clipping due to a poor AC/DC
load ratio. What is this "carpological" behavior of the Ge diode that you are
talking about? Can you define the nature of this behavior?
> > > Then I use Ge diode feeding RC circuit, and this can
> > > directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
> > > any a volume control and while employing time passive poles to give
> > > say -3dB at 30Hz before any power amp which has NFB. I often add in
> > > another 12AU7 gain stage for tone control to boost/cut treble; bass in
> > > AM is usually OK.
> >
> > > To slightly widen AF response the Q of all IFTs may be reduced by
> > > strapping 100k across each coil. It doesn't work in all sets, but may
> > > be tried.
> >
> > The coupling between the two coils in an IFT often needs to be increased
> > somewhat when adding resistors, that may explain why it doesn't work in all
> > sets, if the IFTs are over coupled adding resistors may ne counter
> > productive.
>
> Try things. Predjudice don't belong anywhere when you want to make old
> junk meet modern expectations.
What and whose prejudice are you talking about here? Are you talking about
people who have a "prejudice" against "improving" old radios? I have no such
prejudice. I do wonder about the design basis and effectiveness of
modifications, whether the modifications are being done in the best and most
effective manner.
--
Regards,
Patrick Turner
> In article <66fd4c06-24a6-448a-8e2f-b69870a77...@28g2000pry.googlegroups.com>,
I didn't speak meaningless giberish. But, I do understand that
whatever I do say, and whatever I have posted at my website as
examples of my craftmanship will always be misunderstood, belitlled,
rubbished, dismissed, and pissed upon shat upon from a great height. I
don't care because whoever is downloading the regular 100MB a day from
my site might not believe in you gibberish theory, and when they try
some of my circuits they find the sound is dramatically better than
has been served up by an industry mainly concerned with the cost-of-
production rather than audio fidelity.
As many of the readers here are approaching nearer to their 90th
birthday closer than I am, of course its likely they might find my
answers to questions and discussions of 101 things related to audio or
the world to be gibberish well before I conceed to myself that I don't
understand my position any better, because of increasing possible
senility. When blokes here say they were apprentices in some
electronics company in 1952, but of course try to hide their age, I
have to allow for the inevitable fact that a percentage of them have
gone soft up top, and I'd be utterly bamboozling, no matter how simply
I explained anything.
I suggest you read RDH4 and visit my website more.
I just re-wired a horrible Howard radio set made in about 1935. The
last IFT secondary is biased at +50V, and it feeds a 1/2 12AU7 CF,
with Ge diode, with 680k + 250pF cap to 0V, then 200k plus 100pF LPF
then direct coupled to another 1/2 12AU7 CF which poweres the 500k
volume control. The sound is HEAD and SHOULDERS above any other crappy
radio.
I suggest you try my methods to learn why I use the methods, for
therin lies the answers to all you testy questions.
> The cathode follower
> is going to reduce the loading on the IFT, which is going in exactly the
> opposite direction you seem to want to go when you recommend loading the IFTs
> with 100k resistors!
Resistance loads the IFTs with resistance, and the effect retains
pentode linearity. I don't always use R damping to reduce Q, but its
handy when the set has been changed to work with all mains operated 7
pin tubes in lieu of the 1.4V filament types which are fragile, have
low gm, and gain, and are becoming hard to obtain, and which ppl don't
want to use any more with batteries. The IFTs in such sets usually
have very high Q, and very low and thus poor sounding AF BW so R
damping is fine. With sets that have an RF stage ahead of the mixer so
hence have a total of 6 tuned circuits, side band cutting sometimes
limits AF to 2kHz, ie, bloody awful, OK if you are listening to long
distance talk back, but useless for music. So with so many stages the
IF Q may be reduced quite a bit and there is still plenty of gain for
the set overall for what is most listened to, ie, strong local AM
stations. Some reduction of Q is best if spread over all 4 coils in 2
IFTs. One must remember that the each LC circuit has only 6dB/octave
attenuation away from the Fo. Let's say each IFT LC has Q = 45. The
pass band = +/-5kHz, or 10kHz. Maybe you get -15dB at +/- 20kHz so
that with 4 such tuned circuits the pass band is 5kHz or less, and at
+/- 20kHz the attenuation = -60dB, considered OK, because strong
locals are usually separated by 45kHz at least. The nose shape of well
done IFTs won't be as sharp as just one one coil because the mutual
coupling is supposed to be arranged to flatten the top of the pass
band somewhat to give wide BW. Anyway, the nose shape won't much
affect the skirt attenuation at more than +/- 20kHz away from Fo. One
has to take all these considerations into account before adding R
dampers.
> What do you gain by adding the cathode follower to the
> circuit? What is wrong with simply choosing the diode load to reflect the
> desired load to the secondary of the IFT, 100k or whatever?
Better sound. Try it sometime. Then you'll know.
> Also the coupling
> coefficient between the primary and secondary of each IFT should be properly
> coordinated with the selected loading on the IFTs to achieve the desired
> bandwidth with maximum response flatness. A cathode follower after the diode
> can be useful as one way to eliminate negative peak clipping due to a poor AC/DC
> load ratio. What is this "carpological" behavior of the Ge diode that you are
> talking about? Can you define the nature of this behavior?
Ge diodes have reverse lekage currents. Try using one in a typical
radio circuit and get back to me. Be very careful when observing the
whole depressing performance using a high impedance non loading type
of probe to your CRO.
> > > > Then I use Ge diode feeding RC circuit, and this can
> > > > directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
> > > > any a volume control and while employing time passive poles to give
> > > > say -3dB at 30Hz before any power amp which has NFB. I often add in
> > > > another 12AU7 gain stage for tone control to boost/cut treble; bass in
> > > > AM is usually OK.
>
> > > > To slightly widen AF response the Q of all IFTs may be reduced by
> > > > strapping 100k across each coil. It doesn't work in all sets, but may
> > > > be tried.
>
> > > The coupling between the two coils in an IFT often needs to be increased
> > > somewhat when adding resistors, that may explain why it doesn't work in all
> > > sets, if the IFTs are over coupled adding resistors may ne counter
> > > productive.
>
> > Try things. Predjudice don't belong anywhere when you want to make old
> > junk meet modern expectations.
>
> What and whose prejudice are you talking about here? Are you talking about
> people who have a "prejudice" against "improving" old radios? I have no such
> prejudice. I do wonder about the design basis and effectiveness of
> modifications, whether the modifications are being done in the best and most
> effective manner.
Its far simpler for me to to humbly suggest you try my methods rather
than expect a full explanation. If I give an explanation its read,
perhaps called gibberish, and forgotten. The reader does not solder
the idea into reality. Nothing is learnt. I have wasted my precious
time. I've put all my answers at my website in the form of schematics.
I've explained all this at rec.audio.tubes several times in the last
11 years and I'm getting tired of repeating myself.
There are far better ways to build an AM radio than by slavishly,
blindly and unquestioningly following RDH4. Its a great book, but many
of the best practices with tubes are omitted because manufacturers
could not afford to implement them.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jul 6, 10:07 am, John Byrns <byr...@sbcglobal.net> wrote:
>
> > What do you gain by adding the cathode follower to the
> > circuit? What is wrong with simply choosing the diode load to reflect the
> > desired load to the secondary of the IFT, 100k or whatever?
>
> Better sound. Try it sometime. Then you'll know.
OK, I'm sensing a challenge here, similar to Danger Dave's assertions that
resulted in the ³Power Amplifier Without Power Transformer². The problem is
that with respect to Danger Dave's assertions, SNR was an easy to measure
parameter. Unfortunately your claims for the benefits of using a cathode
follower to buffer the IFT from the detector diode, is ³Better sound², a sort of
vague standard.
Before I take up my soldering iron to demonstrate that an AM detector the equal
of yours can be built without the cathode follower between the IFT and detector
diode, I will need to know how to measure ³Better sound². Can you suggest some
valid objective measurements I can use to measure ³Better sound²?
John L Stewart
John Byrns;934590 Wrote:
> In article
> <542b94e6-d75e-40d4...@34g2000pru.googlegroups.com>,
> Patrick Turner <in...@turneraudio.com.au> wrote:
> > On Jul 6, 10:07*am, John Byrns <byr...@sbcglobal.net> wrote:
> > -
> > >*What do you gain by adding the cathode follower to the
> > > circuit? *What is wrong with simply choosing the diode load to
> reflect the
> > > desired load to the secondary of the IFT, 100k or whatever? *-
> >
> > Better sound. Try it sometime. Then you'll know.-
>
> OK, I'm sensing a challenge here, similar to Danger Dave's assertions
> that
> problem is
> that with respect to Danger Dave's assertions, SNR was an easy to
> measure
> parameter. Unfortunately your claims for the benefits of using a
> cathode
> sort of
> vague standard.
>
> Before I take up my soldering iron to demonstrate that an AM detector
> the equal
> of yours can be built without the cathode follower between the IFT and
> detector
> suggest some
> valid objective measurements I can use to measure łBetter sound˛?
Now John, there are dreamers & there are doers. Which are you?
Pls pick up that soldering iron & show us what you can do.
Cheers to all, John L Stewart who still remembers which end is the
handle!
