A TAP-X owner recently asked me if I would like to have a listen to his unit and and be willing to take some basic measurements with the dScope. I agreed and those are presented below. Unfortunately the listening portion was a little more costly. I missed it so much after shipping it back I had to contact John Chapman and order the parts to build my own.
The unit I measured was balanced, so all measurements were made in balanced mode with the TAP-X set to "54", which is unity gain. I suspect most measurements would actually improve at lower gains but I have not tested to verify this hypothesis.
This is an FFT plot, 2V RMS input. While (auto)transfomers are not active devices, there is some distortion. The good news is that even the highest distortion product is 2nd harmonic and is -108dB below the fundamental. All the really nasty high order stuff is even lower and over 120dB down from the fundamental.
This is the THD+N vs frequency at +4dBu. Since the level is fairly high, the plot is above the noise floor and essentially shows just the distortion.
This is the intermodulation measurement, 0dBu input level at 19 and 20kHz. The 1kHz difference product is about -105dB down and equates to .0005% IMD.
Frequency response. Note the full scale of the plot is +/- .1dB, so the TAP-X has less than .01dB deviation from 10hZ to 10kHz and at worst is up ~.02dB at 30kHz.