Originally Posted by DonH50
Most of the audible differences I have heard/measured are more related to output impedance than distortion, especially at low levels. Hiss can be an issue if you've too much amp (or too low SNR) for the speakers, e.g. high-power amps and horns six feet away are usually a poor match.
I can tell you from a lot of testing with people, All 3 of my amps have the exact same output stage OPS, in fact, when I layout pcb, I use the layout of the last amp and just modify the frontend circuit, I did not even touch the OPS side of it and fab the pcb. Everyone is the same 9 pairs output transistors with 100mA per pair, 3EF stage. The power supply is exactly the same, they all sounded different and people have no problem distinguish the 3 within a minute or so. All have THD below 0.005%@20KHz at over 50W, and 0.0015%@1KHz.You would think at this low THD, they should sound the same, they are NOT. No spec can predict this.
On my 4th amp I spent over 7 months design, text and built. I put in the error correction into the OPS to further up the damping factor and reduce the crossover distortion. I use the same frontend as the 3rd amp unchanged. We could hardly hear the difference in testing!!! It's so disheartening!!!
You would think OPS govern the sound, I don't see that. You would think the front end is just like the front end of the opamp with differential pairs and they should not matter. It matters
!!! I did not believe this, but it's proven from real life testing with people. There is a lot we don't understand.....at least I don't understand. I don't see direct connection between scientific measurement and sound.
That's why it's laughable when people say amps all sound the same as long as they are not clipped. Here I am, working over 3 years, hours a day designing, measuring, testing and listening like as if I am working part time in my real career, NONE of my amps including the PA-7 and Acurus sounds the same. They are all WAY below clipping level. I really question either they don't have the ears or just believe so firmly that it doesn't make a difference that they don't even listen.