+-------------------------------------------------------------------+
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Patrick Turner
> In article <542b94e6-d75e-40d4-9a07-b4c202792...@34g2000pru.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
> > On Jul 6, 10:07 am, John Byrns <byr...@sbcglobal.net> wrote:
>
> > > What do you gain by adding the cathode follower to the
> > > circuit? What is wrong with simply choosing the diode load to reflect the
> > > desired load to the secondary of the IFT, 100k or whatever?
>
> > Better sound. Try it sometime. Then you'll know.
>
> OK, I'm sensing a challenge here, similar to Danger Dave's assertions that
> resulted in the ³Power Amplifier Without Power Transformer². The problem is
> that with respect to Danger Dave's assertions, SNR was an easy to measure
> parameter. Unfortunately your claims for the benefits of using a cathode
> follower to buffer the IFT from the detector diode, is ³Better sound², a sort of
> vague standard.
>
> Before I take up my soldering iron to demonstrate that an AM detector the equal
> of yours can be built without the cathode follower between the IFT and detector
> diode, I will need to know how to measure ³Better sound². Can you suggest some
> valid objective measurements I can use to measure ³Better sound²?
I suggest you renovate your distortion measuring capabilities, and
then compare an average AM adio performance to something designed on
the principles I recommend which bean counters in 1950 were likely to
say were "uanaffordable". No excuses for your allergy to warm your
soldering iron could ever be respected.
You resemble the lad at school who turns up with no completed
homework, and pleads to teacher, "Please sir, the dog ate my exercise
book" or other ficticious sorts of excuses.
I recall having no excuse for incomplete homework. I wore teacher's
wrath. I was deemed lazy. Pity, they said, while saying I was a
walking waste of intelligence. But ya shoulda seen the Meccano models
and boat models I made and train sets and the radio sets and
transmitters. I was busy, and i'd only learn something if I needed
too, then I learnt real good, and real quick.
So I wasa cunt of a kid, fuckin independant. But very busy; Bored
shitless with stuff the teachers and parents wanted me to learn, like
Latin, French, Geology, English, Economics, and I deliberately sang
out of tune to avoid being in the Skool Choir. So when others wasted
time singing hyms and crap I got a precious hour to catch up on math
or physics. It is upon math and physics that the World turns round! I
hated swimming, and on the main comp day I just didn't turn up. I was
a traitor. I was a hopeless football player and had no athletic
ability. I'm mighty proud I didn't catow to all these folks wanting me
to do stuff I just had fuck all interest in. I didn't bother to be
keen at the one and only social dance held by our school where they
bussed in girls from a local non-catholic grammer school. All those
cunts didn't fuck, wouldn't fuck, and would charge such enormous
prices for a fuck if you married one that I could see they were not
worth dancing with. It was useless trying to explain to Christian
Brothers. They'd never had a fuck. I hadn't either, but I knew what
it'd be like, and what the costs would be, and so why try to relate to
posh unfuckable girls prematurely? Those same girls have become old
chooks I'd not wanna know. Later I prooved I was right about
everything and that education had limited benefits. I got a job as a
carpenter's apprentice and spent many years Building A Better
Australia With My Own Bare Hands, and I went to night college for 6
years to get a Building Certificate. I found a use for education - to
facilitate a noble cause far more worthy than the existance teachers
had in mind for me. But being a bloke who worked outside and got mud
on his boots was a mighty horrid thing in the eyes of grammar school
girls or any girls, even catholic ones. I moved outa home and gave up
all the pretense of upper middle class status my parents thought they
had, and then I met the Working Class who I have loved ever since,
despite all their many little failings. In summer I swim about 5km a
week and I ride a bike 200km a week, and I am officially very fit for
my age. Most blokes of my age, 64, are 3/4 fucked in their minds and
bodies.
That's my explanation or excuse for not knowing everything, what's
yours?
Patrick Turner.
Lord Valve
> I got a precious hour to catch up on math
> or physics. It is upon math and physics that the World turns round!
I'm sure you didn't let little things like mistakes of four orders of magnitude slow
your headlong progress in the field.
Lord Valve
American - and fuck you if you don't like it.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
OK, you seem to be saying that it isn't "Better sound" that counts, that it is
measured "distortion" that counts?
I have next to zero interest in "renovating" my distortion measuring
capabilities, how good do my distortion measuring capabilities have to be to
adequately measure the performance of your AM detector?
What is the measured distortion performance of your AM detector circuit as shown
at the following URL?
http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder_files/schem-tun
er-am-tuner-dec-05.gif
At what IF signal level, into the detector, did you measure its performance?
This is important since I do not intend to try and duplicate your entire tuner,
just the detector.
> and then compare an average AM adio performance
Why do I care about the performance of "an average AM adio", why do you even
mention it?
> to something designed on the principles I recommend which bean
> counters in 1950 were likely to say were "uanaffordable".
All I am interested in comparing is "something designed on the principles you
recommend" vs. something designed on the principals I have recommended.
http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.htm
On your web page above, that describes your AM tuner design, you say about the
detector, "The grid of the IFT2 sec is biased at +31V to give some idle current
in the cathode R9, and to trickle a small current in the 1N914 diode via
R16&R17, so that the non-linear turn on character of the diode is avoided, since
it is always slightly turned on by the idle dc flow." Can you explain how this
bias avoids the "turn on character" of the diode, given that to function as a
detector the diode must turn off?! Bias, if correctly applied, will help reduce
the slew rate limiting at high modulating frequencies.
An interesting web page describing a number of AM detectors used in broadcast
modulation monitors and ham radio receivers can be found here.
http://www.tonnesoftware.com/appnotes/demodulator/diodedemod.html
[Much off topic drivel snipped]
--
Alex Pogossov
"John Byrns" <byr...@sbcglobal.net> wrote in message
news:byrnsj-82E926....@news.giganews.com...
> An interesting web page describing a number of AM detectors used in
> broadcast
> modulation monitors and ham radio receivers can be found here.
>
> http://www.tonnesoftware.com/appnotes/demodulator/diodedemod.html
Thanks John,
This is a very interesting article indeed.
Patrick Turner
> In article <31a222db-07a4-427b-b7f2-4b11e5b77...@x41g2000yqd.googlegroups.com>,
> er-am-tuner-dec-05.gif
>
> At what IF signal level, into the detector, did you measure its performance?
> This is important since I do not intend to try and duplicate your entire tuner,
> just the detector.
>
> > and then compare an average AM adio performance
>
> Why do I care about the performance of "an average AM adio", why do you even
> mention it?
>
> > to something designed on the principles I recommend which bean
> > counters in 1950 were likely to say were "uanaffordable".
>
> All I am interested in comparing is "something designed on the principles you
> recommend" vs. something designed on the principals I have recommended.
>
> http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.htm
>
> On your web page above, that describes your AM tuner design, you say about the
> detector, "The grid of the IFT2 sec is biased at +31V to give some idle current
> in the cathode R9, and to trickle a small current in the 1N914 diode via
> R16&R17, so that the non-linear turn on character of the diode is avoided, since
> it is always slightly turned on by the idle dc flow." Can you explain how this
> bias avoids the "turn on character" of the diode, given that to function as a
> detector the diode must turn off?! Bias, if correctly applied, will help reduce
> the slew rate limiting at high modulating frequencies.
>
> An interesting web page describing a number of AM detectors used in broadcast
> modulation monitors and ham radio receivers can be found here.
>
> http://www.tonnesoftware.com/appnotes/demodulator/diodedemod.html
>
> [Much off topic drivel snipped]
You may well delete "drivel" like so many others here who have awful
personalities who cannot cope with being human or nice in any way. Ah
you going senile? Just can't cope with well rounded discussions? Its
OK, you don't have to hide such characteristics.
I've read the tonnesoftware.com site before and there's nothing there
that works better than my circuits using tubes, and I got less wave
form distortion that they show on their oscillograms.
I suggest you try my circuit rather than waste time trying to find
excuses not to use your soldering iron.
I don't really mind if you are rivetted to your lounge room chair in
front of your PC, but when I ever about an idea which might be good I
rush out and try it out to see if I can replicate claims by the
inventor.
If you can't manage to build my detector circuit which uses 1 x 12AU7
tube with a few R&C then maybe you've lost your touch.
And I have extensively discussed all this stuff before with you here
at r.a.t and I refuse to do it all over again.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jul 9, 9:19 am, John Byrns <byr...@sbcglobal.net> wrote:
> >
> > [Much off topic drivel snipped]
>
> You may well delete "drivel" like so many others here who have awful
> personalities who cannot cope with being human or nice in any way. Ah
> you going senile? Just can't cope with well rounded discussions? Its
> OK, you don't have to hide such characteristics.
Your drivel is a fine topic for discussion, the problem is that it is off topic
in this group and you should take it elsewhere.
> I've read the tonnesoftware.com site before and there's nothing there
> that works better than my circuits using tubes, and I got less wave
> form distortion that they show on their oscillograms.
The detectors used in the modulation monitors, the General Radio 1931, Gates
M5693, and Belar AMM-2/3 don't appear to have any visible waveform distortion,
although your eye is probably more sensitive to this type of aberration than
mine. In my defense, back in the days before I became senile, when I worked in
the radio design factory, where we were designing "white goods" radios and
measuring power output at the 5% distortion level, I was consistently able to
set the signal level where 5% distortion occurred without reading the meter on
the distortion analyzer, looking only at the waveform displayed on the CRO.
