'Differenztonfaktor' and tube amp superiority
Reply #4 – 2009-04-29 18:59:49
I did have a longer response, but I think most of it was covered in the time I was writing it and getting sidetracked away from my PC, so I deleted it for brevity.
I did think of a couple of things: 1) The difference in sound might just as well be down to the interaction of the output stage of each amplifier with the crossover and speaker's complex impedance (reactance and resistance). It doesn't appear they've eliminated such effects. Measurements made across a 10 ohm resistor aren't necessarily representative of driving a real speaker load. 2) To isolate and test the perceived quality of intermodulation distortion magnitude couldn't they use a digital source with DAC, say at 24-bits, and apply very simple DSP to the test signal. For example for each input sample, I (normalized to the -1...+1 scale), create an output sample O, where: O = ½(I + a.I²) The half is there to prevent clipping. I guess dividing by (1+a) or something would be better for level matching. Vary a and audition the results against a=0 (no mixing) through a very clean amplifier. Possibly use good headphones instead of a speaker with crossover to help eliminate reactance effects. Being digital it's possible to pre-process the same music samples two ways, level match, then run ABC/HR or similar. That way one could approximately establish the audibility threshold for factor a and by measurement, the equivalent "differenztonfaktor" This method acts like using a mixer (non-linear element that acts as a multiplier) to generate sum and difference frequencies, then attenuating them to limit the effect. You can find out more about this using google. It's widely used in superheterodyne radio receiver designs where it's desired. In audio amplifiers, it's considered undesirable distortion, at least at levels where it's audible.