Quad ESL-63 and low-powered amps; Sun Audio, Atma

So as long as a well designed amplifier is not driven into clipping it should sound the same as another one not driven into clipping.

This is a pretty big assumption. What is meant by ’well designed amplifier’?

Designers have different goals so their amps are going to sound different. The reason is twofold: output impedance and distortion. All amps make audible distortion- that’s probably the first thing that should be a given, but so many people think that since an amp has 0.005% THD its distortion is inaudible, and that simply isn’t so.

The reason for *that* is that the human ear/brain system uses higher ordered harmonics to sense sound pressure and to this end is more sensitive to these harmonics than most good test equipment. This is complicated by the Fletcher-Munson curve, which shows that our greatest sensitivity is in the upper midrange - birdsong frequencies. Many instruments have fundamentals that are much lower than that, so if harmonics due to distortion show up in this region we’re tuned to hear it.

Most amps that have very low THD also have distortion signatures that are almost entirely composed of higher ordered harmonics. Some of this is on account of the feedback used- while it suppresses distortion, it also adds some of its own (see the writings of Norman Crowhurst; this should be no secret as he was writing about this 50 years ago).

The human ear converts all forms of distortion into some form of tonality- we’ve known this since the 1930s (see the Radiotron Designer’s Handbook, 3rd Edition).

My goal as a designer is to minimize the distortion product to which the ear is most sensitive- IMD and the higher harmonics (5th and above). Others seek to have flat frequency response with overall low distortion, but I’m here to say that doesn’t work out so well. If you don’t pay attention to how the ear perceives sound, you will wind up with colorations- specifically brightness and harshness. That’s a coloration as much as the ever loving 2nd harmonic that so many tube amps have. The ear has tipping points where it will favor distortion over frequency response, so you have to be careful!

Just my opinion of course... The majority of the audio world, which is ’mid fi’ to the high end market, does not agree. In that world, a Pioneer is as good as a Pass for a lot less bucks because the specs don’t lie, right? The problem is that we audiophiles have been lied to for a very long time and we each intrinsically know that we still have to audition the product no matter what the spec sheet says. That is because the spec sheet isn’t based on how the ear/brain system works, its based on how the eye works and how a good story can propagate even when its not true. In a nutshell, most spec sheets are an excellent example of the Emperor’s New Clothes; a lesson which seems to be lost on so many these days...
Bottom line: well designed amps are not going to sound like each other. You have to know the designer’s intention. One might want it to look good on paper, another might want it to sound like real music. That’s a **huge** difference, yet both can be ’well designed’!!

I know nearly nothing technically, compared to Atmasphere, but my experience certainly confirms what he said. I have owned many amps that I imagine most people would consider well-designed, and they sound so different from one another. I know this is an age-old debate, but I have always assumed that there are measurable differences that we just have not figured out how to.measure, such a s a measurement that would gauge how an amp performs with music vs test signals. 

Atmasphere, do you think the differences could be measured someday, but we have not figured it out yet?

@atmasphere I am hoping to build your kit monoblocks someday, perhaps to run the 63

@montaldo yes i have tried calling twice today, no answer, no voicemail- it is summer...i sent an email....nice website, boards, etc...certainly looks like expertise and a valuable resource for sure....

 here is Kent email if you want to try that: esolution@estatsolution.com

Atmasphere, do you think the differences could be measured someday, but we have not figured it out yet?

We can measure them now, but we don't go about it right.

Essentially what is needed is a weighting system; one that applies more importance to the higher ordered harmonics since the ear is so much more sensitive to them and places less weight on the lower orders since the ear doesn't care about them so much. Then the spec sheet can tell us how the equipment sounds.

Horrors!

The industry is a long ways away from that right now. The way our ears detect sound pressure is sort of an inconvenient truth. Easy enough to prove though and with really simple test equipment...