I have two main problems with the tonnesoft web page. The first is that he uses
an ideal diode model for some of his simulations, and doesn't say what model he
used in others, although I haven't looked at the web page in two years, and if I
read it again might find that he does specify the diode models he used.
The second problem, which is very relevant to our disagreement over the use of a
cathode follower to buffer the diode from the IFT, is that he assumes that the
demodulators are driven by a zero impedance, effectively all his circuits
include your cathode follower, which confuses the issue.
> I suggest you try my circuit rather than waste time trying to find
> excuses not to use your soldering iron.
I probably shouldn't even be thinking about this cathode follower issue, let
alone thinking about soldering together your detector circuit. I should be
saving my solder for building an improved 25L6 amplifier, however I am curious
about the effect of your apparently pointless cathode follower, and it is
possible that I will be overcome by a desire to show it to be worthless. You
seem to possess a reverse "bean counter" element to your personality which
drives you to include circuit elements that serve no real function, thereby
needlessly driving costs up and reliability down.
> I don't really mind if you are rivetted to your lounge room chair in
> front of your PC, but when I ever about an idea which might be good I
> rush out and try it out to see if I can replicate claims by the
> inventor.
OK, what exactly is your claim for inserting a cathode follower between the
secondary of the IFT and the detector diode? On your web page you make two
claims.
Claim #1 is that "this CF isolate the loading effects of the diode detector from
the secondary of the 6BA6", however you don't say why this isolation might be
desirable? The only reason I can see is to increase the Q of the secondary of
the 6BA6, however increasing the Q doesn't seem to be your goal as you also talk
about adding 100k damping resistors to reduce the Q. You could save the cost of
the cathode follower and the loading resistor by simply using neither.
Claim #2 is that the cathode follower "provides a low impedance output to drive
the 1N914 silicon diode detector." Again you give no hint why this might be
useful, although it does seems to have some potential usefulness, on the other
hand a higher impedance output also has some advantages in detector
design/operation.
> If you can't manage to build my detector circuit which uses 1 x 12AU7
> tube with a few R&C then maybe you've lost your touch.
Looking at the schematic of your detector it appears to use only 1/2 x 12AU7 for
the cathode follower between the secondary of the IFT and the diode, your design
doesn't really use a cathode follower after the diode, instead depending on a
voltage divider, 330k/100k, in conjunction with a relatively high AC coupled
load of 890k, to minimize negative peak clipping. You do use cathode followers
as the audio stage, but they are not connected in a way that they have any
influence on negative peak clipping.
Your design has given me one idea that I have missed up until now. My design
uses a cathode follower directly coupled to the diode detector to eliminate
negative peak clipping. The down side of this approach for those of us who have
been influenced by bean counters is that it requires a negative supply for the
cathode of the cathode follower, impacting both cost and reliability to some
extent. On the plus side one advantage of this approach is that the AGC voltage
can be easily obtained from a tap a little way down the cathode resistor of the
cathode follower.
The idea you have given me is to float the secondary of the IFT and the diode at
a positive potential allowing the direct coupled cathode follower to operate
without a negative supply. The complication is that a separate AGC rectifier is
required, which has the potential to negatively affect the sound, especially
when a delay circuit is used.
> And I have extensively discussed all this stuff before with you here
> at r.a.t and I refuse to do it all over again.
Discussed it yes, unfortunately you have never offered an explanation of the
usefulness of the cathode follower between the IFT and the detector diode,
beyond the subjective claim that it provides the "best sound". Near as I can
tell the only purpose this cathode follower serves is to assuage your prejudices
against "bean counters".
Patrick Turner
> In article <016c81a0-c2ef-4840-9674-26757d07b...@5g2000yqb.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
> > On Jul 9, 9:19 am, John Byrns <byr...@sbcglobal.net> wrote:
>
> > > [Much off topic drivel snipped]
>
> > You may well delete "drivel" like so many others here who have awful
> > personalities who cannot cope with being human or nice in any way. Ah
> > you going senile? Just can't cope with well rounded discussions? Its
> > OK, you don't have to hide such characteristics.
>
> Your drivel is a fine topic for discussion, the problem is that it is off topic
> in this group and you should take it elsewhere.
I see no reason not to include some background information which
indirectly relates to the subject.
Otherwise the group's discussion can be very boring, especially since
all this has probably been discussed before.
>
> > I've read the tonnesoftware.com site before and there's nothing there
> > that works better than my circuits using tubes, and I got less wave
> > form distortion that they show on their oscillograms.
>
> The detectors used in the modulation monitors, the General Radio 1931, Gates
> M5693, and Belar AMM-2/3 don't appear to have any visible waveform distortion,
> although your eye is probably more sensitive to this type of aberration than
> mine. In my defense, back in the days before I became senile, when I worked in
> the radio design factory, where we were designing "white goods" radios and
> measuring power output at the 5% distortion level, I was consistently able to
> set the signal level where 5% distortion occurred without reading the meter on
> the distortion analyzer, looking only at the waveform displayed on the CRO.
Don't worry, when, or if I become senile, i'll not recognise 5% ona
CRO when its there. But most waves at that site showed slew
distortions and cut off distortions and so forth, all of which can
easily be avoided or minimised to be below 1% with my circuit over a
wide AF range and if the output voltage > 1 Vrms. But even where one
listens to short wave where AF detector output < 0.1Vrms, the sound is
good, although rather mauled by having travelled so far and being so
riddles with noise and fading up and down. I have never ever seen any
old radio or any old schematic of what was used for detecting AF for
use in re-broadcasting. AFAIK, not one single commercial example
exists of a radio with "infinite impedance detector" even though the
Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
27.57
RDH4 does not have anything that works as well as what I invented for
myself 14 years ago. Maybe someone else invented it elsewhere, but
almost anything is better than the normal bean counter driven way of
AM detection in old radios.
>
> I have two main problems with the tonnesoft web page. The first is that he uses
> an ideal diode model for some of his simulations, and doesn't say what model he
> used in others, although I haven't looked at the web page in two years, and if I
> read it again might find that he does specify the diode models he used.
I don't care about his modelling or what diodes he used; I just build
my REAL stuff and test it. If it measures well, and better than all
the websites I've seen online, they have wasted their time.
>
> The second problem, which is very relevant to our disagreement over the use of a
> cathode follower to buffer the diode from the IFT, is that he assumes that the
> demodulators are driven by a zero impedance, effectively all his circuits
> include your cathode follower, which confuses the issue.
There is no need to load the secondary of the IFT and one may indeed
connect it to an "infinite impedance" and it will still act like a
tuned transformer as intended. But one may load the IFT. No laws
against that. But loading can't be low, and so I have found its better
to convert the IF signal to low impedance with the CF and then the
diode is not so critical. Point contact GE types with high reverse
voltage ratings are probably best to use, forward voltage drop of only
0.2V.
>
> > I suggest you try my circuit rather than waste time trying to find
> > excuses not to use your soldering iron.
>
> I probably shouldn't even be thinking about this cathode follower issue, let
> alone thinking about soldering together your detector circuit. I should be
> saving my solder for building an improved 25L6 amplifier, however I am curious
> about the effect of your apparently pointless cathode follower, and it is
> possible that I will be overcome by a desire to show it to be worthless. You
> seem to possess a reverse "bean counter" element to your personality which
> drives you to include circuit elements that serve no real function, thereby
> needlessly driving costs up and reliability down.
Most bean counters justify their employment by being able to reduce
the parts and labour needed to make something, and therefore
increasing shareholder profits and most often reducing the sound
quality and reliability in electronics produced by the company.
I do indeed have an anti bean counter attitude, and most of them
should be frog-marched in public through the town square to where they
should be given a pick and shovel to dig their own grave. Where is Pol
Pot when you need the man? But I am not so extreme in practice, and
the radios and amps I build are amoung the best sound and most
reliable on the planet.
You can set out on a wanton voyage of defiance of good sense if you
like, but meanwhile I have clients I like to please and who pay me to
put in the stuff that the companies left out all those years ago.
> > I don't really mind if you are rivetted to your lounge room chair in
> > front of your PC, but when I ever about an idea which might be good I
> > rush out and try it out to see if I can replicate claims by the
> > inventor.
>
> OK, what exactly is your claim for inserting a cathode follower between the
> secondary of the IFT and the detector diode? On your web page you make two
> claims.
>
> Claim #1 is that "this CF isolate the loading effects of the diode detector from
> the secondary of the 6BA6", however you don't say why this isolation might be
> desirable? The only reason I can see is to increase the Q of the secondary of
> the 6BA6, however increasing the Q doesn't seem to be your goal as you also talk
> about adding 100k damping resistors to reduce the Q. You could save the cost of
> the cathode follower and the loading resistor by simply using neither.
OK, RDH4 talks about the ill-effects of diode + R&C detection loading
and although the book was written to promote tube use it didn't go far
enough as I have in this area. Loading should be resistive only, and
then you buffer to the following processes.
>
> Claim #2 is that the cathode follower "provides a low impedance output to drive
> the 1N914 silicon diode detector." Again you give no hint why this might be
> useful, although it does seems to have some potential usefulness, on the other
> hand a higher impedance output also has some advantages in detector
> design/operation.
