Introducing a New Horse to the Stable

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Peter Wieck

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Sep 3, 2019, 4:11:19 PM9/3/19
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Howard Stone's experience with the Radford amp brought this on, so please forgive the rant-like process here.

Guys and Gals:

When introducing a new piece of equipment to the system, please take NOTHING for granted, not even it it is brand-new, fresh from the box. And if it is used, or, much worse, vintage-used please be exceedingly cautious. Equipment failure can be anything from minimally annoying to spectacularly annoying to genuinely dangerous to life and property.

I have no problems running my 56 year old tube system in my office, and leaving it unattended for hours at a time. It has been through my bench, sat for hours on a metered variac, and I created a temperature-table using a heat-gun such that if I see changes over time, I have a pretty good idea where to look for trouble. But when it came to me, I had no such faith.

In all seriousness, if one is going to pursue this hobby at more than an occasional level, one should obtain the basic tools necessary to do so safely both for the equipment and the real-estate. This is not to suggest that such would have prevented Howard's experience - but he very probably would have seen it coming in time to prevent the special effects.

If there is a consensus, I would be glad to take a picture of my (very basic) bench, and (very basic) tooling, with an explanation for each item and the purpose(s) it services.

Thoughts?

Peter Wieck
Melrose Park, PA

Trevor Wilson

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Sep 6, 2019, 5:54:00 AM9/6/19
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**Yeah, one. I don't get the attraction to valve (tube) equipment.
Anything that is done with valves, can be done, better, cheaper and with
more consistency with solid state.

I mean to say: I get why hipsters embrace the stuff. Hell, my business
has undergone a renaissance thanks to hipster. Old Marantz, Yamaha,
Sansui, Accuphase and the others are suddenly desirable and, therefore
valuable and worth repairing. And for an old fart like me, well, I cut
my teeth fixing that stuff. No surface mount, or microprocessors in
sight. Well, not if you exclude cassette decks.

But, Hell, valves start wearing out the minute they're first switched on!

And, before you get started, I've done a few blind tests with valves,
vs. solid state. The very best valve gear is VERY hard to pick from
decent SS gear. It just costs a whole lot more (check out the cost of a
decent, multi-interleaved output tranny sometime - YIKES!). And then, of
course, there's those pesky valve replacements at regular intervals.
I've replaced a full set of valves in a big power amp more than once and
seen the cost run to a couple of grand.

And yes, I've owned and built valve stuff too. Not anymore though. I
have better things to do with my life. Noisy and microphonic valves. No
thanks. You can stick 'em where the Sun don't shine.


--
Trevor Wilson
www.rageaudio.com.au

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Peter Wieck

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Sep 6, 2019, 1:50:23 PM9/6/19
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Almost the entire reason for a hobby is to be able to indulge in pointless behavior without consequence.

Trevor Wilson

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Sep 6, 2019, 7:39:49 PM9/6/19
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On 7/09/2019 3:50 am, Peter Wieck wrote:
> Almost the entire reason for a hobby is to be able to indulge in pointless behavior without consequence.
>

**I have a different aim: That is to attempt to provide for myself and
my clients, a musical experience that is as close to the original event
that is possible to obtain, given the usual room and budgetary
constraints. OH, and the partner, if one is in the picture.

[ASIDE] Many years ago, I was asked to supply and set-up a very nice
system for a local, well-heeled politician. The man was cultured and had
put in place an endowment for budding pianists. When I saw the room the
system was to be installed, my first words were: "Well, the Steinway has
to go." Went down like a lead balloon. We compromised. The Steinway
remained. The sound system woulda sounded better without it.

Howard Stone

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Sep 10, 2019, 6:08:55 AM9/10/19
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I don’t think Trevor, that the problem was caused by the fact that it’s a tube amp. I mean, in principle valve amps are as robust as SS aren’t they - apart from the fact that the tubes wear out,

My aim was to find an amp which I liked, regardless of whether or was valve or SS - the fact that the Radford has valves seemed an implementation detail which I would learn with,

Anyway, I’d like to hear a list of your favourite amps Trevor. Since I still don’t have my Radford!

Trevor Wilson

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Sep 10, 2019, 6:37:54 AM9/10/19
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On 10/09/2019 8:08 pm, Howard Stone wrote:
> I don’t think Trevor, that the problem was caused by the fact that it’s a tube amp. I mean, in principle valve amps are as robust as SS aren’t they - apart from the fact that the tubes wear out,

**Not even remotely close. Assuming good design and build quality, a
valve amp will always be a less reliable product. If only due to the
necessarily higher Voltages involved and consequent extra stress on
insulation and other things. Then there's the output transformers. Two,
large, heavy and expensive components, which, whilst reasonably
reliable, are less reliable than a typical power transformer, thus
adding an extra layer of unreliability. And of course, whilst valve
failures are part of the game with valve products, the failure of a
valve can also result in the failure of some surrounding components. And
over the years I've seen some (ahem) 'interesting' valve amp failures.

>
> My aim was to find an amp which I liked, regardless of whether or was valve or SS - the fact that the Radford has valves seemed an implementation detail which I would learn with,
>
> Anyway, I’d like to hear a list of your favourite amps Trevor. Since I still don’t have my Radford!

**Oh, there's only one. Locally built (Australia) and has been out of
production for a number of years, but sonically stunning. Solid state
(BJT), of course. Many listeners say that it has some of the
characteristics of the finest valve amps (triode, of course), but with
none of the drawbacks. The key? Zero global NFB, critically matched
components (better than 1% for transistors) and over-sized power
supplies, employing multiple, small value capacitors. I have not had
another amp (permanently) in my system since 1980. I have, however, had
a great many in the system for short periods. Some costing a great deal
of money.

Peter Wieck

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Sep 10, 2019, 7:54:22 AM9/10/19
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OK, OK, I will bite! Minor rant to follow:

Tube vs. Solid State on reliability:

There are not so very many 60-year old components in operation these days unmodified since-new. My oldest tube item turned 100 this year and likely works better than when it was new based on a better understanding of antenna systems, optimum tube voltages and so forth. Other than moving parts (CD player), the newest component in my office system was made in 1963. The system runs 9 hours per day, 5 days per week. Oh, and the tubes are original as well.

On the other hand, and given my hobby, I see a large number of SS components that have blown transistors, exploded capacitors and much worse, irrespective of age and source. The well made, well designed stuff is serviceable, distinguishing it from the rest of the garbage out there.

I would make a fairly apt comparison: A tube amplifier is much like a mid-last-century Mercedes or VW - few things were self-adjusting, and they required regular and attentive care-and-feeding. With such, they were good for several hundred thousand miles of reliable service. A contemporary Ford, Cadillac, Plymouth would be considered remarkable were it to survive 100,000 miles without heroic measures. Might run very nicely when running, but that would be your basic solid-state device in comparison.

Put simply, they are different beasts designed with different things in mind, but for the same basic purpose. That one is or is not "BETTER" than the other is not relevant to the purpose in either case.

Now, when I here things like "Zero global NFB" and "Critically matched components", I can smell the snake-oil from a great distance, even the 10,000 miles from here to Australia. I am sure that process also contains descriptives of "interconnects" rolled on the thighs of virgins on Walpurgis Night...

Note that even "critically matched" solid-state components drift after a very short period of time in-service. All of them, such that that "less than 1%" is meaningful for perhaps 12 hours or so.

Being as this is a hobby for me, I get to try things that are otherwise unproductive, unprofitable or impractical. Such as shotgunning a device with single-value capacitors and then comparing it to the same device with carefully screened and matched caps. Or matching driver and output transistors and comparing to a similar device with disparate values. Guys and gals - you would be seriously shocked to discover how little difference some things make that the ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no difference at all.

Trevor Wilson

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Sep 11, 2019, 7:29:51 AM9/11/19
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On 10/09/2019 9:54 pm, Peter Wieck wrote:
> OK, OK, I will bite! Minor rant to follow:
>
> Tube vs. Solid State on reliability:
>
> There are not so very many 60-year old components in operation these days unmodified since-new. My oldest tube item turned 100 this year and likely works better than when it was new based on a better understanding of antenna systems, optimum tube voltages and so forth. Other than moving parts (CD player), the newest component in my office system was made in 1963. The system runs 9 hours per day, 5 days per week. Oh, and the tubes are original as well.
>
> On the other hand, and given my hobby, I see a large number of SS components that have blown transistors, exploded capacitors and much worse, irrespective of age and source. The well made, well designed stuff is serviceable, distinguishing it from the rest of the garbage out there.

**Sure. That has been my observation. I see a lot of junk across my
bench, though I tend to reject many products nowadays. Not worth my
time. FWIW: I have 70 year old radio coming in next week. Obviously,
it's not really worthwhile, but it is a family heirloom, so the client
will spend whatever is necessary. I expect it will be an electrolytic
cap swap. Maybe a valve. We'll see.

>
> I would make a fairly apt comparison: A tube amplifier is much like a mid-last-century Mercedes or VW - few things were self-adjusting, and they required regular and attentive care-and-feeding. With such, they were good for several hundred thousand miles of reliable service. A contemporary Ford, Cadillac, Plymouth would be considered remarkable were it to survive 100,000 miles without heroic measures. Might run very nicely when running, but that would be your basic solid-state device in comparison.

