Hi @Moto, I didn’t know Topping had a balanced DAC. I just ordered to try.
To pencil this out a bit, Amir’s measurement on THDN was achieved at 4.2Vrms, which is 12.5 dBV. That means each leg is driven at 2.1Vrms which is 6.5 dBV on each. The QA403 has inputs of +6 dBV or +12 dBV.
So, I’d first go in single ended at maybe 1 dBV below 6 dBV on the +6 dBV input range and see how that looks (which means D10B out+ into QA403 in+, with in- shorted). Take a look at N-D and also 2H and 3H, and adjust up or down a bit to improve the number. Once you’ve found the optimum, connect the out- to the in-, and you signal should increase 6 dBV but your noise will only increase by 3 dBV. For that reason, click to the +12 dBV range when in balanced mode. Also, when you go balanced, the 2H should disappear.
Some things to be aware of:
Short the inputs on the QA403 and measure the noise on the +6 dBV input range. That noise will probably be around -113 dBV or so (rect, 128K fft size). That is means the noise is about -119 dB below the full scale. That is the best your THD can be considering the noise. In practice, it will degrade as the ADC noise rises as input rises.
Next, using the QA403 generator, start with the generator at 5 dBV, and the input range at +6 dBV. And then slow sneak up (using alt+click on the AMP1 button) until you hear the overload relay start clicking once per second. And then back off 0.1 dB. On my unit, note that I’m in the +6 dBV range, and yet the overload point is 7.3 dBV. The relay will click when overload is within 0.5 dB of dBFS. Now, with this overload point established, set that 0 dBR. And then do an RMS dBR.
On my unit, that gives -120.15 dBr. The point here is that even though the spec max input of the QA403 is 6 dBV, the real max input for that level is about 7.3 dBV. So,if you are trying to squeeze all you can squeeze, then in some cases your input signal could be +6.1 dBV on the +6 dBV range.