Amp designs
Steve Rees
GOOD quality, low- to medium-power Class-A or current-dumping amplifiers?
The more specific the reference the better, but even general leads would
help.
Thanks,
Steve Rees
University of Waterloo
Donald Y Chen
Steve,
I can give some idea who makes these Amps. For a mere $8000+ Mark-Levinson
makes the awesome 25W ML-2A. These are dual mono which is reflected in their
high price. British Fidelity makes some amps. The top of the line is the
175wpc $3500 model supposely a bargain. I suggest you get two.
In the current-dumping arena I only know the Quad 405 and its two
successors. I don't know the price but it's less than $1000.
I hope this information (limited) has been helpful.
K.VENNER
been looking at the Yamaha R-7. Does anyone have any experience
with this receiver? I was looking for 50+ watts of power, digital
tuning with presets, and remot controll. Any other receivers
recommended would be greatly appreciated
Thanks in advance!
Ken Venner
AT&T Bell Labs
WB 1J-306
870-7182
homxc!tigger
J. Abeles (Bellcore, Murray Hill, NJ)
what is a "current-dumping" amplifier?
*** REPLACE THIS LINE WITH YOUR MESSAGE ***
Thomas Krueger
> what is a "current-dumping" amplifier?
Assume you have a 100W (rms) amplifier. If it is 100V at 1A, this is not a
current dumper. If it is 10V at 10A this amp would be a current dumper,
useful for driving speakers with very low impedances. (An example is the
Acoustats with the older matching transformers).
- Tom
--
Thomas Krueger ...ihnp4!uwmcsd1!shop or
University of Wisconsin Milwaukee uwmcsd1!sh...@rsch.wisc.EDU
Computing Services Electronics Shop
3200 N. Cramer St. (414) 963-5172
Milwaukee Wi 53211
J. Abeles (Bellcore, Murray Hill, NJ)
>
> Assume you have a 100W (rms) amplifier. If it is 100V at 1A, this is not a
> current dumper. If it is 10V at 10A this amp would be a current dumper,
> useful for driving speakers with very low impedances. (An example is the
> Acoustats with the older matching transformers).
>
nothing to do with the design of the amplifier (since merely putting
a transformer on the output would change the effective output impedance
from 100 ohms to 1 ohm, to use the above example)?
Jerry Carathers
> I am looking to buy a new receiver and speakers. I have presently
> been looking at the Yamaha R-7. Does anyone have any experience
> with this receiver? I was looking for 50+ watts of power, digital
> tuning with presets, and remot controll. Any other receivers
> recommended would be greatly appreciated
Look at the JVC RX-500B. It has 100 w/ch, digital tuner with 30
presets (15 AM and 15 FM), remote control and 6-band programmable
equalizer.
It's simply the most phenominal receiver I've ever seen and mine has
served me happily for a year and half. They're currently being closed
out at the local retailer in leu of a "new for '87" model not yet
released. Latest price I saw was $460.
When it was released, Audio magazine raved about all aspects of it
including superb FM sensitivity, higher than rated watts per ch, and
exceptionally clean amplifier.
Hope you can find it!
Jerry Carathers
Tektronix, inc.
...tektronix!tekigm2!jerryc
Thomas Krueger
Partially... in a solid-state amplifier, transformers are avoided because
of the phase shifts and high frequency ringing involved are difficult to
compensate for. Therefore, current dumping is not only a function of the
output impedance but also of the power supply current capacity. If the
power supply is capable of putting 2A into the load, no matter what, the
voltage swing on the output would clip as drive to the output stage
increased (along with the output stage melting down). If the amp is
designed with high current output design with a low ouptut impedance
(Accoustat amps come to mind) with a power supply stiff enough (like a lot
of supply capacitance and a large power transformer) the amp will be able
to 'dump' current without self destructing.
As an aside, anyone ever think of desgning large complimentary solid state
amplifiers with no power transformer for 120VAC?
- Tom
--
Thomas Krueger ...ihnp4!uwmcsd1!shop or
University of Wisconsin Milwaukee sh...@csd1.milw.wisc.edu
bo...@convexs.uucp
In net.analog, sarees@watlion writes:
> Can anybody point me toward references (schematics, articles, etc) about
> GOOD quality, low- to medium-power Class-A or current-dumping amplifiers?
