Pass Labs X250 owner manual

Rushing to market in the 70’s with their low TIM distortion designs, companies employed either simpler or more complex circuits to achieve high slew rates. The amplifiers that had simpler circuits with fewer parts tended to sound better than the amplifiers with complex circuits and a lot of parts. They also cost less and broke down less often, not an unimportant benefit.

Thus was a great principle of audio amplifier design reborn. Like the principle of Occam’s razor, if you have two amplifiers with similar performance numbers, the simpler one will sound better. Often the simpler one will sound better even if its measured distortion is higher.

Looking back on my amplifiers, I see a steady progression of simpler and simpler. Like the products of other young designers, my first commercial product had everything but the kitchen sink in it. Now I strive to be like Picasso, who could draw a woman with a single pencil stroke and create a masterpiece.

Supersymmetry is not a single pencil stroke, but I am making progress. Its origin goes back to the late 1970’s when I was examining the virtues and faults of so-called “error correcting amplifiers”, an alternative to conventional feedback. In this approach, two amplifiers, a big one and a small one work together. The big one handles the big job of delivering power to the loudspeaker, and the little one sweeps up after it. The big amplifier, not having to worry about the details, delivers power like a supertanker crossing the ocean. The little amplifier is like a tugboat, which nudges it precisely into port. The concept is a good one, much of the credit going to Peter Walker, but it is a bit more complicated than we might want.

Thoughts about this approach on my part led to the Stasis amplifier, a simpler, if cruder, circuit in which the ocean liner could just about make it into port by itself with only minor damage, and the tugboat was capable of crossing the Atlantic, if not the Pacific. Threshold and Nakamichi have sold lots of these amplifiers for the last 19 years or so, and so it was pretty successful.

Yet it was always in the back of my head that there must be a better solution to the no- feedback performance problem, something even simpler and more elegant. I felt that symmetry and anti-symmetry in the character of signals and circuits held the key, but not having any idea how, I amused myself for the next 15 years by drawing topologies which might do something in this vein. One day in 1993 I drew a picture connecting two transistors, each with local feedback, and the concept fell into place. The following year I received a patent on the design.

The concept is actually very simple. Conventional feedback, local or not, is used to make the output of the circuit look like the input. In this circuit, feedback was not used to make the input look like the output in the conventional sense. Instead it works to make two halves of an already symmetric balanced circuit behave identically with respect to distortion and noise, dramatically lowering the differential distortion and noise but not the distortion and noise of each half of the circuit considered by itself.

If you build such a symmetric (balanced) circuit, you get much of this effect already. If you drive a matched differential pair of transistors without feedback with a balanced signal, you will see a balanced output whose distortion and noise is typically 1/10 that of either device alone, purely out of cancellation. With supersymmetry, the same differential pair’s characteristic can be made so identical that the differential output will have only 1/100 the distortion and noise of either device alone.

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