Si diodes for signal detection at highish F should be driven by low Z
sources lest you get all manner of distortions you don't want. Just
build and measure, and you'll be pleased. If you just won't try such a
tiny little idea with just a tiny handful of parts to do something
that is so simple because its veracity cannot be proved in advance in
words here at this group, then you do indeed possess a moribund mind
and one possessing a high amount of sloth.
> > If you can't manage to build my detector circuit which uses 1 x 12AU7
> > tube with a few R&C then maybe you've lost your touch.
>
> Looking at the schematic of your detector it appears to use only 1/2 x 12AU7 for
> the cathode follower between the secondary of the IFT and the diode, your design
> doesn't really use a cathode follower after the diode, instead depending on a
> voltage divider, 330k/100k, in conjunction with a relatively high AC coupled
> load of 890k, to minimize negative peak clipping. You do use cathode followers
> as the audio stage, but they are not connected in a way that they have any
> influence on negative peak clipping.
>
> Your design has given me one idea that I have missed up until now. My design
> uses a cathode follower directly coupled to the diode detector to eliminate
> negative peak clipping. The down side of this approach for those of us who have
> been influenced by bean counters is that it requires a negative supply for the
> cathode of the cathode follower, impacting both cost and reliability to some
> extent. On the plus side one advantage of this approach is that the AGC voltage
> can be easily obtained from a tap a little way down the cathode resistor of the
> cathode follower.
>
> The idea you have given me is to float the secondary of the IFT and the diode at
> a positive potential allowing the direct coupled cathode follower to operate
> without a negative supply. The complication is that a separate AGC rectifier is
> required, which has the potential to negatively affect the sound, especially
> when a delay circuit is used.
The generation of AGC voltage may be done easily and off the CF output
with a cap and diode with its cathode grounded, with the generated -
Vdc drained off with 1.5M to bias points. It can also be done from the
anode of the IF amp and thus from the primary of IFT. C used is
usually 33pF, and loading effect is negligible.
The anode IF signal is higher than secondary do more AGC is available
and usually there is less sibilance during tuning when the anode is
used for AGC dervivation.
The rules about invention are anything but strict; just do what works
better than you read about in text books or that you see being used in
commercial junk designed bt accountants.
> > And I have extensively discussed all this stuff before with you here
> > at r.a.t and I refuse to do it all over again.
>
> Discussed it yes, unfortunately you have never offered an explanation of the
> usefulness of the cathode follower between the IFT and the detector diode,
> beyond the subjective claim that it provides the "best sound". Near as I can
> tell the only purpose this cathode follower serves is to assuage your prejudices
> against "bean counters".
I give a damn if all I've said misses the point asfayac. Those capable
of invention and who insist that AM radios and audio amps should sound
better and measure better than much of the dumbed down junk that's
sold in shops will understand my website and the attitudes I have.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> On Jul 10, 3:48 am, John Byrns <byr...@sbcglobal.net> wrote:
> > In article
> > <016c81a0-c2ef-4840-9674-26757d07b...@5g2000yqb.googlegroups.com>,
> > Patrick Turner <i...@turneraudio.com.au> wrote:
> >
> > > On Jul 9, 9:19 am, John Byrns <byr...@sbcglobal.net> wrote:
> >
> > > > [Much off topic drivel snipped]
> >
> > > You may well delete "drivel" like so many others here who have awful
> > > personalities who cannot cope with being human or nice in any way. Ah
> > > you going senile? Just can't cope with well rounded discussions? Its
> > > OK, you don't have to hide such characteristics.
> >
> > Your drivel is a fine topic for discussion, the problem is that it is off
> > topic
> > in this group and you should take it elsewhere.
>
> I see no reason not to include some background information which
> indirectly relates to the subject.
I don't see how your so called "background" information relates even
"indirectly" to the subject, it just plain doesn't belong in this group.
> > > I've read the tonnesoftware.com site before and there's nothing there
> > > that works better than my circuits using tubes, and I got less wave
> > > form distortion that they show on their oscillograms.
> >
> > The detectors used in the modulation monitors, the General Radio 1931,
> > Gates
> > M5693, and Belar AMM-2/3 don't appear to have any visible waveform
> > distortion,
> > although your eye is probably more sensitive to this type of aberration
> > than
> > mine. In my defense, back in the days before I became senile, when I
> > worked in
> > the radio design factory, where we were designing "white goods" radios and
> > measuring power output at the 5% distortion level, I was consistently able
> > to
> > set the signal level where 5% distortion occurred without reading the meter
> > on
> > the distortion analyzer, looking only at the waveform displayed on the CRO.
>
> Don't worry, when, or if I become senile, i'll not recognise 5% ona
> CRO when its there. But most waves at that site showed slew
> distortions and cut off distortions and so forth, all of which can
> easily be avoided or minimised to be below 1% with my circuit over a
> wide AF range and if the output voltage > 1 Vrms.
Yes, there are plenty of distortions shown to illustrate the various problems
that can occur in a poorly designed detector, and what causes the distortions.
There are also plenty of undistorted waves shown at that site, with distortions
of less than 1%, to illustrate the good results that can be achieved with
careful design.
> But even where one
> listens to short wave where AF detector output < 0.1Vrms, the sound is
> good, although rather mauled by having travelled so far and being so
> riddles with noise and fading up and down. I have never ever seen any
> old radio or any old schematic of what was used for detecting AF for
> use in re-broadcasting.
I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a.
"Ballempfänger", on the web. It was a rather complex radio although IIRC it
used a straightforward vacuum diode detector.
> AFAIK, not one single commercial example
> exists of a radio with "infinite impedance detector" even though the
> Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
> 27.57
> RDH4 does not have anything that works as well as what I invented for
> myself 14 years ago.
In the US J.W. Miller offered both commercial AM tuners and Radios using the
infinite impedance detector, I have one of their AM Tuners. Altec Lansing also
offered an AM-FM tuner that used the infinite impedance detector in the AM
section. Sargent Rayment also offered a line of tuners and receivers that used
the Selsted & Smith detector in the AM sections. Ampex also built an AM-FM
tuner that used a perverted variant of the Selsted & Smith detector, I also have
one of these.
It's hard to beat a carefully designed diode detector though.
> Most bean counters justify their employment by being able to reduce
> the parts and labour needed to make something, and therefore
> increasing shareholder profits and most often reducing the sound
> quality and reliability in electronics produced by the company.
The opposite approach, of adding components that serve no useful function is
just as bad, actually worse.
Patrick Turner
> In article <b4e1df0a-4abf-4326-b8df-4c189aa17...@28g2000pry.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
>
>
>
>
> > On Jul 10, 3:48 am, John Byrns <byr...@sbcglobal.net> wrote:
> > > In article
> > > <016c81a0-c2ef-4840-9674-26757d07b...@5g2000yqb.googlegroups.com>,
> > > Patrick Turner <i...@turneraudio.com.au> wrote:
>
> > > > On Jul 9, 9:19 am, John Byrns <byr...@sbcglobal.net> wrote:
>
> > > > > [Much off topic drivel snipped]
>
> > > > You may well delete "drivel" like so many others here who have awful
> > > > personalities who cannot cope with being human or nice in any way. Ah
> > > > you going senile? Just can't cope with well rounded discussions? Its
> > > > OK, you don't have to hide such characteristics.
>
> > > Your drivel is a fine topic for discussion, the problem is that it is off
> > > topic
> > > in this group and you should take it elsewhere.
>
> > I see no reason not to include some background information which
> > indirectly relates to the subject.
>
> I don't see how your so called "background" information relates even
> "indirectly" to the subject, it just plain doesn't belong in this group.
Well then we'll just have to agee to disagree. But I like to talk
about Sex, Politics, Religion, and Economics at dinner parties. I find
small talk tedious and boring if not spiced up with some off topics. I
thus get called a "raving left wing Grade A arsole" and so forth on
regular basis for daring to challenge the validity of the limited
perceptions of others which seem to be set in concrete. To un-set the
concrete, and allow flows of ideas I can come across as smooth as
grade 6 sand paper and I don't give a fuck. For flows of ideas,
readers of this rec.audio.tubes site should be active with a soldering
iron and Get Off Fat Arses And Do A Lot Of Something, ie,
GOFAADALOS !!!! :-) :-) And it'd help if they were FREE in how
they come to the site to talk about anything at all.
Someone said to me yesterday, "I'm black or white. I only ever see
black or white" She's the wife of a tech I employ, and can be rather a
stern old lady. "Ah", I said, "ever thought that there might be other
colours, like red, or green? Maybe blue, yellow?" She goes, "Nah,
black or white." Maybe that's why her husband don't talk to her much -
she's intimidating. But I mentioned, "Surely there'd be some green in
how ya see the world, no, because surely there'd be a bitta the Irish
in ya". She still wouldn't smile. Like a lemon. No sense of humour.
What shits me are old people who don't ask any sensible questions and
seek only to disprove others while NEVER constructing something new
and better within the field of their interests, and definately never
trying out anything different to see if it works.
If you now think I have been off topic, no apologies, because its not
wrong to be opinionated.
