This is a normal 2 stage preamp topology. To measure its THD you must build an LCR notch filter first. A notch filter will only reject a single frequency. In this case 1Khz. Here the filter attenuation is -219dB. Once the fundamental is removed all the harmonics and diatorions remain. That is THD+N [Total Harmonic Distortion plus Noise]. After we've determined the THD level ~6.66mV in this case we take its RMS value and get 4.70862mV (0.0066Vx0.707). Now we have to calculate the RMS value of the fundamental frequency. In this case it's 707mV (1Vx0.707). Now we have to divide the distortion/fundamental. So 0.00470862/0.707= 0.00666. Almost there. Now we have to represent this number in percentage. Just multiply it by a hundred. 0.00666x100=0.66%. There you go this amplifier has a rough distortion of 0.66%. To increase the precision you can decrease the time frame by a third (3Khz time frame for 1Khz measurement etc) also design a very narrow notch filter with a very large attenuation of the fundamental. That's it.