**That would be a poor comparison. Having owned a number of vehicles
over the past 50 years, I can assure you that, in general, modern cars
are VASTLY superior in most areas. Ignition systems and fuel handling
systems are the big ones. Before modern EFI, I grappled with
carburettors, that required constant adjustment and let's not even get
into the standard Kettering ignition systems. Such systems require
constant and careful adjustments. As for the body of the vehicle, shall
we discuss the impact of rust? Most (all?) modern cars utilise some kind
of rust prevention at manufacture. Older cars rely on paint. My two
present cars are thoroughly modern vehicles (one is 2 years old and one
is almost 20). Both are perfectly reliable and offer excellent levels of
safety, fuel economy and performance. Both demonstrate the kind of
performance which would be associated with race cars from the 1960s
(though both could manage a tight, twisty race track more quickly, due
to their all wheel drive ability). Without the unreliability.

Anyway, that's all a bit off-topic.

I service equipment from all generations. Older equipment almost always
require replacement of electrolytic caps and carbon composition
resistors (where used). That may be valve or SS.

>
> Put simply, they are different beasts designed with different things in mind, but for the same basic purpose. That one is or is not "BETTER" than the other is not relevant to the purpose in either case.
>
> Now, when I here things like "Zero global NFB" and "Critically matched components", I can smell the snake-oil from a great distance, even the 10,000 miles from here to Australia. I am sure that process also contains descriptives of "interconnects" rolled on the thighs of virgins on Walpurgis Night...

**I did not mention "interconnects". However, you may care to
investigate the possible reasons why many listeners prefer the sound of
valve amps. Consider the amount of global NFB used in typical valve
products. Compared to SS gear, that NFB is either very low, or
non-existent. Consider the action of valve amps under the conditions of
Voltage limiting (clipping) and current limiting. Most SS amps do not
cope with such conditions well, whereas most valve amps clip and current
limit in a benign fashion. SS camps can be designed to act similarly,
but few are. As for the matched components, it's a necessity in the
design I cited. Without matched components, it is impossible to achieve
high reliability, low distortion and zero global NFB. Cumbersome? Sure.

Further and for the record: I have subjected myself to many double blind
tests over the years to verify my preference is uncoloured by vision.


>
> Note that even "critically matched" solid-state components drift after a very short period of time in-service. All of them, such that that "less than 1%" is meaningful for perhaps 12 hours or so.

**The solid state devices are matched at the operational temperature of
the amplifier (60 degrees C) of course. The amplifiers use a demand
responsive fan (infinitely variable speed), which maintains the heat
sink temperatures at 60 degrees C (+/- 3 degrees) at all times. A short
(20 mins) warm-up time is necessary for optimal performance.


>
> Being as this is a hobby for me, I get to try things that are otherwise unproductive, unprofitable or impractical. Such as shotgunning a device with single-value capacitors and then comparing it to the same device with carefully screened and matched caps. Or matching driver and output transistors and comparing to a similar device with disparate values. Guys and gals - you would be seriously shocked to discover how little difference some things make that the ALL-SEEING, ALL-KNOWING gurus will tell you are critical. Often no difference at all.

**Well, that has not been my experience. It depends on the design. A
standard, high global NFB amp, which has been designed to cope with off
the shelf devices will likely show zero difference when fitted with
critically matched devices. HOWEVER, in a past life, I was service
manager for Marantz Australia for several years. During the late 1960s
and 1970s I noted that most Marantz models (even some of the Japanese
built stuff) was specified to use closely matched output and driver
devices (roughly 30% HFE match). I vividly recall the time when, for a
short time, stocks of output devices for the Model 240 (250, 250M, 1200,
1200B) were depleted. I decided to try using unmatched devices in one
repair. Distortion approached 0.5% (rated distortion was 0.1%, but I
would typically measure 0.01%). Clearly a most unacceptable result. I
explained to the customer that they would need to wait for Marantz to
supply the correct devices.

So, matched devices are not as uncommon as you might imagine.

~misfit~

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Sep 11, 2019, 10:17:05 AM9/11/19
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Thanks for your input Peter. If I may ask, do you have an opinion on 'storage capacitors' on an
amplifier power supply? What in your opinion is 'better', a single (or few) very large caps or
multiple smaller caps to the same / similar capacitance?

I have a long term project building my own amp based on PCBs taken from 100w MOSFET (two pairs of
J50 / K135 devices per amp) PA amps made by a New Zealand company in the 1980s. (Craft, Gary
Morrison's company before he went on to become head designer at Plinius until 2005 when he left to
set up Pure Audio). I got my hands on a rack of four of these mono amps and preliminary testing
using a clean source and good speakers suggest they will make a great stereo amp.

I need to put together a power supply to feed two of these and have some new 10,000uF caps but was
wondering if multiple smaller caps would be better. (In the PA amps they only had 2,200uF but
obviously weren't called on to reproduce much bass.)

As it is I'll be using fly leads from the rectifier PCB to the caps, then to the amps and I'm
building my own case. I was thinking of maybe using my 10,000uF caps as well as maybe some smaller
ones, perhaps 1,000 in a bank, the best of both worlds. (There are also 100uF electros across the
rails on the amp PCBs that I'll be replacing.) That said I could also just go to multiple

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Peter Wieck

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Sep 11, 2019, 10:45:16 AM9/11/19
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OK... getting down to basics:

Electrolytic capacitors are essentially chemical engines. The materials developed over the last 20 years have greatly improved along with longevity and reliability, but they remain chemical engines. A single very large capacitor will, therefore, necessarily be slower than multiple smaller capacitors in parallel, all other things being equal. The limiting factor being real-estate in most cases.

Generally, I try to run multiple caps in parallel where real-estate permits, with a small-value, high-voltage film cap across each as a snubber. This is a preference, not a requirement.

For something as brute-force as a power-supply for audio purposes, the difference(s) will be manifest only at or near clipping, or when the amps are fed signal with extreme Peak-to-Average content. A cap bank will be able to deliver a *marginally* faster transient than a single very large cap. NOTE: If you are going to have the capacity (pun intended) to overdrive your output devices for these transients, you might need to install some sort of speaker protection. Solid-state devices often do not clip nicely.

~misfit~

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Sep 12, 2019, 6:18:50 AM9/12/19
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Thank you Peter.

I already have speaker protection ready to install. I bought two of these
<https://www.aliexpress.com/item/1350315871.html> Probably not the best option but I don't know how
to make such things myself.

Trevor Wilson

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Sep 12, 2019, 6:19:00 AM9/12/19
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**Those old MOSFETs were pretty ordinary devices (not very linear).
Evidenced by the fact that Plinius amps have always used BJTs. As Peter
has stated, multiple small value caps will usually provide a superior,
higher speed power supply. However, I would posit that those old MOSFETs
are so horrible (modern MOSFETs are far superior), that it may not be
worth the effort. Craft amps used huge amounts of global NFB, required
due to very low bias currents and the necessity to reduce the huge
levels of distortion caused by the 'knee' at low currents (A Class A, or
high bias MOSFET amp would have been much better). Anyway, the huge
levels of global NFB means that PSRR (Power Supply Rejection Ratio) will
be quite high, thus the influence of power supply changes will be
relatively small.

One more thing: Decent amounts of capacitance placed close to the output
devices is far more influential than caps placed some distance away. In
fact, long(ish) cables AFTER the main filter caps can be a serious
limiting factor on the effectiveness of a power supply in a Class A/B
amplifier. This is because the inductance of the wires can be a factor.

Peter Wieck

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Sep 12, 2019, 9:32:16 AM9/12/19
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https://www.eeweb.com/tools/parallel-wire-inductance

This website will allow you to calculate inductance by giving it the gauge, type and nature of the wire you are using. You will find, pretty quickly, that the distances involved in the typical component of say 40 cm (16") square are such that the actual inductance realized will be infinitesimal in "real life".

Again, getting to practical matters: there are common-sense applications and techniques for wiring electronics, much dependent on the nature and use intended. Part of my hobby is the restoration of vintage Zenith TransOceanic tube radios - and wire location/component location can be critical for high-band Short Wave sensitivity. It is common sense to shield power-supplies in pre-amplifiers, especially those that contain phono or NAB pre-amp sections. And, wire-dressing is always good practice. But worrying about straight-wire inductance at audio frequencies is much akin to worrying about skin-effect...

Now, Trevor clearly has a 'thing' about negative feedback, which is entirely his choice, and doubtless for sufficient and good reasons. But, again, in the real world, negative feedback, done properly, has many more advantages than disadvantages. Done badly - Ouch! Keep in mind that in its most practical application, it dates back to 1927, and was patented by Bell Labs in 1937. So, it is a pretty well established technique, such that any thoughtful designer not totally strangled by the bean-counters will get it right very nearly all of the time.

https://en.wikipedia.org/wiki/Negative-feedback_amplifier#Two-port_analysis_of_feedback

Go down to the "distortion" section. As brief as it is, it conveys some very good information.

In point-of-fact, part of the TIP-Mod for your 120 involved increasing capacitance within the feedback loop to reduce bass roll-off.

