I came across an interesting article on the audibility of distortion, 05/73 Stereo Review. It might have been the first of the "Carver Challenges", but with a different focus. We are familiar with Bob's exploits at Audio Critic (cloning the transfer function of the Mark Levinson ML-2) and Stereophile (cloning the transfer function of a Conrad Johnson tube amp). In this test Bob and the editors wanted to test audibility of distortion using a) test tones; b) music. Participants were Julian Hirsch, Craig Stark, Larry Klein, and Ralph Hodges. The test was not double blind, but a 'critical listening' session. I will list the 'findings' as they were presented.
1) the Phase Linear 400 was compared to 'an excellent 300 watt amplifier made by a competitor'. The brand is not mentioned, but from a photo, along with the recent power amp comparison SR did a couple issues earlier, it is clear the amp was a Crown DC-300. The Phase had been modified to include a switch that would 'inject' pre-measured amounts of crossover distortion into the output, which was fed into a monophonic (single) AR-LST speaker.
First was a note on level matching:
When we were finally able to get the output levels of the two power amplifiers matched, there was absolutely no audible difference when switching between them while listening to either white noise or music. During adjustments...it was demonstrated dramatically that minute differences in volume level (sound quantity) ...are interpreted by the ear as 'obvious' differences in sound quality. Everyone was startled by this effect...
2) fed by an HP generator delivering a 60Hz tone, and driven to 1 watt, the amplifier was stepped up to 0.05%; 0.07%; 0.1%; 0.15%; 0.2%, at which point distortion became audible. At 0.7% the distortion became a separate buzz on top of the 60Hz tone. Increasing power output to 4 watts increased hearing sensitivity of distortion to 0.15%.
3) mixed 60Hz and 7KHz tones required the distortion to be at 2% before it was audible.
4) mixed 60Hz, 3KHz, 7KHz tones. Distortion was heard at 4%.
5) with simple music, 6% distortion became audible on voice. Guitar and cymbals showed evidence of audible distortion at 12%.