Measuring electronics (e.g., amplifiers) with REW

This topic is gaining popularity, yet there is still a fair bit of uncertainty out there, so I thought I would seek clarifications.

First, the set-up:

1) Akitika 1 kHz oscillator as the source, connected to the amplifier using a single RCA cable. The other amplifier input is shorted.

2) A cable from the amplifier output to the 3-pin XLR input 1 of a Scarlett 2i2 audio interface.

There is no volume knob leading into the amplifier.

There are 3 knobs on the 2i2: 1) A big knob marked "Monitor", 2) a gain knob for input 1, 3) and a gain knob for input 2.

I calibrated the audio interface in REW and the preferences pane looks as follows:




During the calibration process, I had to turn up the "Monitor" knob so that input level would match output level, as indicated by the Out/In gauges.

Does that mean the "Monitor" knob should be left at the same position for the calibration to remain valid?

Now, in the RTA window,




Should there be an extra calibration in place for the input?

and




Should I uncheck "Get fundamental from generator"? Since the Akitika oscillator generates the signal instead of the REW generator?




Should "Ref resistance" be adjusted to match the value of the dummy load?




Finally, here, how to use this "FS sine Vrms" and the associated "Calibrate level" button, when the source of the signal is the Akitika oscillator?

Some people say it should be the Vrms value being output by the Akitika oscillator, some people say it should be the Vrms value being output by the amplifier itself, across the dummy load, before attenuation.



Arguably, not everyone can be right.

I wouldn't mind getting the facts and writing a guide for this.

Another theory on the Internet: “FS sine Vrms” should be the maximum Vrms the input of the 2i2 can accept.

trobbins said:

Please clarify what the 2i2 output channel is doing, as you seem to want to use an independent signal source ( Akitika 1 kHz oscillator) ?

Click to expand...

Hi Tim,

In my set-up, the left and right output of the 2i2 are unused.
As you said, the oscillator is the signal source for the amp and the (amp + load + attenuator) output is the input to the 2i2 input 1.

Based on your reply, I should measure the RMS voltage coming out of the (amp + load + attenuator) and use it to calibrate the input in the RTA window.

Correct?

Isn’t that the same as taking the maximum input level in the 2i2 specs sheet and entering it in the box without clicking the Calibrate button?