Trevor Wilson

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Sep 12, 2019, 4:28:35 PM9/12/19
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On 12/09/2019 11:32 pm, Peter Wieck wrote:
> https://www.eeweb.com/tools/parallel-wire-inductance
>
> This website will allow you to calculate inductance by giving it the gauge, type and nature of the wire you are using. You will find, pretty quickly, that the distances involved in the typical component of say 40 cm (16") square are such that the actual inductance realized will be infinitesimal in "real life".

**Some of the products I work on are significantly larger than 40cm. One
(badly designed) amplifier, from a well-known 'high end' manufacturer,
used power supply wires which were approximately 60cm long! Whilst it
made servicing easier (allowing the output device heat sinks to be
removed from the amplifier and still be fully operational, the
additional inductance damaged the ability of the output stage to deliver
fast transients. A bunch of capacitance close to the output devices made
things much better.

>
> Again, getting to practical matters: there are common-sense applications and techniques for wiring electronics, much dependent on the nature and use intended. Part of my hobby is the restoration of vintage Zenith TransOceanic tube radios - and wire location/component location can be critical for high-band Short Wave sensitivity. It is common sense to shield power-supplies in pre-amplifiers, especially those that contain phono or NAB pre-amp sections. And, wire-dressing is always good practice. But worrying about straight-wire inductance at audio frequencies is much akin to worrying about skin-effect...'

**No. Skin effect is only a worry at RF and for power line companies,
where VERY long cables can lead to significant losses (hence the rising
popularity of DC transmission systems).


>
> Now, Trevor clearly has a 'thing' about negative feedback, which is entirely his choice, and doubtless for sufficient and good reasons.

**Let me be very clear about several things:

* NFB is fine. In fact, NO audio amplifier can work without it.
* GLOBAL NFB is also fine. When properly applied.
* I have a personal preference for the amplifiers I use, which employ
lots of local NFB and no global NFB. Others may have a different opinion.
* As part of my education into the world of zero global NFB amplifiers,
I subjected myself to a couple of single (unfortunately) blind tests,
between two, otherwise identical, amplifiers. One employed zero GNFB and
one employed a modest amount of GNFB. I preferred the zero GNFB one.
Since that time, I subjected several (10) of my clients to the same test
(DBT). The zero GNFB models was preferred every time. Except one.
* Once more: I would posit that part of the reason why some listeners
prefer valve amplifiers, is due to the fact that global NFB levels are
very low, or non-existent.


But, again, in the real world, negative feedback, done properly, has
many more advantages than disadvantages.

**Again: No issue with NFB. In fact, no issue with GNFB, when done well.


Done badly - Ouch! Keep in mind that in its most practical
application, it dates back to 1927, and was patented by Bell Labs in
1937. So, it is a pretty well established technique, such that any
thoughtful designer not totally strangled by the bean-counters will get
it right very nearly all of the time.

**Sure. I learned about GNFB back when I was a teenager, having just
built my second amplifier. A mighty 10 Watt/ch, push pull amp using 6V6
output valves. It had no GNFB. It also had a gain control before the
phase splitter. After reading an old article in a local electronics
magazine about NFB, I decided to try it. With the in-loop gain control,
I found I could vary the amount of NFB right up to the point of
oscillation. Backed off a fraction, I found that GNFB improved the sound
quality significantly.

>
> https://en.wikipedia.org/wiki/Negative-feedback_amplifier#Two-port_analysis_of_feedback
>
> Go down to the "distortion" section. As brief as it is, it conveys some very good information.

**Indeed. All good and well, but that does prompt the question as to why
your preference is for an old valve amp, which employs far less GNFB
than a typical SS amp?

>
> In point-of-fact, part of the TIP-Mod for your 120 involved increasing capacitance within the feedback loop to reduce bass roll-off.
>

**Huh?

~misfit~

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Sep 13, 2019, 6:02:42 AM9/13/19
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I hooked a pair of them up to a preamp while still using their original power supplies and was very
pleased with the sound so decided to go ahead with the build.

> Craft amps used
> huge amounts of global NFB, required due to very low bias currents and the necessity to reduce the
> huge levels of distortion caused by the 'knee' at low currents (A Class A, or high bias MOSFET amp
> would have been much better). Anyway, the huge levels of global NFB means that PSRR (Power Supply
> Rejection Ratio) will be quite high, thus the influence of power supply changes will be relatively
> small.

Unfortunately I don't own a 'scope so am unable to check a lot of stuff. When I listened to them
with the original power supplies (designed for PA use) they sounded sweet and clean at low and
moderate volume levels but seemed to run out of power at higher volumes, especially when there was
a lot of bass.

> One more thing: Decent amounts of capacitance placed close to the output devices is far more
> influential than caps placed some distance away. In fact, long(ish) cables AFTER the main filter
> caps can be a serious limiting factor on the effectiveness of a power supply in a Class A/B
> amplifier. This is because the inductance of the wires can be a factor.

Thanks. The fly-leads will only be 6" tops and I'll be using at least 1.5 square mm multistrand
copper conductors. If space allows I'll put a ~1,000uF cap right at the amplifier PCB as well (or
as large as I can get away with). I may end up building a wooden case as I don't have a suitable
metal one and wood's something I have experience and the tools for.

I still haven't finalised my design yet. I might end up feeding them a few more volts than they
were getting from their original power supplies (my only suitable toroidial transformer is 10v AC
higher than original) so may parallel up a third pair of output devices onto the heatsinks using
one of the other amps as a donor. I haven't decided yet, as I said it's a long-term project and I'm
learning as I go.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Trevor Wilson

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Sep 14, 2019, 9:58:44 AM9/14/19
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**I haven't listened to Craft (hi fi) amps in many years. What I heard
back then was pleasing. Very wide bandwidth (ca. 1MHz), as I recall.

>
>> Craft amps used huge amounts of global NFB, required due to very low
>> bias currents and the necessity to reduce the huge levels of
>> distortion caused by the 'knee' at low currents (A Class A, or high
>> bias MOSFET amp would have been much better). Anyway, the huge levels
>> of global NFB means that PSRR (Power Supply Rejection Ratio) will be
>> quite high, thus the influence of power supply changes will be
>> relatively small.
>
> Unfortunately I don't own a 'scope so am unable to check a lot of stuff.
> When I listened to them with the original power supplies (designed for
> PA use) they sounded sweet and clean at low and moderate volume levels
> but seemed to run out of power at higher volumes, especially when there
> was a lot of bass.

**That could be due to a number of factors. Including:

* Insufficient Voltage output.
* Insufficient current output.
* Insufficient power supply.
* An unreasonable speaker impedance.

Don't forget: Those meaty looking 2SJ50/2SK135 output devices are only
rated for a meagre 7 Amps each and 100 Watts PDiss. By comparison, a
typical output BJT of the same time period was rated at a far more
respectable 20 Amps and 200 Watts PDiss (MJ15003/MJ15004). Present
production variants are rated at 25 Amps and 250 Watts.

So, a little Ohm's Law should tell you if you are demanding more current
than the output devices are capable of delivering. 14 Amps is, by high
end audio standards, a relatively modest current ability for a (say) 100
Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
benign, you should be OK. Fortunately, it is real hard to damage
MOSFETs, by 'asking' them to deliver more current than they are rated for.


>
>> One more thing: Decent amounts of capacitance placed close to the
>> output devices is far more influential than caps placed some distance
>> away. In fact, long(ish) cables AFTER the main filter caps can be a
>> serious limiting factor on the effectiveness of a power supply in a
>> Class A/B amplifier. This is because the inductance of the wires can
>> be a factor.
>
> Thanks. The fly-leads will only be 6" tops and I'll be using at least
> 1.5 square mm multistrand copper conductors. If space allows I'll put a
> ~1,000uF cap right at the amplifier PCB as well (or as large as I can
> get away with). I may end up building a wooden case as I don't have a
> suitable metal one and wood's something I have experience and the tools
> for.

**Wiring sounds good. And yeah, caps placed close to output devices is a
very good thing. A wooden case, not so much. Wood is an excellent
thermal insulator, which means heat may not escape too easily.

>
> I still haven't finalised my design yet. I might end up feeding them a
> few more volts than they were getting from their original power supplies
> (my only suitable toroidial transformer is 10v AC higher than original)
> so may parallel up a third pair of output devices onto the heatsinks
> using one of the other amps as a donor. I haven't decided yet, as I said
> it's a long-term project and I'm learning as I go.

**Well, the MOSFETs are rated for a decent 160 Volts, so a few more rail
Volts should be OK. And yes, more output devices won't hurt (refer to
Ohm's Law as before). Pay attention to the drive capabilities of the
preceding stages though.

dpierce.ca...@gmail.com

unread,
Sep 15, 2019, 5:49:48 PM9/15/19
to
On Saturday, September 14, 2019 at 9:58:44 AM UTC-4, Trevor Wilson wrote:
> So, a little Ohm's Law should tell you if you are demanding more current
> than the output devices are capable of delivering. 14 Amps is, by high
> end audio standards, a relatively modest current ability for a (say) 100
> Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
> benign, you should be OK.

Hmm, that's not what a little Ohm's law tells me.

100 Watts into 8 Ohms is a tad over 3.5 amps. Let's say it's a VERY
robust 100 watt amplifier, delivering 200 Watts into 4 Ohms requires
about 7 amps, and, let's pretend it has essentially ZERO output
impedance and an effectively limitless power supply, you're not reaching
14 amps until you're driving 400 watts into 2 ohms.