>
> The more specific the reference the better, but even general leads would
> help.
Audio Amateur magazine published a construction article on the Pass A-40
in 1978 (I think). That is a 40 watt/channel class A amp. Audio Amateur's
sister company, Old Colony, sells A-40 kits. I believe you can buy a complete
kit for ~$500.
You can get back issue ordering information from:
Subscription Department
The Audio Amateur
Post Office Box 576
Peterborough, New Hampshire 03458
K<bob>
deep_thought() Bob Miller
{ Convex Computer Corporation
sleep(years_to_seconds(7500000)); Richardson, Texas
return 42;
} {ihnp4,cbosgd,allegra,sun}!convex!bobm
Disclaimer: The above is not just the opinion of the author; it is the opinion
of all sentient beings in this universe and all other known universes. The
author's employer may, however, not be sentient.
Andrew Koenig
> output impedance but also of the power supply current capacity.
My understanding is that "current dumping" is a trademark of the
Acoustical Manufacturing Company in Huntington, England, and refers
to a particular circuit configuration they use in their power
amplifiers (QUAD 403).
The idea is that the output of the amplifier is the difference of the outputs
of two separate parts. One, called the current dumper, has a lot of
power capacity but only mediocre linearity. The difference between the
output of the current dumper (divided by a constant) and the input is
then amplified by a smaller amplifier on which considerable care is taken
to minimize distortion. This is easier to do here because this
amplifier is smaller.
Its proponents claim that this is a way to get a lot of power with
very low distortion and only a little more design effort than would
be necessary for a much smaller amplifier.
I am not qualified to evaluate this claim.
> As an aside, anyone ever think of desgning large complimentary solid state
> amplifiers with no power transformer for 120VAC?
I think it would be an incredible safety hazard.
Phil Gustafson
> As an aside, anyone ever think of desgning large complimentary solid state
> amplifiers with no power transformer for 120VAC?
>
Yes. They usually stop thinking about it when they consider that the
speaker wires for such an amplifier would pose a lethal shock hazard.
phil
--
---------------------------------------------------------
All opinions except attributed quotations are mine alone.
Satirical comments may not be specifically identified as such.
--
Phil Gustafson Voice: (408)435-8600
Saber Technology Corp.
2381 Bering Drive Mail: decwrl!sun!saber!phil
San Jose, CA 95131 idi!saber!phil
Erez Levav
> In article <16...@homxc.UUCP>, tig...@homxc.UUCP (K.VENNER) writes:
> > I am looking to buy a new receiver and speakers.
>
> presets (15 AM and 15 FM), remote control and 6-band programmable
> equalizer.
>
>
Maybe I count wrong, but I have a RX-500B, with 16/16 presets and 7 band eq
I paid $350 in a local store, and I the lowest price I saw was ~$300 in mail
order (maybe it was 2 months ago in SR?). I saw a picture of the new remote
that JVC supplies with the new line replacing the RX-?00's, I like the one I
have much better... (it controls only the receiver, but it has almost all the
functions on the remote). About half a year ago I posted my impressions from
the RX-500B, if you want I can send you an e-mail copy.
Another receiver I think is worth looking into is the Marantz SR-940. Very
similar specs and the lowest price i saw was $299.
