Sokrates Measurements



	Common Mode Rejection:

This picture shows the common mode gain of the input stage. Since the differential gain is unity, the common mode rejection ratio (CMRR) is the inverse of the common mode gain.

>100dB suppression in the 50Hz...100Hz region is a pretty darn good value for an OPamp based solution. So the resistor matching work has paid off. With an audio transformer you could reach up to around 120dB CMRR in that frequency range.
But also the close to 80dB @ 20KHz are outstanding and such a high value is typically not even reached by transformers (but please take note that this measurement has been performed under lab conditions with no source imbalance).

Phono Amp:

With Arta it seems to be impossible to measure the freq. response of the entire phono section from the stylus to the output of the RIAA amp using a special test LP. Also, since I have no idea how the disc that I could use (Clearaudio Cartridge "Break In" Test Record) has been cut, I cannot know exactly what amplitude and frequency response to expect if I used the swept sine or pink noise track. Instead of dealing with two unknowns, I simply measured the FR of the equalization amp and compared it to the RIAA and IEC responses. Those values are available here.
I have not implemented the IEC amendment but added a 20Hz 3rd order Butterworth rumble filter and a 50KHz 2nd order Butterworth LP to avoid slew-rate issues from scratches and debris further down the signal path.


	As can be seen, there is a close to perfect match above 100Hz. Below 100Hz the biggest deviation from the RIAA response is around 0.5dB.


	Also, and that is actually more important than the absolute deviation, the left and right channel are very consistent.

The left graph below illustrates the noise of the entire phono section. For that I used track 4 (no signal) of the test LP.
The increased noise floor mainly below 1KHz is purely noise from the movement of the stylus in the groove (~30dBV) ! If the stylus is lifted, the noise floor decreases but then nasty frequency components from the mains are picked up. Anyway, this shows that the noise contributions of the OPamps, the eq. and filter stages are very reasonable.


	In total, I am very satisfied with the operation of the phono section.

Main Pre-Amp Section (buffer + output stage):

All measurements have been recorded using my sound interface in dual channel mode, which imposes a load of 1MW to the output stage of Sokrates. This is a very small load but in the real application the output will not see a resistance smaller than 20KW, which is not a significant load for an OPamp either. As a result the distortion will not rise noticeably.
0dBFS corresponds to 3.2V peak.
Also these results are very satisfying and this is yet another proof that there is no good reason to be afraid of OPamps in general.


	Intermodulation Distortion:






	Wideband multitone test with 1/3 oct. spaced sine signals.


	Harmonic Distortion (generator @ -6dBFS):








		Last updated 03-Feb-2019