So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
presents a broadband 2 ohm impedance or, conversely, what kind of
musical content would generate that kind of power requirement over
the pretty narrow band of frequencies where a loudspeaker has the
kind of pathological impedance curve that would dip to as low as
2 ohms.

(Yes, there exist SOME rare examples of loudspeakers with
2 ohm impedance, but such are confined to a VERY narrow
band of frequencies)

Okay, let's pretend we have real examples of the above. Let's assume
such a speaker has a moderately low efficiency, say the equivalent
of, oh, 86 dB SPL/1W/1M. We're blowing in 400 watts that means the speaker
is putting out 112 dB 1 meter way, a stereo pair, assuming the two
channels are uncorrelated, that's 115 dB. Really? This is a serious
requirement?

But wait, you explicitly stated:

"Provided the driver impedance is relatively benign"

and you specified 8 Ohms. So let's assume it's a nominal 8 ohm
impedance 3-way speaker using at least a 2nd-order crossover
network. The impedance will be around 6.5 ohms below system
cutoff (DC resistance of woofer voice coil), will rise to
perhaps 30 Ohms at and around system cutoff, then drop down
to perhaps 15% above the DC resistance above there, start
rising again until the woofer-midrange crossover starts working,
maybe betting to 10-12 ohms, then dip to perhaps 60% of the rated
impedance, so about 4.8 ohms, rise again to about 12 ohms or so
at the mid-tweeter crossover point, drop down to about 10-15% above
the tweeter DC resistance (which, for the purpose of argument, we'll
take to be a nominal 4 Ohm tweeter, so about 4.5 Ohms, after which
it starts rising again.

So, minimum impedance of about 4.8 ohms will occur over perhaps
a 2-octave band around 1 kHz, then about 4.5 ohms around 5 kHz.

Let's take your 14 amps, produce a musical signal where ALL the energy
is concentrated from about 500-2000 Hz and from about 4000-8000 Hz
ALONE, and see what 14 Amps does.

Well, since

P = I^R

And we'll assume the impedance at these points is largely resistive,
which is is, then:

P = 14^2 * 4.5

882 watts. And to do that, the amplifier must be capable of outputting

E = I R

E = 14 * 4.5

63 volts RMS.

Really?

Oh, wait! Everyone knows that under transient conditions, the loudspeaker
impedance can actually go well below the lowest impedance of the
speaker for brief moments due to back EMF, Otala said so.

Oh, wait! Everyone who knows that is wrong and has yet to advance any
confirmed data sowing this to be the case and, by the way, Otala DID
NOT say so: he basically said that the peak current requirement under
actual transient conditions is exactly what is expected from the actual
measure steady state impedance, and the only thing he really said
that's even remotely like this is that the peak current requirements
are greater than predicted by the "nominal" impedance of the loudspeaker.

Give me a shovel, Mr. Ohm wants to go back to sleep.

~misfit~

unread,
Sep 15, 2019, 9:43:18 PM9/15/19
to
So three pairs per side should be fine for a reasonably powerful amp? I've studied the PCB and the
output devices are paralleled (along with a resistor for each) so it wouldn't be hard to add a
third device to each (on very short flyleads - or even daughterboards - mounted to the same heatsink).

The speakers I'm intending to use with this are Sony SS-K90EDs.
Like these:
<https://www.stereo.net.au/forums/topic/260972-fs-sony-ss-k90ed-speakers-rare/>

> So, a little Ohm's Law should tell you if you are demanding more current than the output devices
> are capable of delivering. 14 Amps is, by high end audio standards, a relatively modest current
> ability for a (say) 100 Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
> benign, you should be OK. Fortunately, it is real hard to damage MOSFETs, by 'asking' them to
> deliver more current than they are rated for.

That's one of the things I like about MOSFETs.

>>> One more thing: Decent amounts of capacitance placed close to the output devices is far more
>>> influential than caps placed some distance away. In fact, long(ish) cables AFTER the main filter
>>> caps can be a serious limiting factor on the effectiveness of a power supply in a Class A/B
>>> amplifier. This is because the inductance of the wires can be a factor.
>>
>> Thanks. The fly-leads will only be 6" tops and I'll be using at least 1.5 square mm multistrand
>> copper conductors. If space allows I'll put a ~1,000uF cap right at the amplifier PCB as well (or
>> as large as I can get away with). I may end up building a wooden case as I don't have a suitable
>> metal one and wood's something I have experience and the tools for.
>
> **Wiring sounds good. And yeah, caps placed close to output devices is a very good thing. A wooden
> case, not so much. Wood is an excellent thermal insulator, which means heat may not escape too easily.

I have a couple of big heatsinks for the amplifier modules that will sit either side of the case,
fins outwards in free air. They'll easily handle the power dissipation being 4x bigger than the
'sinks used on the PA amp. Also I'll ventilate the top and bottom of the 'box' (if I end up going
with wood).

>> I still haven't finalised my design yet. I might end up feeding them a few more volts than they
>> were getting from their original power supplies (my only suitable toroidial transformer is 10v AC
>> higher than original) so may parallel up a third pair of output devices onto the heatsinks using
>> one of the other amps as a donor. I haven't decided yet, as I said it's a long-term project and
>> I'm learning as I go.
>
> **Well, the MOSFETs are rated for a decent 160 Volts, so a few more rail Volts should be OK. And
> yes, more output devices won't hurt (refer to Ohm's Law as before). Pay attention to the drive
> capabilities of the preceding stages though.

Thanks for this Trevor, I have saved it for future reference. My 300 VA toroid that I'm thinking of
using with this outputs 50v AC so +/- 70v DC when rectified. The original PA transformers were 40v AC.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Trevor Wilson

unread,
Sep 16, 2019, 5:52:54 AM9/16/19
to
On 16/09/2019 7:49 am, dpierce.ca...@gmail.com wrote:
> On Saturday, September 14, 2019 at 9:58:44 AM UTC-4, Trevor Wilson wrote:
>> So, a little Ohm's Law should tell you if you are demanding more current
>> than the output devices are capable of delivering. 14 Amps is, by high
>> end audio standards, a relatively modest current ability for a (say) 100
>> Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
>> benign, you should be OK.
>
> Hmm, that's not what a little Ohm's law tells me.
>
> 100 Watts into 8 Ohms is a tad over 3.5 amps. Let's say it's a VERY
> robust 100 watt amplifier, delivering 200 Watts into 4 Ohms requires
> about 7 amps, and, let's pretend it has essentially ZERO output
> impedance and an effectively limitless power supply, you're not reaching
> 14 amps until you're driving 400 watts into 2 ohms.

**Well, no. The RMS current is certainly 3.5 Amps, but output devices
only 'care' about PEAK currents. The peak current is, of course, 3.5 X
1.414 ~ 5 Amps.

With a 4 Ohm load, the peak current required is 10 Amps. For 2 Ohms, it
is 20 Amps.

Assuming a 100 Watt amp. For a (say) 200 Watt amp, those peak current
figures become 7 Amps, 14 Amps and 28 Amps respectively. WAY past the
ability of two pairs of old Hitachi MOSFETs to deal with.

>
> So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
> presents a broadband 2 ohm impedance or, conversely, what kind of
> musical content would generate that kind of power requirement over
> the pretty narrow band of frequencies where a loudspeaker has the
> kind of pathological impedance curve that would dip to as low as
> 2 ohms.

**I have a few here that are tougher than that. Some of the Peerless
XXLS drivers dip to the low 2 Ohm region. Most ESLs fall lower than that
at HF.

>
> (Yes, there exist SOME rare examples of loudspeakers with
> 2 ohm impedance, but such are confined to a VERY narrow
> band of frequencies)

**If those frequencies happen to be in an area where the amplifier is
required to deliver a lot of power, then it matters a lot. In the bass,
for instance.
**I pay no attention to such things. I just look at the actual
impedance/phase angle curves. Like the ones at the end of this post.

