Hi,
Yes i didnt think you'd want to buy one. That's very understandable.
The adjustments for the nulling would of course include amplitude, so that you can null out the first harmonic whatever amplitude it happens to come out of the amplifier under test (AUT) at. If you input say 1v and you get 2v out, then you've got to be able to null out 2v or if attenuated back down to 1v then you've got to be able to null that out. The amplifier could very well introduce some phase shift too however, so you would have to be able to adjust the phase shift. Just in case the amp puts out an inverted signal you would also need a 180 phase shifter which could be done with an op amp as an inverter.
A regular low pass filter provides a phase shift, and a two or three stage low pass filter provides even more of a phase shift. You would possibly use that plus an op amp amplifier to adjust gain. The low pass filter resistor(s) could be a potentiometer and that would give you phase adjustment, or you could look into an all pass filter. Either way, you would get phase shift adjustment, and adjusting the output attenuation (perhaps a pot too) you could null out the signal generators frequency out of the output. That would leave just the junk which would be taken to be the distortion.
To test you could probably introduce a small phase shift with another low pass (in place of the AUT) and see if you can null it out. You might also start with say a 1kHz signal and mix in some 60Hz or 50Hz signal of lower amplitude, then see if you can null out the 1kHz and see only the 60Hz (or 50Hz) signal. Just in case you want to test your setup first.
A low pass filter can be made with a resistor and capacitor. Changing the value of cap and/or resistor changes the phase shift. The amplitude also goes up or down too though depending on the desired phase shift, so you have to readjust the amplitude.
I think the best way to look for a null is with a scope. Maybe a band pass filter too so you can concentrate on the first harmonic.