#include <any/disclaimer> ~~~
{> <}
Erez Levav __ ^ __
University of Wisconsin / \
| />
ARPA: er...@rsch.wisc.edu \/
BITNET: erez%rsch....@wiscvm.wisc.edu
UUCP: {allegra,harvard,ihnp4,seismo,topaz}!uwvax!erez
new...@topaz.berkeley.edu
quickly discovered that even I could design/build equipment far more
suited to the emerging requirements of post-Beatles sound systems
than could be bought from the likes of Altec. (Actually, there really
*wasn't* anyone but Altec making this stuff- maybe Langevin and the
remnants of the Ampex-JBL-Cinemascope rpoject, and other film-industry
derived stuff, but otherwise nothing advanced beyond what Bell Labs had
designed for the talkies and record-cutting). It wasn't just a matter
of attaining higher power levels- it was the fact that you could build
a twenty-input mixer that was quieter than a five-ton (mostly iron)
recording console for less than the cost of a store-bought stepped
ladder fader for a single input. Anyway, building high-power (and
indestructible) solid-state amps was of course a priority- one needed
dozens, basically one-per-driver in the brave new world of hi-fi
sound reinforcement. For tyro designers like me, the then-common
output and driver devices were a problem- they were restricted to
low-voltage regimes and therefore low (100 watts into 8 ohms with
2N3055's) output powers (given output transformerless design for
all the obvious cost, quality and elegance reasons). Worse, you
couldn't get power transformers wound for you locally in the heavy currents
necessary for power several such low-voltage output modules from a
single bipolar DC supply. So the question of how to build r eally
hefty power amps was always an issue (we didn't wanna just buy the
Crown DC300s that were just becoming proven- no $$).
J ust then I became aware of a San Diego company that made kilowatt
solid state amps for driving sonar transducers. I was most impressed with
their careful, systematic analysis (what I'd now recognize as quintessentially
engineer-like) of what were truly the worst-case fault modes their
amps might encounter, and the circuit topologies they'd adopted to
cope. They dealt with not just roadies who shorted speaker cables
(or the undersea nuke-war equivalent), not just totally reactive loads,
but the case where another transducer actively drives power back into
the victim amp in the worst possible phase-way.
Oh, yeah, the point: They directly rectified the three-phase (400 Hz, i guess)
line power to generate the DC supplies, using a rectifier bridge two
components (actually, however many are appropriate to the star or whatever
configuration the were using) of which were SCR's, so the DC supply
was efficiently regulated. BUT: they used a humongous OUTPUT transformer
(many kilowatts at up to 100KHz) to couple to the load! Not a very cost-effective
tradeoff for an audiophile these days. It was a lesson to me, though. Plodding
through the careful engineering design of the whole amp, I learned that
only dilettantes and amateurs would decree a priori that "you can't
get x type of performance using y (e.g., transformers) type design-- an
engineer, even in a field as close to the ultimate limits of human knowledge as
--gasp-- *audio*, simply asks "What are the performance requirements", and then
tries to build something that doesn't violate any of the constraints.
By the way, I suppose you could safely build a line-powered "powered speaker".
that is one where the amp was safely integrated into the speaker cabinet,
but then you'd still be wise to isolate the input. Now *I* would be happy
with a well-laid out instrumentation amp on a clean circuit board, but
nervous nellies and product liability insurers would probably be happier
with a transformer. And the kind of transformer that nut.audiophiles
would accept would take PhilR twenty megabyte-long postings just to extol,
and who knows how long to wind.....
G...@psuvma.bitnet
>
>As an aside, anyone ever think of desgning large complimentary solid state
>amplifiers with no power transformer for 120VAC?
great idea though, because you get no isolation from the power lines.
All the noise, spikes, and dropouts will come right through, unless you
use a hefty isolation transformer.
-------
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Sunil Gupta | Bitnet: got@psuecl
P enn | got@psuelc2
S tate |
U niversity | UUCP : ihnp4!psuvax1![psuecl, psuecl2]!got
E ngineering | : ihnp4!psuvax1!psuvma.bitnet!got
C omputer |
L ab | ARPA : got%psuecl...@wiscvm.arpa
"I dares ya to step across this line! _______" - Bugs
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Berry Kercheval
>In article <16...@homxc.UUCP>, tig...@homxc.UUCP (K.VENNER) writes:
>
>Look at the JVC RX-500B. It has 100 w/ch, digital tuner with 30
>presets (15 AM and 15 FM), remote control and 6-band programmable
>equalizer.
>
>It's simply the most phenominal receiver I've ever seen
I have a very similar JVC receiver -- the RX-740 I think it's called.