>
> Oh, wait! Everyone who knows that is wrong and has yet to advance any
> confirmed data sowing this to be the case and, by the way, Otala DID
> NOT say so: he basically said that the peak current requirement under
> actual transient conditions is exactly what is expected from the actual
> measure steady state impedance, and the only thing he really said
> that's even remotely like this is that the peak current requirements
> are greater than predicted by the "nominal" impedance of the loudspeaker.
>
> Give me a shovel, Mr. Ohm wants to go back to sleep.
>

**Here are some real-world speaker impedance plots. All have parts of
their curve below 4 Ohms. Most of them are speakers I have personal
experience with. I owned a pair of Westlakes for awhile and I have a
pair of Martin Logan Quest Z as a workshop system. Both are tough loads
for any amplifier.

https://www.stereophile.com/content/westlake-bbsm-6f-loudspeaker-measurements

https://www.stereophile.com/content/wilson-audio-specialties-cub-loudspeaker-measurements

https://www.stereophile.com/content/aerial-acoustics-20t-v2-loudspeaker-measurements

https://www.stereophile.com/content/wilson-audio-specialties-yvette-loudspeaker-measurements

https://www.stereophile.com/content/bampw-800-diamond-loudspeaker-measurements

https://www.stereophile.com/content/bw-802d-loudspeaker-measurements

https://www.stereophile.com/content/bw-signature-800-loudspeaker-measurements

https://www.stereophile.com/content/bw-nautilus-801-loudspeaker-measurements

https://www.stereophile.com/content/boston-acoustics-m350-loudspeaker-measurements

https://www.stereophile.com/content/bowers-wilkins-702-s2-loudspeaker-measurements

https://www.stereophile.com/content/bowers-amp-wilkins-804-diamond-loudspeaker-measurements

https://www.stereophile.com/content/dunlavy-audio-laboratories-sc-iv-loudspeaker-measurements

https://www.stereophile.com/content/dynaudio-evidence-temptation-loudspeaker-measurements

https://www.stereophile.com/content/focal-maestro-utopia-iii-loudspeaker-measurements

https://www.stereophile.com/content/focal-sopra-no3-loudspeaker-measurements

https://www.stereophile.com/content/focal-jmlab-nova-utopia-be-loudspeaker-measurements

https://www.stereophile.com/content/infinity-irs-beta-loudspeaker-measurements

https://www.stereophile.com/content/innersound-eros-mkiii-electrostatic-loudspeaker-measurements

https://www.stereophile.com/content/kef-reference-5-loudspeaker-measurements

https://www.stereophile.com/content/martinlogan-quest-z-loudspeaker-measurements

And, of course, the pathological loads I measured some years ago:

http://www.rageaudio.com.au/index.php?p=1_12

Trevor Wilson

unread,
Sep 16, 2019, 5:53:07 AM9/16/19
to
**Again: It depends on the maximum Voltage output. 3 pairs allows for a
peak current ability of 21 Amps.

> I've studied the PCB and the output devices are paralleled (along with a
> resistor for each) so it wouldn't be hard to add a third device to each
> (on very short flyleads - or even daughterboards - mounted to the same
> heatsink).

**Sure. However, make certain the drive circuitry can cope.

>
> The speakers I'm intending to use with this are Sony SS-K90EDs.
> Like these:
> <https://www.stereo.net.au/forums/topic/260972-fs-sony-ss-k90ed-speakers-rare/>

**OK.

>
>
>> So, a little Ohm's Law should tell you if you are demanding more
>> current than the output devices are capable of delivering. 14 Amps is,
>> by high end audio standards, a relatively modest current ability for a
>> (say) 100 Watt @ 8 Ohms amplifier. Provided the driver impedance is
>> relatively benign, you should be OK. Fortunately, it is real hard to
>> damage MOSFETs, by 'asking' them to deliver more current than they are
>> rated for.
>
> That's one of the things I like about MOSFETs.

**Well, a properly designed BJT amp should demonstrate the same
robustness and reliability.

>
>>>> One more thing: Decent amounts of capacitance placed close to the
>>>> output devices is far more influential than caps placed some
>>>> distance away. In fact, long(ish) cables AFTER the main filter caps
>>>> can be a serious limiting factor on the effectiveness of a power
>>>> supply in a Class A/B amplifier. This is because the inductance of
>>>> the wires can be a factor.
>>>
>>> Thanks. The fly-leads will only be 6" tops and I'll be using at least
>>> 1.5 square mm multistrand copper conductors. If space allows I'll put
>>> a ~1,000uF cap right at the amplifier PCB as well (or as large as I
>>> can get away with). I may end up building a wooden case as I don't
>>> have a suitable metal one and wood's something I have experience and
>>> the tools for.
>>
>> **Wiring sounds good. And yeah, caps placed close to output devices is
>> a very good thing. A wooden case, not so much. Wood is an excellent
>> thermal insulator, which means heat may not escape too easily.
>
> I have a couple of big heatsinks for the amplifier modules that will sit
> either side of the case, fins outwards in free air. They'll easily
> handle the power dissipation being 4x bigger than the 'sinks used on the
> PA amp. Also I'll ventilate the top and bottom of the 'box' (if I end up
> going with wood).

**OK.

>
>>> I still haven't finalised my design yet. I might end up feeding them
>>> a few more volts than they were getting from their original power
>>> supplies (my only suitable toroidial transformer is 10v AC higher
>>> than original) so may parallel up a third pair of output devices onto
>>> the heatsinks using one of the other amps as a donor. I haven't
>>> decided yet, as I said it's a long-term project and I'm learning as I
>>> go.
>>
>> **Well, the MOSFETs are rated for a decent 160 Volts, so a few more
>> rail Volts should be OK. And yes, more output devices won't hurt
>> (refer to Ohm's Law as before). Pay attention to the drive
>> capabilities of the preceding stages though.
>
> Thanks for this Trevor, I have saved it for future reference. My 300 VA
> toroid that I'm thinking of using with this outputs 50v AC so +/- 70v DC
> when rectified. The original PA transformers were 40v AC.

**+/- 70VDC suggests a maximum power output of around 250 Watts @ 8
Ohms. If you plan on attempting to obtain that much power
(continuously), then you will need two of those toroids.

~misfit~

unread,
Sep 16, 2019, 9:01:53 AM9/16/19
to
I'm not exactly sure of how to do that?
I intend to use the system in my lounge so won't want crazy SPLs, the speakers likely wouldn't
handle that much power anyway. I actually do have two of the toroids but that would make for a big
amplifier case - and surely then I'd need to consider adding *two* more pairs of output MOSFETs per
amplifier?

I was thinking that, as I don't listen to dubstep or extremely bass-heavy music, using one toroid
and a lot of capacitance (in the region of 20,000 to 50,000 uF per rail) would be enough to handle
transients. If not then I might as well build a pair of monoblocks.

I've got a few coffee-cup sized Mepco/Electra 14,000 uF / 100v caps but they're not new... I also
have 8 new 10,000 uF / 100v Elna caps that are only about 1/4 of the size.

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Peter Wieck

unread,
Sep 16, 2019, 11:11:40 AM9/16/19
to
IF all you are doing is wanting to power speakers at a reasonable listening level in a reasonably sized room, and you do not want ear-bleed levels, your major task in this case is managing transients.

Your plan to use a large amount of capacitance will address that fairly nicely. And if your amp "breathes" at high volumes (you will absolutely know if that happens), then and only then consider (a) beefier power-supply(ies).

I run a pair of AR3a speakers (4-ohm, nominal) in a fairly small room (17 x 14 x 10 (feet)) with a 60 wpc/rms amp, and it does fine at any listening level I would care to use. It has about 8,000 uF/channel of capacitance.

Trevor Wilson

unread,
Sep 16, 2019, 7:54:49 PM9/16/19
to
**You need to examine the drive circuitry, the components used and then
calculate if those components can cope with the extra load caused by
extra MOSFETs. It will PROBABLY be OK, but I don't know.
**As Peter has correctly stated, provided you don't need the full
continuous power capacity of the amplifier at all times, then one
transformer will likely be plenty. From my perspective, I am a purist.
If I am presented with an amplifier rated at (say) 200 Watts/channel,
then that amplifier needs to be able to deliver 200 Watts/channel
INDEFINITELY and, possibly more importantly, it needs to be able to
deliver roughly 40% of it's maximum power without thermal distress. With
one transformer in your amplifier chassis, it would fail such a test.
But, your amplifier is not a commercial item. You can make it anything
you want.

>
> I was thinking that, as I don't listen to dubstep or extremely
> bass-heavy music, using one toroid and a lot of capacitance (in the
> region of 20,000 to 50,000 uF per rail) would be enough to handle
> transients. If not then I might as well build a pair of monoblocks.

**A worthy consideration.

>
> I've got a few coffee-cup sized Mepco/Electra 14,000 uF / 100v caps but
> they're not new... I also have 8 new 10,000 uF / 100v Elna caps that are
> only about 1/4 of the size.

**The amplifier I presently use has a 5.5kVA (yes, 5,500VA), split wound
(one winding for each channel), double C core power transformer,
followed by 92 X 3,300uF filter capacitors. The result is to ensure
that, under full power operation (at any impedance higher than 2 Ohms)
ripple is kept below 100mV. So, discussions of 10,000uF per rail doesn't
excite me. It's what I expect to see in a mass market product from
Yamaha or NAD. However, as Peter and I have both suggested, in a high
global NFB amp, such as yours, huge lumps of filter capacitance will not
be pivotal to performance. Placing a decent amount near the output
devices will be beneficial though.

dpierce.ca...@gmail.com

unread,
Sep 19, 2019, 6:00:57 AM9/19/19
to
While much of what you say below is, strictly speaking, technically
correct under some very specific cases, it's misleading and could well
not be applicable in reality.

I've not got a lot of time to spend on this, so let me just
take on a couple of your points.