> > Don't worry, when, or if I become senile, i'll not recognise 5% ona
> > CRO when its there. But most waves at that site showed slew
> > distortions and cut off distortions and so forth, all of which can
> > easily be avoided or minimised to be below 1% with my circuit over a
> > wide AF range and if the output voltage > 1 Vrms.
>
> Yes, there are plenty of distortions shown to illustrate the various problems
> that can occur in a poorly designed detector, and what causes the distortions.
> There are also plenty of undistorted waves shown at that site, with distortions
> of less than 1%, to illustrate the good results that can be achieved with
> careful design.
Because my home brew test signal of modulated RF waves isn't perfect,
the distortion within the envelope is probably > 1%, and hence if I
measured the THD of the AF after its been through the AM radio i would
get THD of detected AF > 1%, certainly at say 95% modulation.
So to see how much THD is introduced by the radio set one may use a
dual trace CRO to display both test
signal at the radio input and the detected AF and overlay the waves to
see how they compare. Using as much of the screen as possible, its not
difficult to see if there is less than 1% of additional THD added by
the radio set. All that takes time but its a very simple thing and I
cannot understand why you would not have tried out all sorts of AM
detector ideas like I have to see what works best for you. What the
fuck impedes your progress to your workshop and soldering iron? If you
have a broken leg, or are impaired or disabled in some way, then let's
hear about it.
>
> > But even where one
> > listens to short wave where AF detector output < 0.1Vrms, the sound is
> > good, although rather mauled by having travelled so far and being so
> > riddles with noise and fading up and down. I have never ever seen any
> > old radio or any old schematic of what was used for detecting AF for
> > use in re-broadcasting.
>
> I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a.
> "Ballempfänger", on the web. It was a rather complex radio although IIRC it
> used a straightforward vacuum diode detector.
Nothing wrong with vacuum diode detectors if used wisely. They might
be used instead of how I have used Ge diodes with CF. They all work
best with low Z signal sources.
I googled Telefunken AM rebroadcast receiver "Ballempfänger"
Nothing to be seen.
>
> > AFAIK, not one single commercial example
> > exists of a radio with "infinite impedance detector" even though the
> > Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
> > 27.57
> > RDH4 does not have anything that works as well as what I invented for
> > myself 14 years ago.
>
> In the US J.W. Miller offered both commercial AM tuners and Radios using the
> infinite impedance detector, I have one of their AM Tuners. Altec Lansing also
> offered an AM-FM tuner that used the infinite impedance detector in the AM
> section. Sargent Rayment also offered a line of tuners and receivers that used
> the Selsted & Smith detector in the AM sections. Ampex also built an AM-FM
> tuner that used a perverted variant of the Selsted & Smith detector, I also have
> one of these.
I've never seen any of these things in Oz or any of their schematics.
Infinite Z detector isn't bad, but gives very low output.
>
> It's hard to beat a carefully designed diode detector though.
Depends. Most are distortion generators.
>
> > Most bean counters justify their employment by being able to reduce
> > the parts and labour needed to make something, and therefore
> > increasing shareholder profits and most often reducing the sound
> > quality and reliability in electronics produced by the company.
>
> The opposite approach, of adding components that serve no useful function is
> just as bad, actually worse.
Each unto their own. everyone has a different way of doing things, and
commercial competition stifles anyone who dares to use one more nut,
bolt or resistor than the fucking opposition brand. The result is that
they all have competitions to seewho can dumb shit down the most while
maintaining enough sales for the shareholders to dine well.
If it makes you ill to use one more cathode follower than someone
somewhere said could be superflous, then don't. But I will. In my
vicinity there are no shareholders or bean counters, and my clients
get a good deal without having to pay for someone's Cadillac or
expensive lunches, neither of which contribute to sound quality.
Patrick Turner.
> Regards,
>
> John Byrns
>
> Surf my web pages at, http://fmamradios.com/- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
Unfortunately it is also somewhat subjective.
> and I
> cannot understand why you would not have tried out all sorts of AM
> detector ideas like I have to see what works best for you.
I already know that a well-designed diode detector works best for me; at least
since I regressed to tubes, in my IC days I would have answered differently.
You say that you łhave tried out all sorts of AM detector ideas˛, besides your
spin on the diode detector, and your ill fated attempts at building a syncrodyne
receiver, what have you tried?
> What the
> fuck impedes your progress to your workshop and soldering iron?
Two things, first the fact that AM detectors are of academic interest only, as
there has been no AM broadcasting around this area, that needs anything better
than the crudest AM detector, for at least 15 years or so. Second, I think I am
more interested in putting my soldering iron to work on the John Stewart
inspired łimproved 25L6˛ amplifier. There are three binary design decisions to
be made before I purchase a chassis and begin punching it.
> If you
> have a broken leg, or are impaired or disabled in some way, then let's
> hear about it.
Ah, you think you can drag me into an off topic discussion, it won't work, if
you want to explore that kind of discussion, please take it to email.
> > > But even where one
> > > listens to short wave where AF detector output < 0.1Vrms, the sound is
> > > good, although rather mauled by having travelled so far and being so
> > > riddles with noise and fading up and down. I have never ever seen any
> > > old radio or any old schematic of what was used for detecting AF for
> > > use in re-broadcasting.
> >
> > I found a schematic for an old Telefunken AM rebroadcast receiver, a.k.a.
> > "Ballempfänger", on the web. It was a rather complex radio although IIRC
> > it
> > used a straightforward vacuum diode detector.
>
> Nothing wrong with vacuum diode detectors if used wisely. They might
> be used instead of how I have used Ge diodes with CF. They all work
> best with low Z signal sources.
>
> I googled Telefunken AM rebroadcast receiver "Ballempfänger"
> Nothing to be seen.
You weren't persistent enough, for a general description go here:
http://www.radiomuseum.org/r/telefunken_rundfunk_ballempfaenger_b.html
If you aren't a member, I'm not, you won't be able to see the schematic. The
schematic is available from several other web sites though, you can find it in
two parts here:
http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger-E1-1.gif
and here:
http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger-E1-2.gif
While the design looks like the łbean counters˛ were kept at a safe distance,
there still isn't a cathode follower in sight!
> > > AFAIK, not one single commercial example
> > > exists of a radio with "infinite impedance detector" even though the
> > > Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
> > > 27.57
> > > RDH4 does not have anything that works as well as what I invented for
> > > myself 14 years ago.
> >
> > In the US J.W. Miller offered both commercial AM tuners and Radios using
> > the
> > infinite impedance detector, I have one of their AM Tuners. Altec Lansing
> > also
> > offered an AM-FM tuner that used the infinite impedance detector in the AM
> > section. Sargent Rayment also offered a line of tuners and receivers that
> > used
> > the Selsted & Smith detector in the AM sections. Ampex also built an AM-FM
> > tuner that used a perverted variant of the Selsted & Smith detector, I also
> > have
> > one of these.
>
> I've never seen any of these things in Oz or any of their schematics.
>
> Infinite Z detector isn't bad, but gives very low output.
That's an interesting comment as the output level depends on the input just as
with a diode detector. My experiments with the circuit suggest that it does
exploit the characteristics of the low mu triodes to improve operation at low
carrier levels.
> > It's hard to beat a carefully designed diode detector though.
>
> Depends. Most are distortion generators.
Those are not łcarefully˛ designed, it is easy to screw up a diode detector, it
is equally easy to build a good one but it costs a little gain. This and the
output transformer are the areas where the work of łbean counters˛ can be most
easily seen.
> > > Most bean counters justify their employment by being able to reduce
> > > the parts and labour needed to make something, and therefore
> > > increasing shareholder profits and most often reducing the sound
> > > quality and reliability in electronics produced by the company.
> >
> > The opposite approach, of adding components that serve no useful function
> > is
> > just as bad, actually worse.
>
> Each unto their own. everyone has a different way of doing things, and
> commercial competition stifles anyone who dares to use one more nut,
> bolt or resistor than the fucking opposition brand. The result is that
> they all have competitions to seewho can dumb shit down the most while
> maintaining enough sales for the shareholders to dine well.
Hardly, commercial products today are overflowing with every imaginable feature,
each of which adds to the parts count. Competition today seems to be based more
on features, with cost as a secondary issue, why make something more expensive
than it needs to be to provide the required features and performance?
> If it makes you ill to use one more cathode follower than someone
> somewhere said could be superflous, then don't. But I will. In my
> vicinity there are no shareholders or bean counters, and my clients
> get a good deal without having to pay for someone's Cadillac or
> expensive lunches, neither of which contribute to sound quality.
An unneeded cathode follower doesn't contribute to sound quality either. I
won't bother to ask who pays for your lunches and transportation, least it take
us off topic again, but I am reasonably sure they are accounted for in the price
of the amplifiers you build.
--
Patrick Turner
> > dual trace CRO to display both test
> > signal at the radio input and the detected AF and overlay the waves to
> > see how they compare. Using as much of the screen as possible, its not
> > difficult to see if there is less than 1% of additional THD added by
> > the radio set. All that takes time but its a very simple thing
>
> Unfortunately it is also somewhat subjective.
>
> > and I
> > cannot understand why you would not have tried out all sorts of AM
> > detector ideas like I have to see what works best for you.