I am VERY happy with it. I got for about $180 it at the Pacific
Stereo liquidation sale after they filed Chapter 11. The only
problem is that it was their demo unit and came "as-is" -- no manual,
no box, no warranty card, just the receiver. I helped the salesman
yank it out of the demo room. Now if only I could figure out how that
*%@# programmable equalizer works....
--berry
Kok Chen
>
> My understanding is that "current dumping" is a trademark of the
>
Right on! This is the first proper description of "current dumping (TM)" that
has appeared so far. In fact, what <ark> has described is the way Quad does
FEED-FORWARD. The error signal from the cheapo high-current section is fed
to a lower power section to cancel at the speakers (instead of feeding the
error signal BACK to the input of the high-current amplifier). I, for one,
would be interested in hearing more on feed-forward power amp topologies.
One idea that I kicked around has been to difference the main and feed-
forward paths by feeding the output of the main driver to one speaker wire,
and the output of the error-signal amplifier to the second speaker wire (like
a diff amp). Two possible flaws in this: 1) won't work with electrostatics
that require single ended inputs, and 2) the finiteness of the first driver's
output impedance will introduce some positive feedback to the second driver.
Another idea: the feed-forward signal (the input to the second amp) can come
from an auxiliary cable that goes straight to the speaker terminals (like in
remote-sensing power supplies), instead of to the output of the first driver.
I haven't tried out any of these ideas yet, except on paper (I do have a
spare DH-120 that I don't mind modifying and blowing up, though, if I ever
find time :-). Anyone see major flaws in the logic?
Kok Chen
Imagen Corporation (we don't manufacture amplifiers; in fact, we don't
even do anything remotely analog!)
David N. Horn
interesting design of amplifier introduced by Quad (name of a Hi Fi manufacturer
in England headed by Peter Walker) about ten years ago.
It had two amplifiers for each channel, one high power but slow amplifier to
do most of the work, and one low power but fast. A comparator sensed the
difference between the input signal and the output divided by the gain (i.e.
the distortion), and the fast amplifier made up the difference.
The technique was also known as "feed-forward".
A number of articles on current dumping were published in "Wireless World".
By the way, Peter Walker has a very good reputation for producing top quality
products way ahead of their time.
Dave Horn, AT&T Bell Labs, Holmdel, vax135!dh
new...@topaz.berkeley.edu
uring Co. (Quad) has an excellent reputation (is his name Walker? All I remember
is the company). During a period of more-than-usually obsequious sycophancy,
I asked Ray Dolby if there was anyone in the audio world whose commercial
style he admired in the same way that I found *his* approach admirable.
Without hesitation he mentioned Peter Walker[?] (we had already agreed on
Henry Kloss), but couldn't seem to think of anyone else (like Ike when asked
to list a single achievement of Richard Nixon as Veep, I'm sure if I'd given him
a week he'd have thought of one.)
As to why I found (still find) Ray Dolby a bird so rare in the commercial
audio aviary, if you have to ask...
...I guess I'll have to expatiate, later.
Doug Maisel
(415) 848-5247
56 Panoramic Way
Berkeley, CA 94704
Les Szepesi
> One idea that I kicked around has been to difference the main and feed-
> forward paths by feeding the output of the main driver to one speaker wire,
> and the output of the error-signal amplifier to the second speaker wire (like
>
> Kok Chen
> Imagen Corporation (we don't manufacture amplifiers; in fact, we don't
> even do anything remotely analog!)
The biggest problem I see with this is that all the current coming from the
output of the "main driver" must also pass throught the output of the "error-
signal amplifier". (KCL) This now implies an error amplifier with very good
linearity, THD, etc that also can source and sink large currents. The whole
point of the original topology was "To build a low power, quality amp is
cheaper and easier than to build a high power, quality amp."
Les Szepesi
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Everett, WA 98206 USA ucbvax >!lbl-csam /
(206) 356-6362 hplabs/
Dave Brower
>
>As to why I found (still find) Ray Dolby a bird so rare in the commercial
>audio aviary, if you have to ask...
>
> ...I guess I'll have to expatiate, later.
Please do. And consider this proposition: Ray Dolby may be the
last great analog design engineer.
-dB
--
{amdahl, sun, mtxinu, cbosgd}!rtech!daveb