Measuring output power of class D amp

[diydidi]

Hi all. I want to use my scope to measure output power of a unfiltered class D amp. It has floating outputs. I'll obviously have to un-ground my scope as I dont have a diiff probe. BUT, thats ok. My main problem is the high freq riding on top of the inputted sine wave. Anyone have a nice filter circuit to get rid of this?

[J M Fahey]

I measure them all day long without need for special equipment.
1) "the outputs are floating" *really means that *the load* is floating; or to be more precise, no load terminal is grounded *but* each of them receives a signal which *still is* referenced to ground.
It isn't as if signal came from a floating transformer winding referenced to nothing !!!!!!!!
So I just clip my scope fron one hot terminal to ground (nothing special here) and drive it to barely clipping, then back up a little.
Then check the other terminal, same thing.
If I have a regular run of the mill dual trace scope, can check both sides at the same time, go figure, always referenced to ground and with no wild grounding or floating setups needes, at all.
Then I measure both voltages (referred to ground) and add them up. Piece of cake.
No unsafe ungrounding, no diff probes, nothing special.
2) suppose you are measuring audio power with, say, a 1KHz tone.
suppose your amp switching frequency is, say, 150KHz.
Use an RC filter at, say, 15KHz , such as, for example , 1Kohm/.01uF .
It will be easily driven by your power amp, will not be loaded by your scope's high input impedance (usually 1M) , will attenuate the 150KHz signal by around 20dB (or 10X) which should be more than enough.
3) why unfiltered Class D? Not a good practice hitting your speakers with that kind of HF energy, not to mention FCC will kill you for polluting the airwaves.
Just sayin'

[Steve Conner]

The official filter is a massively complicated design that goes by the name of "AES17". http://ap.com/products/accessories/aes17_filters

All published distortion measurements on Class-D amps are (or should be) made with a filter meeting the AES17 spec. In practice, something like JM's RC filter is more practical for a one-off job.

Some cheap powered speakers use an unfiltered Class-D amp, relying on the voice coil inductance to smooth the current, and the lack of an extension speaker jack to ensure EMC. But we are talking something like the speakers in a cheap TV set here.

[km6xz]

Yeah, all that Juan wrote but also you probably have a good diff amp scope input if you have a dual trace scope. Connect a and b channel probes to the output leads and press the 2nd channel Invert button and set the vertical amp mode switch to ADD. Set both defection factor selectors to the same appropriate range.
Some small Class D modules use the inductive reactance of the speaker/earphone as a low pass filter so if you are using a dummy load use your own filter. The filter's performance is an important design element of a self-contained Class D amp so use the one recommended by the application notes of the semiconductor's manufacture

[diydidi]

Thanx guys! Yes after posting this thread, i did some experimenting. Did the dual scope, add, invert one channel thing, but the trace still has the hi freq on it. So I did the filter ( 1k, 4,7nF) on each line. Perfect clean trace..