On Monday, September 16, 2019 at 5:52:54 AM UTC-4, Trevor Wilson wrote:
> On 16/09/2019 7:49 am, dpierce.ca...@gmail.com wrote:
> > On Saturday, September 14, 2019 at 9:58:44 AM UTC-4, Trevor Wilson wrote:
> >> So, a little Ohm's Law should tell you if you are demanding more current
> >> than the output devices are capable of delivering. 14 Amps is, by high
> >> end audio standards, a relatively modest current ability for a (say) 100
> >> Watt @ 8 Ohms amplifier. Provided the driver impedance is relatively
> >> benign, you should be OK.
> >
> > Hmm, that's not what a little Ohm's law tells me.
> >
> > 100 Watts into 8 Ohms is a tad over 3.5 amps. Let's say it's a VERY
> > robust 100 watt amplifier, delivering 200 Watts into 4 Ohms requires
> > about 7 amps, and, let's pretend it has essentially ZERO output
> > impedance and an effectively limitless power supply, you're not reaching
> > 14 amps until you're driving 400 watts into 2 ohms.
>
> **Well, no. The RMS current is certainly 3.5 Amps, but output devices
> only 'care' about PEAK currents. The peak current is, of course, 3.5 X
> 1.414 ~ 5 Amps.
>
> With a 4 Ohm load, the peak current required is 10 Amps. For 2 Ohms, it
> is 20 Amps.
>
> Assuming a 100 Watt amp. For a (say) 200 Watt amp, those peak current
> figures become 7 Amps, 14 Amps and 28 Amps respectively. WAY past the
> ability of two pairs of old Hitachi MOSFETs to deal with.

All of your calculations ASSUME several things, none of which are
likely to be true in real usage.

1. Your continuous-to-peak current calculations using a factor of
sqrt(2) ASUMMES that the excitation is pure sine. That's surely
not the case in real life, I'm sure you'd agree. Yes, the actual
crest fact may be greater than 3dB, but, again, you ASSUME that
in actual practice those peak current are REQUIRED> I suggest
they are not, in absence of any supporting evidence they are.

2. The comprehensive list of impedance curves is, yes, useful but
the interpretation of them as applied to your case ignores the
VERY important details and thus is overly simplistic and mis-
leading. Let me take just a couple of example from the list:

a. Westlake BBSM-6F
Yes, the impedance gets to 2 ohms, but look at the
broadband sensitivity 92dB/2.83V: less power is needed to
achieve a given sound pressure level, continuous, peak or
otherwise. It illustrates that you simply can't look at one
particular measurement in isolation.

And, not that it may be relevant, this is specifically
marketed towards studio use at high (deafening?:-) level.

b. The acoustat impedance curve you posted on the RageAudio
site: indeed, the impedance drops WAY down to under 1 Ohm.
BUT it does it over a VERY narrow bandwidth, and it does
it at 15 kHz. It's above 4 ohms over the entire audio range
from 10Hz to 9kHz. Exactly what kind of musical material
would require one to dump a LOT of power between 9 kHz and
20+kHz? Over the majority of the audio bandwidth, the impedance
is 6 ohms or higher. Even if you assume (quite unrealistically)
that the energy is distributed across the spectrum, equal
energy per octave, your impedance problems ar confined to about
1 octave out of 10, suggesting that if you need 100 watts in that
one octave, you'll need another 900! for everything else.

Ah, but what about short-term transients, you ask. Look at the
spectral distribution of such in actual music: sorry, you're
still NOT generating a lot of power over such a narrow bandwidth
(Sorry, Mr. Fourier, you can lay back down, we shan't be needing
you just yet).


> >
> > So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
> > presents a broadband 2 ohm impedance or, conversely, what kind of
> > musical content would generate that kind of power requirement over
> > the pretty narrow band of frequencies where a loudspeaker has the
> > kind of pathological impedance curve that would dip to as low as
> > 2 ohms.
>
> **I have a few here that are tougher than that. Some of the Peerless
> XXLS drivers dip to the low 2 Ohm region. Most ESLs fall lower than that
> at HF.

Yes, over VERY narrow bands and at high frequencies, where your
power requirements are not anywhere near as large as at significantly
lower frequencies.

~misfit~

unread,
Sep 20, 2019, 6:52:29 AM9/20/19
to
On 17/09/2019 11:54 AM, Trevor Wilson wrote:
> On 16/09/2019 11:01 pm, ~misfit~ wrote:
>> On 16/09/2019 9:53 PM, Trevor Wilson wrote:
>>>
>>> **Sure. However, make certain the drive circuitry can cope.
>>
>> I'm not exactly sure of how to do that?
>
> **You need to examine the drive circuitry, the components used and then calculate if those
> components can cope with the extra load caused by extra MOSFETs. It will PROBABLY be OK, but I
> don't know.
>
>>
>> I intend to use the system in my lounge so won't want crazy SPLs, the speakers likely wouldn't
>> handle that much power anyway. I actually do have two of the toroids but that would make for a
>> big amplifier case - and surely then I'd need to consider adding *two* more pairs of output
>> MOSFETs per amplifier?


[ Massively pruned of old quotes because your mod just
couldn't take it any more. --dsr ]



>
> **As Peter has correctly stated, provided you don't need the full continuous power capacity of the
> amplifier at all times, then one transformer will likely be plenty. From my perspective, I am a
> purist. If I am presented with an amplifier rated at (say) 200 Watts/channel, then that amplifier
> needs to be able to deliver 200 Watts/channel INDEFINITELY and, possibly more importantly, it needs
> to be able to deliver roughly 40% of it's maximum power without thermal distress. With one
> transformer in your amplifier chassis, it would fail such a test. But, your amplifier is not a
> commercial item. You can make it anything you want.
>
>>
>> I was thinking that, as I don't listen to dubstep or extremely bass-heavy music, using one toroid
>> and a lot of capacitance (in the region of 20,000 to 50,000 uF per rail) would be enough to
>> handle transients. If not then I might as well build a pair of monoblocks.
>
> **A worthy consideration.
>
>>
>> I've got a few coffee-cup sized Mepco/Electra 14,000 uF / 100v caps but they're not new... I also
>> have 8 new 10,000 uF / 100v Elna caps that are only about 1/4 of the size.
>
> **The amplifier I presently use has a 5.5kVA (yes, 5,500VA), split wound (one winding for each
> channel), double C core power transformer, followed by 92 X 3,300uF filter capacitors.

Wow! Decades ago I used to work with a touring band doing stage lighting and (some) sound mixing
and that's a more capable amplifier power supply than were in some amps we used at medium-sized gigs.

> The result
> is to ensure that, under full power operation (at any impedance higher than 2 Ohms) ripple is kept
> below 100mV. So, discussions of 10,000uF per rail doesn't excite me. It's what I expect to see in a
> mass market product from Yamaha or NAD. However, as Peter and I have both suggested, in a high
> global NFB amp, such as yours, huge lumps of filter capacitance will not be pivotal to performance.
> Placing a decent amount near the output devices will be beneficial though.

Thanks for your input Trevor. It helps me to decide how to go about building my 'franken-amp'.
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Trevor Wilson

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Sep 21, 2019, 9:02:17 AM9/21/19
to
**Whoaa. Hang on a sec. Let's get down to basics. The peak to average
figure available in music is not the issue here and is certainly not
what I am talking about.

Let us assume, for the sake of simplicity, that the music played IS
close to pure sine waves (maybe the listener is a pipe organ lover). Now
I certainly recognise that 99.9999% of music is not sine waves
(apologies to Fourier), but I need to make an important point. Let's say
we have (again, for simplicity and ignoring SOA considerations for a
moment) an amplifier that uses the most excellent Toshiba
2SC5200/2SA1943 output devices. Let's say an RMS output current of 15
Amps is required. This would not be healthy for the output devices, as
their maximum current rating is 15 Amps (peak). Yes, it can
(theoretically, SOA considerations aside) deliver a DC current of 15
Amps. However, a sine wave of 15 Amps means that the peak current will
be in excess of 21 Amps. Examine the SOA curve of the 2SC5200:


https://toshiba.semicon-storage.com/ap-en/product/bipolar-transistor/bipolar-transistor/detail.2SC5200.html

You may care to note that even if the current flowing through the device
exceeds 15 Amps for as little as ONE MILLISECOND (in truth it is far
less), the device will be destroyed.

It is for this reason that peak current MUST be taken into account when
designing any solid state amp.

>
> 2. The comprehensive list of impedance curves is, yes, useful but
> the interpretation of them as applied to your case ignores the
> VERY important details and thus is overly simplistic and mis-
> leading. Let me take just a couple of example from the list:
>
> a. Westlake BBSM-6F
> Yes, the impedance gets to 2 ohms, but look at the
> broadband sensitivity 92dB/2.83V: less power is needed to
> achieve a given sound pressure level, continuous, peak or
> otherwise. It illustrates that you simply can't look at one
> particular measurement in isolation.

**Indeed. I can assure you, however, having owned a pair for some years,
that they are utterly ruthless in exposing an amplifier's ability to
deal with tough loads. Even at modest listening levels.

>
> And, not that it may be relevant, this is specifically
> marketed towards studio use at high (deafening?:-) level.

**I can assure you that I do not and have not, for many years listened
to music at silly levels. Back when I was in my teens and early 20s,
yes. But not now and not when I owned the Westlakes.

>
> b. The acoustat impedance curve you posted on the RageAudio
> site: indeed, the impedance drops WAY down to under 1 Ohm.
> BUT it does it over a VERY narrow bandwidth, and it does
> it at 15 kHz. It's above 4 ohms over the entire audio range
> from 10Hz to 9kHz. Exactly what kind of musical material
> would require one to dump a LOT of power between 9 kHz and
> 20+kHz? Over the majority of the audio bandwidth, the impedance
> is 6 ohms or higher. Even if you assume (quite unrealistically)
> that the energy is distributed across the spectrum, equal
> energy per octave, your impedance problems ar confined to about
> 1 octave out of 10, suggesting that if you need 100 watts in that
> one octave, you'll need another 900! for everything else.
>
> Ah, but what about short-term transients, you ask. Look at the
> spectral distribution of such in actual music: sorry, you're
> still NOT generating a lot of power over such a narrow bandwidth
> (Sorry, Mr. Fourier, you can lay back down, we shan't be needing
> you just yet).

