I’m using the QA402 for the development of a preamplifier with phono input. Measuring the phono inputs with the QA402 has some challenges which need to be considered.
A phono preamp has a gain of typcally +40dB for a MM cartridge or +60dB for a MC cartridge (@ 1kHz). Therefore the input to the preamp needs to be low level (e.g. 5mV @ 1kHz = -46dBV). The QA402 output stage has an output attenuator followed by an output driver stage. For such small output signals, the output driver stage adds noise (or the internal -30dB attenuator adds the noise). Therefore, the THD+N and SNR measurements are not accurate. Using a -40dB (9.9k Ohm + 100 Ohm) or -60dB (10k Ohm + 10 Ohm) passive attenuator to feed the preamp input gives better results.
The frequency response of the phono preamp is non linear according to RIAA (or IEC). The weighting feature of the the QA402 allows easy evaluation of the frequency response deviation of the preamp. The standard RIAA playback curve file of the QA402 has relatively low frequency resolution. Therefore the accuracy is not very good if measurements with high resolution (high number of FFT bins) are made. I used a higher resolution RIAA playback file (about 80 frequency/gain pairs) for my measurements. I can share this file if someone is interested.
Frequency response of the QA402
A second little issue is the frequency response of the QA402. At 20kHz, the frequency response is down -0.25dB. While this can be compensated using the right channel as a reference for linear gain amplifiers, I have not tried this yet for the phono preamp. As an alternative, I consider adapting the weighting file accordingly which would allow to make a two channel frequency response measurement of the preamp.
Overall the QA402 is an excellent device for measuring audio equipment such as my preamp. Well done Matt and team!