Take two SS amplifiers that produce a perfectly linear response from 20Hz to 20kHz. How is it they can sound different? My hypothesis is the answer lies in the amount and type of harmonic distortion, and what mix of harmonics dominates the distortion profile. Conventional class AB SS amps tend to be low in distortion overall, with the third harmonic the highest in level. That’s why it’s often difficult to tell them apart in a blind, level-matched comparison.
The less conventional sliding bias and zero global feedback designs (Threshold, Coda, Ayre etc) that produce more “bloom” in the midrange tend to have a more complex distortion profile in which the second harmonic is often nearly as high or even higher in amplitude than the third. I think it’s for that reason that such amps can sound remarkably similar to a push-pull AB tube amp. The same applies to some class D amps. A couple class D amps I recently had in house (Bel Canto E1X and Axxess Forté 1), sounded remarkably similar in the high freqs to an average KT88 or EL34 push-pull amplifier. Where they differed from a typical tube amp is they didn’t have the elevated midrange and upper bass response. Turns out those amps have higher 2nd order distortion than a typical SS amp, especially the Forté 1, which happens to be 2nd order dominant according to one magazine’s bench test.
Parasound’s John Curl amps also tend to be second-order dominant. IME, some of that midrange bloom comes through in their sound when matched with a transparent preamp. Though they are generally low in distortion and noise, they sound considerably warmer than an amp like the Benchmark AHB2. After living with an AHB2 and many other designs, I have come to the conclusion that our ears (if still healthy) are far more sensitive to distortion and noise than was claimed by many industry objectivists. That isn’t to say it’s the best sounding amp subjectively, rather, that you can hear the difference when A/Bd against more conventional designs.
Personally, I find that relatively affordable amps can be remarkably good these days, when they have the right synergy with the paired speakers. The amplifier sound quality/cost ratio is quickly catching up to that of DACs and sources. We are already past the days where one need spend the equivalent of a good used car to get true high performance amplification.
