Friday, November 4, 2016

Amplifier Voicing

I've been beginning to think that there is indeed a basic subjective difference among amplifiers which was aptly described as Yin and Yang by Harry Pearson.

The Aria amplifiers designed by Michael Elliot (best known as the founder of Counterpoint who made many innovative tube audio products in the 1980's) are available in a upgraded XL version with custom voicing.  When you get the custom voicing option (which adds $1499 to the price) Michael Elliot comes to your home to do the voicing himself, by making small changes to 4 resistors which are initially matched to 0.1%.  He says:

By making subtle shifts in the operating points of devices in the amps, I can shift the "tonal center" of the sound upwards, to emphasize upper harmonics, speed, and transient impact, or downwards, to provide a rich, pure midrange with velvety soft highs. 

 I find this idea to be very interesting.  I do seem to be able to categorize my amplifiers as Yin (the dark, the receptive, the mother womb) and yang (the bright, the fire, the rising white light).

But while most (including Elliot) claim to make their default "neutral," I am thinking there may be no neutral, just a possibly smaller tilt one way or the other.  Neutral is a point so small you never land on it.

Another thing I agree with Michael, high input impedance is essential a line level (and good amplifiers).
On this page he says higher is always better, never accept a line input stage with impedance less than 50k.  He chooses 100k for his line stage, and a mind boggling 470k for the Whole Tone power amplifiers.  My intuition would be to go for 75k on both.  Power amp inputs are not in most cases higher than line preamp inputs.  In most cases, preamps are actually buffered attenuators, reducing level somewhat.  Thus the power amp should not have a higher impedance corresponding to higher input levels.  But his super high impedance on the power amp probably helps move the effective voicing from slightly dark (typical of tube amps) to neutral.

Here's a discussion of OTL amps, which features the designer of Atma Sphere amps describing many things.  Most of the alleged downsides of OTL are prejudice which followed just a few bad apples.  Just a few of the dozens of "totem pole" designs by Julius Futterman and others were unstable.  Most have been rock solid, and other manufacturers using the Circlotron circuit, like Atma Sphere, have no stability problems at all.  And with Atma Sphere there is no loop feedback at all, total "class a2" operation--with a tad of sliding bias. I'd be very interested in how well those work with my Acoustats.  The only downside for me is rather high price.  If I had the time, the optimal approach would be a direct drive tube amp like the one designed by a friend of mine.  I don't think the direct drive amp originally made by Acoustat was very good.

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