>
> I already know that a well-designed diode detector works best for me; at least
> since I regressed to tubes, in my IC days I would have answered differently.
With ICs you get a dc coupled amp and its possible to use an output
diode pointed at an RC circuit with the audio voltage and Vdc fed back
to the FB input port of the IC, and thus get what is supposed to make
the most linear detector imaginable because there is MUCH more NFB
being applied than when using a cathode follower like I do. The IC
input impedance at IF frequency needs to be high though. I have
circuits for doing all that but the CF I use is just fine.
>
> You say that you ³have tried out all sorts of AM detector ideas², besides your
> spin on the diode detector, and your ill fated attempts at building a syncrodyne
> receiver, what have you tried?
I've tried a lot of variations on normal arrangements with IF coil
feeding an anode in a vari-mu pentode IF amp and following RC circuit.
The CF seems best to me.
I tried to build a version of the DG Tucker Synchrodyne circuit
published in Wireless World in about 1947. Maybe its now online some
place, and what prevented progess was the making of a suitable
balanced synchronous demodulator. Probably the best way is to use a
toroidal transformer with the core µ just right for range of RF
frequencies. But I never had time to explore how to make such a
tranny. The tucker circuit uses lots more tubes than a superhet and
needs much more practical expertise to get running without loud
whistles while tuning. It was a potentially excellent thing but it was
never to become popular because the superhet was king. Then tried
making a synchrodyne based on a self oscillating 6BE6, and there are
some simple circuits of those around, just RF input tube and 6BE6
needed only. That sort of worked a bit but monkey chatter and whistles
and controlling oscillations just right were easier said than done so
I abandoned the idea and proceeded with a good superhet. AGC is
applied to RF vari-mu cascode triode input amp and 6AN7mixer, then
6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for
local current FB. AF bandwidth is 10kHz, with minimum distance between
IF coils, but even with max distance AF BW is about 6kHz. Tone control
boosts and cuts F above 2kHz. 12AU7 CF detector used, and after
comparing the sound of my kitchen AM radio to countless others I have
repaired, my own design is very much better to listen to. I have a
Marantz AM/FM tuned hooked up to allow switching to FM in mono, and
when the same ABC news program is being broadcast at the same time on
local ABC AM stations and FM stations the AM radio I have sounds
clearer than the FM produced by the SS tuner. There is a subtle
difference, and a pleasant one. The AM from the Marantz is fucking
awful.
I try not to give AM radio much attention in my life because I have
1,001 projects to complete for customers and several amp projects fr
myself in progress. But I get "trapped by AM radios" which ppl want
fixed. The majority are collectors items with outrageously poor
performance if they are restored to original condition, and because
ppl use compact flourescent lamps and 101 other things which create
noise, a ferrite rod antenna must be used and the MW RF input coild
junked. Then inputs for tuner and CD must be put in with a switch to
people are not stuck having to listen to the appalling programme
material such as talk back and commercial garbage laced with adds. The
speakers and amps need rebuilding to meet modern expectations. Thre
are some who might junk all the tubes and put in a board full of SS
parts but then that betrays tubes, and I don't do that trick.
> > What the
> > fuck impedes your progress to your workshop and soldering iron?
>
> Two things, first the fact that AM detectors are of academic interest only, as
> there has been no AM broadcasting around this area, that needs anything better
> than the crudest AM detector, for at least 15 years or so. Second, I think I am
> more interested in putting my soldering iron to work on the John Stewart
> inspired ³improved 25L6² amplifier. There are three binary design decisions to
> be made before I purchase a chassis and begin punching it.
So it seems unlikely you'll ever listen to the benefits of my ideas.
Binary decisions eh. In a bind are you?
>
> > If you
> > have a broken leg, or are impaired or disabled in some way, then let's
> > hear about it.
>
> Ah, you think you can drag me into an off topic discussion, it won't work, if
> you want to explore that kind of discussion, please take it to email.
I guess you think some things are best kept out of the public gaze.
Its remarkable how many people are secretive about themselves, I'm one
who isn't. I have very little to feel ashamed of or embarrassed
about.
>
> > I googled Telefunken AM rebroadcast receiver "Ballempfänger"
> > Nothing to be seen.
>
> You weren't persistent enough, for a general description go here:
>
> http://www.radiomuseum.org/r/telefunken_rundfunk_ballempfaenger_b.html
>
> If you aren't a member, I'm not, you won't be able to see the schematic. The
> schematic is available from several other web sites though, you can find it in
> two parts here:
>
> http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
>
> and here:
>
> http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
>
> While the design looks like the ³bean counters² were kept at a safe distance,
> there still isn't a cathode follower in sight!
>
Those links open but the pictures won't open to reveal what you are
talking about.
I doubt I am missing anything.
>
>
>
>
> > > > AFAIK, not one single commercial example
> > > > exists of a radio with "infinite impedance detector" even though the
> > > > Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
> > > > 27.57
> > > > RDH4 does not have anything that works as well as what I invented for
> > > > myself 14 years ago.
>
> > > In the US J.W. Miller offered both commercial AM tuners and Radios using
> > > the
> > > infinite impedance detector, I have one of their AM Tuners. Altec Lansing
> > > also
> > > offered an AM-FM tuner that used the infinite impedance detector in the AM
> > > section. Sargent Rayment also offered a line of tuners and receivers that
> > > used
> > > the Selsted & Smith detector in the AM sections. Ampex also built an AM-FM
> > > tuner that used a perverted variant of the Selsted & Smith detector, I also
> > > have
> > > one of these.
>
> > I've never seen any of these things in Oz or any of their schematics.
>
> > Infinite Z detector isn't bad, but gives very low output.
>
> That's an interesting comment as the output level depends on the input just as
> with a diode detector. My experiments with the circuit suggest that it does
> exploit the characteristics of the low mu triodes to improve operation at low
> carrier levels.
The infinite Z detector should work very well if a CF is used to
buffer the IFT output to produce a low Z drive which might then be
applied to a diode and then to resistance to then be able to extract
the average voltage across the resistance, and without the diode
having to charge a cap. Kinda like a half wave rectifier in a PSU
where the aim is to have the AF signal linear to average voltage. One
can use a choke and cap after the diode in the same way.
I've not needed to explore all that because I find the CF driving the
diode which charges the RC works just fine. The second CF can also be
rigged as a low gain AF amp with RC coupllng but R is a bias R of say
1M5 taken to a tap on Rk, so it appears as maybe 5M0 to the detector
circuit so cut off at high AM% is avoided.
The anode then can easily give gain = 4. it does mean that where one
has say 2Vrms of AF at the detector, you have 8Vrms at 12AU7 anode,
and that's getting high, tempting high THD, so I prefer having two CF,
with the second just to buffer the RC detector circuit and stop the
vol control loading. some Gain for the poweramp is easily created in a
tone control stage.
>
> > > It's hard to beat a carefully designed diode detector though.
>
> > Depends. Most are distortion generators.
>
> Those are not ³carefully² designed, it is easy to screw up a diode detector, it
> is equally easy to build a good one but it costs a little gain. This and the
> output transformer are the areas where the work of ³bean counters² can be most
> easily seen.
Indeed.
>
> > > > Most bean counters justify their employment by being able to reduce
> > > > the parts and labour needed to make something, and therefore
> > > > increasing shareholder profits and most often reducing the sound
> > > > quality and reliability in electronics produced by the company.
>
> > > The opposite approach, of adding components that serve no useful function
> > > is
> > > just as bad, actually worse.
>
> > Each unto their own. everyone has a different way of doing things, and
> > commercial competition stifles anyone who dares to use one more nut,
> > bolt or resistor than the fucking opposition brand. The result is that
> > they all have competitions to seewho can dumb shit down the most while
> > maintaining enough sales for the shareholders to dine well.
>
> Hardly, commercial products today are overflowing with every imaginable feature,
> each of which adds to the parts count. Competition today seems to be based more
> on features, with cost as a secondary issue, why make something more expensive
> than it needs to be to provide the required features and performance?
I have a Marshall JCM2000 60W guitar amp pulled to bits on the bench.
It has an enormous lot of features and 5 times the number of R&C used
in 1960, and maybe 15 opamps and it is a complete PIA to work upon.
Nobody much gives a shit about the concerns I have, such as it should
be simple and easy. But some guitar amps are excellently configured
with ultra simplicity, real class A, and hardly any features. The
really good musos don't need features; they know how to entertain
without needing gear features to cover shortcomings.
>
> > If it makes you ill to use one more cathode follower than someone
> > somewhere said could be superflous, then don't. But I will. In my
> > vicinity there are no shareholders or bean counters, and my clients
> > get a good deal without having to pay for someone's Cadillac or
> > expensive lunches, neither of which contribute to sound quality.
>
> An unneeded cathode follower doesn't contribute to sound quality either. I
> won't bother to ask who pays for your lunches and transportation, least it take
> us off topic again, but I am reasonably sure they are accounted for in the price
> of the amplifiers you build.
I make all payments around here and I hear the benefits of my circuits
as do my customers. I make very low wages with all I do because the
general public never pays artisans very much. I like my work, so I do
my work. Toilet cleaners get paid more. It worries me not, money ain't
everything.
Patrick Turner.
