Amplifier distortion measurement with REW

[Greg Dunn]

I have a few Class D amps I'd like to measure just as a learning exercise. My plan is to use REW to generate the stepped sine wave input and then capture the output signal as well, to get a THD+N measurement over the 20-20k bandwidth. I know this will require an attenuator because the output levels will be significantly above the sound card input limits. The amps are common ground and already have a filter at the output to reduce the PWM signal. Are there any other issues I should be aware of before proceeding?

 

[Greg Dunn]

Just to complete the thread, the answer seems to be "it works." I put an L-pad on the output of the amp, designed to minimize loading on it and reduce the signal to within the linear range of the measuring device (Scarlett 2i2) This is a test of the Crown XLS 2002 at approximately 20W output (I don't have the parts on hand to build a 40 dB attenuator to handle 100V).

Looks good to me!

 

[Antonio Di Motta]

Please note that some class D amplifiers work with balanced power output and therefore you cannot have reference to the ground signal, if you connect a branch of the output to ground damage the amplifier. Normally the class D amplifiers that have the output referred to ground are those that are powered in dual mode, for example + 30V 0 -30V.
Therefore, if you need to make a measurement on an amplifier that has an unbalanced output, you must use an operational amplifier to perform the balanced conversion - unbalanced.

 

[Greg Dunn]

This one has a normally grounded output common on both channels so there's no problem. Now, if I put it in bridge mode I will need to use the differential input of the Scarlett to measure the output. I plan to test that next time, after I construct a balanced attenuator.

 

[Greg Dunn]

I wanted to test this amp in bridge mode as well, which means I could no longer use the common ground. I wired up a simple balanced attenuator; though I didn't have resistors with the appropriate power rating to create one with sufficient attenuation to test the amp at high power, so I had to settle for a proof of concept measurement at 2W. The Scarlett mic preamp (XLR differential input) worked very nicely to measure the bridged amp outputs, and the amp was unruffled. I also belatedly realized that REW had a display option to show distortion directly on the axes.

Anyway, very good results from the Crown as hoped. REW is my new tool to measure electronics as well as speakers.