Things rattle around in your head (they do in mine anyway), then eventually they fall out.
I'd really like a straightforward way of measuring THD, preferably one not involving spending a great deal of money.
It occurred to me to write a program to create some .wav files with tones at 50Hz, 1k and 20k, with the left and right tracks 180 degrees out of phase.
Then build a simple very-low-distortion summer with 2 inputs with amplitude controls.
Pass one channel through the amp and thence into the summer. Pass the other direct into the summer. Adjust the amplitudes for the minimum residual.
I'm fortunate in possessing a USB oscilloscope. It does maths and has a function showing instantaneous RMS. This would make it fairly easy to adjust for the minimum residual. You'd set the RMS value of the DUT output fundamental to a whole number, after that it's just arithmetic.
The minimum vertical resolution of the 'scope is 10mV/div., so some additional gain after the summer might be desirable.
Maybe I'd be better using a second PC and soundcard set up as a spectrum analyser. I don't know if I could get one PC running 2 pieces of software to talk to 2 soundcards.
Obviously there's a phase error (delay) caused by passing the signal through the amp. This might not be the end of the world, though. I'd prefer not to be messing with the relative phase of the 2 channels since the outputs will be closest to identical if they are inversions of one another, but writing a program to output the signals to a soundcard in real time with phase shifting in small increments is only a bit more difficult than writing a program to output .wav files.
Anyway I'm going to take a stab at it and see how far I get.
I've drawn up a circuit for the summer. I'll get around to a PCB in the next day or so.
The first thing is to see how good the nulling is between the channels without a test item in place.
Anybody got any thoughts or comments? Anybody know what order the delay through the amplifier is likely to be? Of course I can measure that with a square wave.