**Oh, I agree, you won't be generating much power. However, VI limiting
systems in most amplifiers don't care about such things. They just
operate anyway. Mostly. And to the detriment of sound quality. And, more
seriously, when using almost any valve amplifier with the Accustats,
will guarantee a suck-out at the impedance dip. That is likely to be
audible.

>
>
>>>
>>> So, a couple of questions that Mr. Ohm may ask; what kind of loudspeaker
>>> presents a broadband 2 ohm impedance or, conversely, what kind of
>>> musical content would generate that kind of power requirement over
>>> the pretty narrow band of frequencies where a loudspeaker has the
>>> kind of pathological impedance curve that would dip to as low as
>>> 2 ohms.
>>
>> **I have a few here that are tougher than that. Some of the Peerless
>> XXLS drivers dip to the low 2 Ohm region. Most ESLs fall lower than that
>> at HF.
>
> Yes, over VERY narrow bands and at high frequencies, where your
> power requirements are not anywhere near as large as at significantly
> lower frequencies.

**The Peerless XXLS drivers are LF drivers, requiring prodigious currents:

http://www.loudspeakerdatabase.com/Peerless/XXLS-P835017/Peerless_XXLS-P835017.pdf

I have passive radiator based subwoofer in the workshop right now. I
will run a real world impedance plot for the system when I get some time.

~misfit~

unread,
Sep 21, 2019, 10:34:44 PM9/21/19
to
On 20/09/2019 10:52 PM, ~misfit~ wrote:
> On 17/09/2019 11:54 AM, Trevor Wilson wrote:
>> On 16/09/2019 11:01 pm, ~misfit~ wrote:
>>> On 16/09/2019 9:53 PM, Trevor Wilson wrote:
>>>>
>>>> **Sure. However, make certain the drive circuitry can cope.
>>>
>>> I'm not exactly sure of how to do that?
>>
>> **You need to examine the drive circuitry, the components used and then calculate if those
>> components can cope with the extra load caused by extra MOSFETs. It will PROBABLY be OK, but I
>> don't know.
>>
>>>
>>> I intend to use the system in my lounge so won't want crazy SPLs, the speakers likely wouldn't
>>> handle that much power anyway. I actually do have two of the toroids but that would make for a
>>> big amplifier case - and surely then I'd need to consider adding *two* more pairs of output
>>> MOSFETs per amplifier?
>
>
>    [ Massively pruned of old quotes because your mod just
>      couldn't take it any more. --dsr                ]

Thanks. I actually seriously considered doing the same but, as a relative newbie here didn't want
to upset any pie-carts. :)

[ Always feel free to trim quotes in your reply down to
the points you are actively addressing. Usenet is
archived all over the world, it's really easy to
reconstruct threads. --dsr ]

Mat Nieuwenhoven

unread,
Oct 13, 2019, 5:00:32 PM10/13/19
to
On 12 Sep 2019 20:28:33 GMT, Trevor Wilson wrote:

<snip>
>
>**Let me be very clear about several things:
>
>* NFB is fine. In fact, NO audio amplifier can work without it.
>* GLOBAL NFB is also fine. When properly applied.
>* I have a personal preference for the amplifiers I use, which employ
>lots of local NFB and no global NFB. Others may have a different opinion.
>* As part of my education into the world of zero global NFB amplifiers,
>I subjected myself to a couple of single (unfortunately) blind tests,
>between two, otherwise identical, amplifiers. One employed zero GNFB and
>one employed a modest amount of GNFB. I preferred the zero GNFB one.
>Since that time, I subjected several (10) of my clients to the same test
>(DBT). The zero GNFB models was preferred every time. Except one.
>* Once more: I would posit that part of the reason why some listeners
>prefer valve amplifiers, is due to the fact that global NFB levels are
>very low, or non-existent.
>
>
> But, again, in the real world, negative feedback, done properly, has
>many more advantages than disadvantages.
>
>**Again: No issue with NFB. In fact, no issue with GNFB, when done well.

How do modern switching amps (class D) stack up for HiFi use? Aren't
most PA systems now fully digital? Do they actually use FB? If I look
at the spec sheet of the TDA7492 it doesn't look like it. Do they
sound worse than a good analog amp?

The class-D amps typically have a series inductance between the
switching elements and the speakers, does that influence transients?
Even a tweeter has already 15-20 microHenry of inductance.

Mat Nieuwenhoven






Trevor Wilson

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Oct 13, 2019, 5:26:48 PM10/13/19
to
On 14/10/2019 8:00 am, Mat Nieuwenhoven wrote:
> On 12 Sep 2019 20:28:33 GMT, Trevor Wilson wrote:
>
> <snip>
>>
>> **Let me be very clear about several things:
>>
>> * NFB is fine. In fact, NO audio amplifier can work without it.
>> * GLOBAL NFB is also fine. When properly applied.
>> * I have a personal preference for the amplifiers I use, which employ
>> lots of local NFB and no global NFB. Others may have a different opinion.
>> * As part of my education into the world of zero global NFB amplifiers,
>> I subjected myself to a couple of single (unfortunately) blind tests,
>> between two, otherwise identical, amplifiers. One employed zero GNFB and
>> one employed a modest amount of GNFB. I preferred the zero GNFB one.
>> Since that time, I subjected several (10) of my clients to the same test
>> (DBT). The zero GNFB models was preferred every time. Except one.
>> * Once more: I would posit that part of the reason why some listeners
>> prefer valve amplifiers, is due to the fact that global NFB levels are
>> very low, or non-existent.
>>
>>
>> But, again, in the real world, negative feedback, done properly, has
>> many more advantages than disadvantages.
>>
>> **Again: No issue with NFB. In fact, no issue with GNFB, when done well.
>
> How do modern switching amps (class D) stack up for HiFi use?

**Provided the switching frequency is high enough and the load impedance
is benign, then they should work well. I was particularly impressed with
the interesting design from Devialet. A small Class A stage is used in
much the same way that Quad did several decades ago for their Current
Dumping™ products. They used a Class A stage, combined with a Class C
power stage.

Aren't
> most PA systems now fully digital?

**No such thing.

Do they actually use FB?

**EVERY amplifier uses NFB. Every single one. Regardless of technology
or claims from manufacturers.

If I look
> at the spec sheet of the TDA7492 it doesn't look like it. Do they
> sound worse than a good analog amp?

**I see a loop feedback mechanism in the block diagram. I see some
audibly significant problems with the amplifier. Max THD is cited aas
0.4% and the frequency response is poor, compared to even modest Class
A/B amplifiers. The low switching frequency ensures that low impedance
(<4 Ohms) loads are not well catered for.

>
> The class-D amps typically have a series inductance between the
> switching elements and the speakers, does that influence transients?

**Of course.

> Even a tweeter has already 15-20 microHenry of inductance.
>
> Mat Nieuwenhoven
>
>
>
>
>
>


--
Trevor Wilson
www.rageaudio.com.au

Trevor Wilson

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Oct 14, 2019, 2:58:58 PM10/14/19
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On 14/10/2019 8:00 am, Mat Nieuwenhoven wrote:

> Even a tweeter has already 15-20 microHenry of inductance.

**Not so. I haven't measured one in quite some time, but the EMIT HF
drivers, used in many Infinity speakers exhibit far lower inductance
figures than that. If I had to guess, I'd estimate the inductance figure
to be around 5 X 10^-6H. I'll see if I can locate one to measure. Then,
of course, is the sadly deleted Audax HD-3P Piezo HF driver. And any
number of ELS HF drivers.


--
Trevor Wilson
www.rageaudio.com.au

--

Mat Nieuwenhoven

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Oct 16, 2019, 6:50:03 AM10/16/19
to
On 14 Oct 2019 18:58:55 GMT, Trevor Wilson wrote:

>On 14/10/2019 8:00 am, Mat Nieuwenhoven wrote:
>
>> Even a tweeter has already 15-20 microHenry of inductance.
>
>**Not so. I haven't measured one in quite some time, but the EMIT HF
>drivers, used in many Infinity speakers exhibit far lower inductance
>figures than that. If I had to guess, I'd estimate the inductance figure
>to be around 5 X 10^-6H. I'll see if I can locate one to measure. Then,
>of course, is the sadly deleted Audax HD-3P Piezo HF driver. And any
>number of ELS HF drivers.

Some data from the German magazine "Hobby Hifi", from various
manufacturers:

Omnes Audio AMT50 (Air Motion Transformer): 10 uH/20 kHz
Audaphon APR 1.0, band tweeter: 54 uH/20 kHz
Monacor DT-352NF, dome tweeter: 45 uH/20 kHz
SB Acoustics TW29B, dome tweeter: 18 uH/20 kHz
Scan Speak D3404-552000, elliptical dome tweeter: 18 uH/20 kHz
Tang Band 25-2234SD, inverse dome tweeter: 24 uH/20 kHz

Multiple have a copper covering in the magnetgap to reduce the
increase of impedance at the higher frequencies, and to reduce
distortion.