John Byrns
Patrick Turner <in...@turneraudio.com.au> wrote:
> >
> > I already know that a well-designed diode detector works best for me; at
> > least
> > since I regressed to tubes, in my IC days I would have answered
> > differently.
>
> With ICs you get a dc coupled amp and its possible to use an output
> diode pointed at an RC circuit with the audio voltage and Vdc fed back
> to the FB input port of the IC, and thus get what is supposed to make
> the most linear detector imaginable because there is MUCH more NFB
> being applied than when using a cathode follower like I do. The IC
> input impedance at IF frequency needs to be high though. I have
> circuits for doing all that but the CF I use is just fine.
HP had an interesting AM demodulator in their AM/FM modulation meter. The AM
detector used transistors, not ICs. A rough description as I remember it is as
follows. The transistors were configured in what was basically a darlington
configuration. Feedback was taken from the collector through a capacitor to a
pair of parallel diodes back to a summing junction at the base. The two diodes
were connected in opposite directions, with a pair of parallel connected complex
cognate networks in series with one diode at the end connected to the transistor
base. The modulation output was taken across the network that has the form of a
low pass filter IIRC.
If you are willing to use an IC you can get a whole synchronous detector circuit
in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips.
There are instructions on the web that describe how to do this.
> > > What the
> > > fuck impedes your progress to your workshop and soldering iron?
> >
> > Two things, first the fact that AM detectors are of academic interest only,
> > as
> > there has been no AM broadcasting around this area, that needs anything
> > better
> > than the crudest AM detector, for at least 15 years or so. Second, I think
> > I am
> > more interested in putting my soldering iron to work on the John Stewart
> > inspired ³improved 25L6² amplifier. There are three binary design
> > decisions to
> > be made before I purchase a chassis and begin punching it.
>
> So it seems unlikely you'll ever listen to the benefits of my ideas.
>
> Binary decisions eh. In a bind are you?
Me thinks you misinterpreted what I meant by the term "Binary decisions".
Here are the three decisions I have to make, perhaps I should flip a coin and be
done with it.
1. The output stage will be of the distributed load type with 50% of the load in
the cathode circuit, and 50% in the plate circuit. The decision yet to be taken
is should I use the McIntosh output circuit, or an Acoustical style of output
circuit? This decision will have some impact on the layout of the power supply.
2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull
parallel, or a pair of 7695s. Actually this is a ternary choice, the KT55 is
another possible choice for the output tubes, while these are cool tubes,
finding some would probably not be easy. The output tube choice has a major
impact on the chassis size, and layout, so I have to settle it before I purchase
a chassis and begin drilling and punching it.
3. The driver tube must also be specified, the binary choice was between the
6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to
the list. These tubes all use the same socket, so they can all be tried with
only a change of cathode resistor.
> > > I googled Telefunken AM rebroadcast receiver "Ballempfänger"
> > > Nothing to be seen.
> >
> > You weren't persistent enough, for a general description go here:
> >
> > http://www.radiomuseum.org/r/telefunken_rundfunk_ballempfaenger_b.html
> >
> > If you aren't a member, I'm not, you won't be able to see the schematic.
> > The
> > schematic is available from several other web sites though, you can find it
> > in
> > two parts here:
> >
> > http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
> >
> > and here:
> >
> > http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
> >
> > While the design looks like the ³bean counters² were kept at a safe
> > distance,
> > there still isn't a cathode follower in sight!
> >
>
> Those links open but the pictures won't open to reveal what you are
> talking about.
>
> I doubt I am missing anything.
I think you are just intimidated by the excellent German engineering in this
receiver, to eliminate any possible excuse of your not being able to open it, I
have posted a copy of the schematic on my web pages at this URL:
http://fmamradios.com/stuff/E1.pdf
Patrick Turner
> In article <af933490-3b7d-46d5-af2c-143121897...@q29g2000prj.googlegroups.com>,
> Patrick Turner <i...@turneraudio.com.au> wrote:
>
>
>
> > > I already know that a well-designed diode detector works best for me; at
> > > least
> > > since I regressed to tubes, in my IC days I would have answered
> > > differently.
>
> > With ICs you get a dc coupled amp and its possible to use an output
> > diode pointed at an RC circuit with the audio voltage and Vdc fed back
> > to the FB input port of the IC, and thus get what is supposed to make
> > the most linear detector imaginable because there is MUCH more NFB
> > being applied than when using a cathode follower like I do. The IC
> > input impedance at IF frequency needs to be high though. I have
> > circuits for doing all that but the CF I use is just fine.
>
> HP had an interesting AM demodulator in their AM/FM modulation meter. The AM
> detector used transistors, not ICs. A rough description as I remember it is as
> follows. The transistors were configured in what was basically a darlington
> configuration. Feedback was taken from the collector through a capacitor to a
> pair of parallel diodes back to a summing junction at the base. The two diodes
> were connected in opposite directions, with a pair of parallel connected complex
> cognate networks in series with one diode at the end connected to the transistor
> base. The modulation output was taken across the network that has the form of a
> low pass filter IIRC.
It'd be nice to have a schematic John. Then 1,000 words are told with
1 picture.
All sorts of things are possible, but the humble pair of CFs for me is
the most appropriate in a tube radio.
If I had the time I'd investigate further but I really have little
need, and I'm perpetually busy fixing people's radios, amps and
speakers. I dunno about you, but I must work or I starve, and find
time to ride 200km a week on a bike.
> There are three binary design
> > > decisions to
> > > be made before I purchase a chassis and begin punching it.
>
> > So it seems unlikely you'll ever listen to the benefits of my ideas.
>
> > Binary decisions eh. In a bind are you?
>
> Me thinks you misinterpreted what I meant by the term "Binary decisions".
Er, no, its just my Cents of Hugh Mer.
>
> Here are the three decisions I have to make, perhaps I should flip a coin and be
> done with it.
>
> 1. The output stage will be of the distributed load type with 50% of the load in
> the cathode circuit, and 50% in the plate circuit. The decision yet to be taken
> is should I use the McIntosh output circuit, or an Acoustical style of output
> circuit? This decision will have some impact on the layout of the power supply.
So how long have you been wondering what the F to do? Who is t say
you'll flip a coin tommorrow, then later un-decide what the coin said,
and keep flipping until a better outcome appears. Could Flipper help?
The McIntosh method gives the best measurements, and you don't need
bifilar OPT windings. EAR did a quasi McI amp in the EAR 509 with some
PL509 HORRIBLE output tubes, and it was to get 100W with high NFB and
its sounded terrible, and using a pair of KT88 with UL or CFB of 20%
and aiming to get 50W max with high % of class A1 working, say 15W max
at least, will give excellent performance with one less gain stage.
But during the next 15 years you'll have plenty of time to consider.
>
> 2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull
> parallel, or a pair of 7695s. Actually this is a ternary choice, the KT55 is
> another possible choice for the output tubes, while these are cool tubes,
> finding some would probably not be easy. The output tube choice has a major
> impact on the chassis size, and layout, so I have to settle it before I purchase
> a chassis and begin drilling and punching it.
Its often not what you use, but how you use it that counts.
>
> 3. The driver tube must also be specified, the binary choice was between the
> 6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to
> the list. These tubes all use the same socket, so they can all be tried with
> only a change of cathode resistor.
Good luck.
I guess we may never know how your project ends up at the present rate
of progress.
>
>
>
>
>
> > > > I googled Telefunken AM rebroadcast receiver "Ballempfänger"
> > > > Nothing to be seen.
>
> > > You weren't persistent enough, for a general description go here:
>
> > >http://www.radiomuseum.org/r/telefunken_rundfunk_ballempfaenger_b.html
>
> > > If you aren't a member, I'm not, you won't be able to see the schematic.
> > > The
> > > schematic is available from several other web sites though, you can find it
> > > in
> > > two parts here:
>
> > >http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
>
> > > and here:
>
> > >http://www.tsf-radio.org/schph.php?f=tome00/Telefunken/Ball-Empfanger...
>
> > > While the design looks like the ³bean counters² were kept at a safe
> > > distance,
> > > there still isn't a cathode follower in sight!
>
> > Those links open but the pictures won't open to reveal what you are
> > talking about.
>
> > I doubt I am missing anything.
>
> I think you are just intimidated by the excellent German engineering in this
> receiver, to eliminate any possible excuse of your not being able to open it, I
> have posted a copy of the schematic on my web pages at this URL:
>
> http://fmamradios.com/stuff/E1.pdf
>
Not a bad radio schematic, but I'm not going to adopt any of the
German techniques. Too complex. This radio has UMPTEEN PARTS which
don't need to be there just to get strong local AM stations with hi-fi
quality.
While you won't accept my use of one lousy cathode follower, you are
putting a radio set under my nose which has maybe 200 parts I don't
need to use to attain my simple goals.
AM radios tend to waste huge chunks of my time with very little
profit, and I just do what works for me, and this includes far more
simplicity than what the Germans may have done.
I have several communication radios and trancievers which all need re-
builds. An old radio ham I knew died and his folks sold all they
could, but then there was still a small mountain of collectable stuff
left over in various states of dis-repair. So they rang me to collect
what would have gone to a tip. Took me a week to collect and sort it
then another week to extend my storage areas with shelving to keep the
old junk.
