Richard Clark $10,000 Amplifier Challenge - Why Couldn't Anyone Pass this Test??

+1 Everyone 

Good thread with good points by all. There is much knowledge among you Audiogoners!

Then to just complicate what I said above, the current output of an amp comes into as well, to keep the said control over the speaker.

This is a common myth.

An amplifier can have a very low output impedance without having much ’current’.

Ralph you jumped the gun mate, read again, I never tied output impedance and current output together in the same sentence.

Not in that sentence, surely! I don't think I jumped the gun; the statement is a common myth and I showed why in my post. You don't need a lot of current to 'control' a speaker, what you need is an output impedance that is sufficiently low, and you don't need 'current' to get that.

 Also to add, an amplifier "with low output impedance", but with not much "current ability" like otl's, have no chance driving a pair of Wilson Alexia's and like, which drop to an EPDR of  .9ohm around 100hz (the power region) without serious problems.

As an OTL manufacturer I might be recognized as having a bias, but here is the simple truth of the matter: The output section of an OTL has to do the same sorts of things that a solid state amp might do- and that is drive a speaker. It has to be able to make current to do that. Our amps don't have a particularly low output impedance owing to little or no feedback, but if they did, driving this particular speaker really isn't that hard. All of the Wilsons have been fairly easy loads for tube amps in general despite their impedance!  We've had very good results with the Sophias, which are a similar load. I've heard that combo many times. I've not heard the Alexias yet. FWIW, the sales manager of Wilson had our amps for some years until an outside organization tried to take over Atma-Sphere about 13 years ago. At that point he sold them to be out of the possible politics.

To give you an example of how much current is available in one of our amps, while tampering :) with a set of MA-1s many years ago I noticed that if the output section was deprived of bias, all by itself (IOW not including the filament circuit) it could blow a 15Amp fuse without damage to the power tubes! Certainly, with that sort of current, the power tubes would overheat fairly quickly, but for short (no pun intended) periods of time they can survive much larger amounts of current than one would expect. Our driver section has the ability to drive the output tube grids to about 15Volts positive with respect to the cathodes (which means there can be quite a lot of grid current; this is 15V more than the output section would have on the girds if deprived of bias!) and the output section can be linear in this region. IOW we operate the amplifier class A2.

IOW, we have the current, but not the low output impedance. Weird, huh?

The fact that the amp does not act as a voltage source is simply because to do so requires about 20 db of feedback. We've certainly built amps that way but never got them to sound as good. Proper application of feedback is a lot trickier than most engineers think! You can't just apply it according to the formulas and expect it to work, because the formulas don't cover everything. This article (be sure to read part 2 as well) does a good job of covering the problems and also proper application, which as far as I can tell is not executed by about 95% of **all** amps employing feedback:

http://www.normankoren.com/Audio/FeedbackFidelity.html

If the principles laid out in this article were applied in audio today, it would advance the art. So far, its been simpler for us to achieve our design goals by being pragmatic and recognizing that our amps won't drive all loads. But if we can get the amp and speaker to work together, the combination has the ability to cross the line between music and hifi.

Roger said that though the OTL/QUAD combination was indeed considered by many to be a good synergistic match, he considered it to be the opposite---an OTL being the absolute worst amp design for use with that speaker.

Roger and I don't agree on that one- we have a lot of customers with Quads. I've heard his RM9 (an excellent amplifier BTW) against our amps on ESL63s a lot as a friend used (may he rest in peace) to have them.

@geoffkait  wrote: " Is A Stradivarius Violin Easier To Hear? Science Says Nope"

Maybe not to the average listener, but to a trained, professional violinist? Yes, absolutely! 

Many years ago I worked at the Banff Centre, which is where professional artists, musicians, & conductors go to hone their skills. Over the years, the Banff Centre has built up a sizeable collection of Steinway grand pianos (mainly through bequeathments). At last count (~2016), they had approx. 115 Steinways. Every Steinway had a name.

Musicians and conductors would be in residency at the Centre for anywhere from 3 weeks to 4 months. Some would specifically request their favourite Steinway by name.  

In 2004 I asked one of the piano techs (who have a contract with the Smithsonian in Washington to repair their Steinways) if someone could actually hear the difference between the pianos.

The tech told me that he did a blind sound test with Oscar Peterson in 1974. Oscar, who was the director of the new Jazz Program that summer, was being very picking in selecting a piano for the summer, so the tech arranged to have 20 pianos brought together onstage for Oscar to test. The tech blindfolded Oscar and then played each piano for him.

Oscar was able to correctly identify all 20 pianos by name, blind-folded. 

You obviously didn't read the article. They WERE trained musicians. That's the whole point. He-loo!

In fact I would love it if I could make the output impedance of our amps lower. The problem I have with solid state is that many semiconductors have a non-linear aspect about them that causes them to have higher ordered harmonics (at low levels, but as I pointed out earlier, the ear is very sensitive to that sort of thing) and hard clipping. The only devices that I have found that don’t are the static induction transistors made by Sony. IMO They had a chance to really set the audio world forward, but in true Sony fashion (which is to come up with an innovation and then shoot themselves in the foot) failed to make a full complement of driver and voltage amplifier devices to go with their rather amazing output devices.

Then when you add in high levels of feedback (there are various forms of feedback and I’m speaking in general terms), you end up with micro changes in transient values of the harmonic structure, all smeared out of time and out of level, shape, expression, and so on.

This is sometimes...the new detail found in some recordings. People mistake this false data as real and write the given amplifier up as being revealing.

Yet one good listen to the given amp by an aware person who has heard differently (and possibly understands what is going on), will drive said person from the given room with hands clamped on ears.

The brain and aural learning, generally moves forward. It is the cognitive speed and lack of aural projection (parallel aural paths in forms of known neural discernment and learning)..which makes the difference in this question and answer pairing (in it’s evolution). Ie, the brain has a masking and pre-load issue that it uses to keep aural experiences from taking too long to discern -it pastes in learned data on new and current aural experiences. Learning to defeat that mechanism takes time and awareness.

Or, for the intrepid audio explorer -"what has been heard, cannot be unheard".

Complex crossovers with low impedance considerations..exacerbate said issues, and make the given high feedback amplifier even more illiterate with fine transient function - and increase false harmonic structure. Which drives those who hear it, even faster.... from the given room. This falseness is, more importantly -- covering up real detail and real harmonics.

Using feedback in audio is tricky, at best.

This is cognitive function like learning and intelligence. It takes time to learn to discern these things, aurally. Some people ’get it’ sooner, some people ----never do.