Mat Nieuwenhoven


dpierce.ca...@gmail.com

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Oct 16, 2019, 8:59:26 AM10/16/19
to
Mat's data is far closer to prevailing reality here. Trevor cites but
two potential examples: one of which never had wide distribution
and is essentially unobtainable, the other is a proprietary unit
that is, at very best, rarely found in even restricted distribution.

And even looking at the real data on the Audax, it exhibits significant
inductive behavior over its bandwidth. From 5 kHz to 7 kHz, the impedance
has a significant inductive component, then again from about 12 kHz to
almost 16 kHz, it's impedance has an inductive component to it.

The point being is that the vast majority of tweeters and speaker
systems, that is, those that are now and have been available to any
segment of consumer or pro audio market you care to choose, have
impedances in the HF region that exhibit a predominantly inductive
component. One can cherry pick counterexamples, to be sure, but
do note, that particular cherry tree is nearly devoid of usable
fruit.

Like the prior discussion on impedance in this thread, yes, there
can be found isolated examples of speakers whose arguably pathological
properties can be found to conform to a particular narrow view. But
the reality and the practicality of the actual situational use in
situ is often very different than the conclusions that might be drawn
from such a viewpoint.


Peter Wieck

unread,
Oct 16, 2019, 12:43:04 PM10/16/19
to
a) Argument-by-exception is, generally, fallacious.
b) Anything including a coil that carries current will be inductive.
c) Most well-designed speakers using conventional drivers that include voice-coils will account for this in their design.
d) Many crossover designs include inductors of various natures typed.
e) And those well-designed speakers that incorporate the exceptions will also account for those option.

Comes down to the question of: Does driver/speaker inductance in *this* particular speaker coupled with *that* particular amplifier matter at *this* range of frequencies and volumes?

Theory is all well and good, but how things operate in the real world at the living/listening room level are, or at least should, be the primary issue.

Mat Nieuwenhoven

unread,
Oct 19, 2019, 8:03:53 AM10/19/19
to
On 13 Oct 2019 21:26:46 GMT, Trevor Wilson wrote:

<snip>
>
> Do they actually use FB?
>
>**EVERY amplifier uses NFB. Every single one. Regardless of technology
>or claims from manufacturers.
>
> If I look
>> at the spec sheet of the TDA7492 it doesn't look like it. Do they
>> sound worse than a good analog amp?
>
>**I see a loop feedback mechanism in the block diagram. I see some
>audibly significant problems with the amplifier. Max THD is cited aas
>0.4% and the frequency response is poor, compared to even modest Class
>A/B amplifiers. The low switching frequency ensures that low impedance
>(<4 Ohms) loads are not well catered for.

The TDA7492 has a switching frequency of typically 310 kHz. How is
this related to bad handling of a 4 ohm load, and why is that
dependent on the load resistance? And this IC is specified for 4 ohm
or more.

The low frequency fall off is deliberate (page 24 of the ST spec),
easily fixed by increasing the input size capacitor. The
THD-versus-frequency plot is indeed not impressive, THD rises in
spots to 0.2 %. You don't happen to have a link to a similar plot
from a good quality analog amp? Also, a link for a similar FFT plot ?
I am curious.

The IC is also dirt cheap, on a board for less than 10 $. For that
price, it is superb value for money.

There is indeed a feedback path from the OUTx pins to the second amp
inside.

<snip>
For a professional product, see e.g.
https://icepower.dk/products/other/a-series/ . Its datasheet is at
https://icepower.dk/download/2414/ . It supports loads down to 2.7
ohm loads. Again I wonder what the switching frequency has to do with
load.

I wonder if the phase plot can be matched by any analog amp, or even
the output resistance of <50 mOhm .

Mat Nieuwenhoven




Trevor Wilson

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Oct 19, 2019, 10:07:33 PM10/19/19
to
On 19/10/2019 11:03 pm, Mat Nieuwenhoven wrote:
> On 13 Oct 2019 21:26:46 GMT, Trevor Wilson wrote:
>
> <snip>
>>
>> Do they actually use FB?
>>
>> **EVERY amplifier uses NFB. Every single one. Regardless of technology
>> or claims from manufacturers.
>>
>> If I look
>>> at the spec sheet of the TDA7492 it doesn't look like it. Do they
>>> sound worse than a good analog amp?
>>
>> **I see a loop feedback mechanism in the block diagram. I see some
>> audibly significant problems with the amplifier. Max THD is cited aas
>> 0.4% and the frequency response is poor, compared to even modest Class
>> A/B amplifiers. The low switching frequency ensures that low impedance
>> (<4 Ohms) loads are not well catered for.
>
> The TDA7492 has a switching frequency of typically 310 kHz. How is
> this related to bad handling of a 4 ohm load, and why is that
> dependent on the load resistance? And this IC is specified for 4 ohm
> or more.

**The output filter will affect phase and frequency response, when used
with <8 Ohm loads. A switching frequency closer to (or exceeding 1Mhz)
is desirable to ensure phase/frequency response errors are not extant
with lower impedance loads.

>
> The low frequency fall off is deliberate (page 24 of the ST spec),
> easily fixed by increasing the input size capacitor.

**Yes, but the problems are also obvious at the other end of the
frequency spectrum. The problems with this chip will be audibly obvious,
if it is attempted to be used in a high quality system.


The
> THD-versus-frequency plot is indeed not impressive, THD rises in
> spots to 0.2 %. You don't happen to have a link to a similar plot
> from a good quality analog amp? Also, a link for a similar FFT plot ?
> I am curious.

**Here's one:
http://www.ti.com/lit/ds/symlink/lm3886.pdf

Here is one of my all-time favourite Class A/B chips (sadly no longer
manufactured, but excellent performance):

http://pdf.datasheetcatalog.com/datasheet/philips/TDA1514.pdf

Either will comfortably outperform your cheap 'n cheerful Class D chip.
At the cost of efficiency, price and heat sinking, of course.


>
> The IC is also dirt cheap, on a board for less than 10 $. For that
> price, it is superb value for money.

**No argument from me. Telephones, TV sets, portable audio equipment can
all make good use of such chips. For QUALITY audio, however, forget it.
Far too many audibly significant problems. Same as cheap tube amps - too
many audible problems to bother with.

>
> There is indeed a feedback path from the OUTx pins to the second amp
> inside.

**Of course. See my previous comment. ALL amplifiers use some kind (or
kinds) of NFB.

>
> <snip>
> For a professional product, see e.g.
> https://icepower.dk/products/other/a-series/ . Its datasheet is at
> https://icepower.dk/download/2414/ . It supports loads down to 2.7
> ohm loads. Again I wonder what the switching frequency has to do with
> load.

**The last ICEpower amp I had on my bench sounded horrible. It's been a
few years. Perhaps they're better now. That said, the issue with
switching frequency is pivotal the the high frequency performance when
used with low impedance loads. The output filter is almost always a
simple 6dB/octave affair. As such, it can introduce phase shift and/or
frequency response errors down into the audio band. Raising the
switching frequency to >1MHz means that almost any speaker system can be
used without causing such problems. IMO, once switching frequencies
reach (say) 1.5MHz, then Class A and Class A/B amps will no longer need
to exist. Right now, at the present state of the art, Class A and Class
A/B amps still provide superior performance, when used with low
impedance loads. Of course, this means that for subwoofers, Class D is
the only sane choice.

>
> I wonder if the phase plot can be matched by any analog amp, or even
> the output resistance of <50 mOhm .

**Of course. Achieving an output impedance of 50mOhm is easy enough. The
catch is achieving that output impedance at 20kHz.

~misfit~

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Oct 19, 2019, 10:07:36 PM10/19/19
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So would it be worth my while to buy one or two of these and play with them?
<https://www.aliexpress.com/item/32796154933.html> I have a few ~100w laptop power bricks around
that I could use to feed power to them (that I could supplement with a large local capacitor...).

I realise it's in a different class to the links you provided but I don't have a big budget. That
said I don't have money to waste either...

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

Trevor Wilson

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Oct 20, 2019, 4:31:14 PM10/20/19
to
**Depends on what you are trying to achieve. For 4 Bucks, it represents
very good value for money, for an amplifier that can make some noise. It
ain't 'proper' hi fi, but it will certainly outperform many highly
prized (and very expensive) valve amps. It cannot hope to perform as
well as any competently designed Class A/B solid state amp though.
Still, it is FOUR BUCKS!

~misfit~

unread,
Oct 21, 2019, 5:47:05 AM10/21/19
to
Thanks Trevor. Yeah US$4 + some postage, not a lot of money to spend to find out. I had zero
interest in this sort of thing until I saw the post I replied to then started to wonder it it might
be worth converting some of the less valuable speakers in my speaker collection (that I need to
part with soon) to powered speakers. The sort of thing a smart phone can be plugged into...

That might make them a bit more profitable to sell on the current market. The ones I'm thinking of
are old-ish but not classic. T'was just a passing thought. I see there are also Bluetooth enabled
versions of TDA7492-based power amps for sale for not a lot more than that one.

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy little classification
in the DSM"
David Melville

This is not an email and hasn't been checked for viruses by any half-arsed self-promoting software.

dpierce.ca...@gmail.com

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Oct 21, 2019, 10:02:12 AM10/21/19
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