Past few days I was playing around with my measurement gear and noticed something a bit unexpected with the E50 - it seems that the level of H3 in both channels depends on whether the other channel is playing the same signal or not.
Let me illustrate; first the results when both channels are playing the test tone:
View attachment 340134
H2 and H3 are both around -130dB and therefore look pretty similar to what
@amirm measured in
#1.
However, if I drive only one channel and keep the other silent, H3 in the measured channel goes up by ~10dB!
View attachment 340135
View attachment 340137
Note that I get the same results even if only one channel is physically connected between the DAC and ADC.
Enabling the disconnected playback channel in the DAC (e.g. by enabling the secondary generator channel) improves the measured H3 in the connected DAC channel by ~10dB (compared to only one channel being driven). This IMHO shows it is not an issue in the ADC (nor the Scaler), but how the DAC itself functions.
These remain stellar measurements even so, just notice that SINAD when driving a single channel now becomes distortion-dominated (rather than noise-dominated) and should be around 118dB.
My theory is that the E50 THD calibration was optimized for best results when both channels are driven and that this is an expected artifact of it, but I'd be interested to hear thoughts of others who may be more experienced in the topic.
@amirm Have you perhaps seen such behaviour in other DACs before? Thanks!