Interface calibration and Preamp distortion (THD) measurements (not room acoustics)

[rpschultz13]

Hi, new to REW. I'm using REW through a Motu M4 on a Mac.
I'm testing an acoustic guitar preamp that I've designed, it has TS input/output.

• I read (AI) that going from unbalanced to balanced I should float the RING of a TRS on the OUTPUT of the M4 into preamp, but simply use a TS cable between the output of the preamp and the INPUT of the M4.
• I calibrated it at -12dBFS, I measured 72.5 mV. This seems low, and in the generator it won't let me put in a signal more than 289 mV, which is 0 dBFS. M4 says it can output +16 dBU which is +/-3.6V. Why won't it let me output more? Not sure I need to, just wondering.
• After doing this calibration, I measured THD at 3 different preamp gain levels (top left knob), see below.

I really have no idea what I'm doing. What should I do differently or better. What else should I be looking for?

Thanks!

 

[rpschultz13]

Wow I gotta say doing bode plots on REW is much easier than my scope and AWG!

 

[John Mulcahy]

I calibrated it at -12dBFS, I measured 72.5 mV. This seems low, and in the generator it won't let me put in a signal more than 289 mV, which is 0 dBFS. M4 says it can output +16 dBU which is +/-3.6V. Why won't it let me output more? Not sure I need to, just wondering.

That does seem low. The generator calibration relies on your own measurement of output level, so assuming that was the 72.5 mV it should be accurate. Perhaps the Motu doesn't like half the balanced output being shorted to ground by the TS cable.