Going right over it all and all would take weeks before even one radio
set could be declared to be "good at what they do", without all sorts
if intermittencies and noises because of old switches and contacts and
crap. But listening on SW is a bit boring, and even if one joins in
discussions with radio amateurs after building one's own little old
radio station, one ends talking to old guys about their latest surgery
and health bothers and old age problems. Kinda puts me to sleep,
listenin' to them droning on nerdishly, saying so many words, but
trying to be un-personal, and not saying much at all. There are many
who just like to broadcast, and their audience never really exists.
They can't convey their creativity with pictures very well.
Rather than put up with droners, I have my own life to do, and a
living to be earned, and fitness to be maintained.
Patrick Turner.
Patrick Turner
> >A drawback - a despised electrolytic as a shaping component.
>
> Btw, there's another means of achieving a modest 'LF shelf', using a
> concertina, and that's to boostrap the gain stage load off the
> concertina cathode. Gain drops at LF as the boostrap rolls off.
This has been done in some Dynaco schematics which have the concertina
cathode driving the top of a RLdc via an electro cap, therefore
supplying positive FB to the input pentode by means of increasing the
RL the pentode anode "sees" so that pentode gain goes much higher
therefore making a given amount of GNFB much more effective. Basically
while peter robs paul and you light a fire under both, they tend to
become very active indeed.
If the input tube is a lowly triode who's gain is determined by its
lower µ the PFB effect is minimal, and whether there is a bootstrap or
not won't change VLF gain very much, so the shelving networks seen
throughout my website are perhaps the better way with triodes.
> It has the added advantage of boosting stage gain and theoretically
> lowering distortion from the (idealized) 'infinite impedance' of the
> bootstrap
The bootstrap from concertina cathode could be taken from an
additional triode cathode follower which is driven directly off the
cathode so concertina R values remain unchanged, ie, anode RL =
cathode RL. The CF can then be used to drive a two resistor R divider
to V1 anode. One has to be careful that cut off distortion with the AC
coupling does not happen.
>
> The concertina resistors need adjusting to re-balance the load, and
> that causes an offset toward B+, but that can be an advantage as well
> since direct coupled concertinas on modest B+ rails can end up with
> rather low anode voltage on the gain stage and the offset bumps that
> up.
Yes, indeed. I would say I prefer the LTP driver with cathode CCS with
say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced
pair with very low THD. Whatever the input tube is, pentode or
triode, it has only to produce a low signal in SE mode so THD remains
lower than if you have an input tube needing to make slightly more Va
than is applied to each output tube grid.
There's plenny of headroom for the function of the shelving network.
The LTP does not need to be directly
driven from input gain tube anode, so the LTP can have much more
headroom than found in most samples of amps like Leak and Manley Labs
et all.
Patrick Turner.
Patrick Turner
> On Fri, 15 Jul 2011 02:03:07 -0700 (PDT), Patrick Turner
>
>
>
>
>
> <i...@turneraudio.com.au> wrote:
> >snip
>
> >> >A drawback - a despised electrolytic as a shaping component.
>
> >> Btw, there's another means of achieving a modest 'LF shelf', using a
> >> concertina, and that's to boostrap the gain stage load off the
> >> concertina cathode. Gain drops at LF as the boostrap rolls off.
>
> >This has been done in some Dynaco schematics which have the concertina
> >cathode driving the top of a RLdc via an electro cap, therefore
> >supplying positive FB to the input pentode by means of increasing the
> >RL the pentode anode "sees" so that pentode gain goes much higher
> >therefore making a given amount of GNFB much more effective. Basically
> >while peter robs paul and you light a fire under both, they tend to
> >become very active indeed.
>
>
> >If the input tube is a lowly triode who's gain is determined by its
> >lower µ the PFB effect is minimal, and whether there is a bootstrap or
> >not won't change VLF gain very much, so the shelving networks seen
> >throughout my website are perhaps the better way with triodes.
>
>
> Sometimes 'just a little more' is all it takes and, at least with the
> ones I've done, the capacitor values are usually, or can be, small
> enough to use film and avoid electrolytics.
>
> My "Stealth AX" amp does it sort of 'in reverse'. The gain triode Rl
> is under the concertina splitter, for the 'infinite impedance'
> bootstrap, and the signal is cap coupled to the concertina grid.
>
> >> It has the added advantage of boosting stage gain and theoretically
> >> lowering distortion from the (idealized) 'infinite impedance' of the
> >> bootstrap
>
> >The bootstrap from concertina cathode could be taken from an
> >additional triode cathode follower which is driven directly off the
> >cathode so concertina R values remain unchanged, ie, anode RL =
> >cathode RL. The CF can then be used to drive a two resistor R divider
> >to V1 anode. One has to be careful that cut off distortion with the AC
> >coupling does not happen.
>
>
> >> The concertina resistors need adjusting to re-balance the load, and
> >> that causes an offset toward B+, but that can be an advantage as well
> >> since direct coupled concertinas on modest B+ rails can end up with
> >> rather low anode voltage on the gain stage and the offset bumps that
> >> up.
>
> >Yes, indeed. I would say I prefer the LTP driver with cathode CCS with
> >say 6SN7/6CG7/12BH7/12AU7 or EL84. Then the LTP acts as a balanced
> >pair with very low THD.
>
> and it's more tubes.
It depends. Have a look at
http://www.turneraudio.com.au/300w-1+2-schem-input-driver-output-jan06.htm
Here I have an LTP which produces less than 0.5% THD at two phases of
85Vrms from each driver triode.
To do a similar thing with concertina, you'd have a triode making
170Vrms Va-k, and one might do that using an EL84 in triode, and the
drive voltage to the Concer grid would be 95Vrms. The previous stage
has to make a shirt&trouser load sized signal, maybe with 5% THD. Now
the concer stage OLG THD might be 10%, and with the CLG gain reduction
to about 2, the 10% is reduced to 1.2%, but basically you end up with
more 2H than I get with LTP which has natural cancelling of 2H. Now in
a typical Williamson, the balanced amp needs to make only 2 phases of
about say 32Vrms to power a couple of 6550 in triode. So the concer
makes only two phases of 2V and input tube makes only 2.2V at
clipping. Now with the Willy, the input concer stages usually have
such LOW signals at normal listening levels, say less than 0.2Vrms,
the THD is down at 0.05% and is reduced by the GNFB to utterly
negligible levels. The same goes for the use of the LTP in my amps.
So the discussion of "what is best" becomes of academic interest only.
In small amps with UL output stages with EL84, the SE input triode
with SE concer stage is all one needs and sounds/measured fine. If you
use an extra triode and make an LTP, you might find the amp can be
made more sensitive, and the THD will be almost indentical to the
concer stage but with less 2H present. Both ideas work fine where
signals are low, but in my high powered amps the concer drive stage is
not so hot. McIntosh amps have balanced driver stages.
Most makers use LTP or balanced with larger tubes.
> > Whatever the input tube is, pentode or
> >triode, it has only to produce a low signal in SE mode so THD remains
> >lower than if you have an input tube needing to make slightly more Va
> >than is applied to each output tube grid.
>
> Distortion is also increased by the extra tubes.
>
> I happen to like the LTP too but that doesn't mean everything else is
> crap and, like all design tradeoffs, "it depends" (on everything
> else).
There's more than one way to do things.
>
> A concertina is enough to drive a pair of 6BQ5s all by itself without
> interposing another set of tubes and while that may not be your cup of
> tea it makes for a perfectly fine little amp.
I agree entirely.
>
> An LTP driven single ended has only half the gain of Williamson's
> concertina double driven short tail and while I know you think
> converting that to an LTP 'reduces distortion' it's at a place in the
> amp likely to be of little consequence since the vast majority of
> distortion is in the output stage. So far that's a "why not?" but the
> LPT does cause overload interaction on positive grid drive, as does a
> concertina *if* it's the thing doing the driving (but it's buffered in
> the Williamson). It might not seem like such a big deal but I've seen
> that cause HF instability on both the concertina and LPT and is the
> reason some add a series grid resistor between the concertina and
> output tubes when driving them directly.
Usually HF instability is due to the OLG phase shift characteristic
and once you have the correct gain shelving and zobels the HF becomes
entirely stable. And of course the grid stoppers are always a good
idea.
>
> >There's plenny of headroom for the function of the shelving network.
> >The LTP does not need to be directly
> >driven from input gain tube anode, so the LTP can have much more
> >headroom than found in most samples of amps like Leak and Manley Labs
> >et all.
>
> What 'headroom'? The shelving network is zero loss in band.
>
> Or do you mean for your shelf's 'peaking', which a bootstrap roll off
> doesn't cause?
>
> All I said is there was "another means," and I don't know that I'd do
> it for 'just that', but it's an interesting aspect to consider if one
> also wants the added gain.
Observation of the error signal at V1 anode output when feeding the
amp a level input signal between 1Hz to 100Hz will show that the LF
shelving network is OK. The reference 1kHz signal should produce half
maximum l Vo. Then if you drop the input F and plot the response at
all electrodes, you'll understand. And what will help matters is that
you have passive input CR input filter with pole at 8 Hz. You just
don't need to try to have the amp vainly try to make a big effort with
a 3Hz signal.
Patrick Turner.