Had a few conversations with Amir and I think I have got to the bottom of the 60Hz issue. Its an interesting one
!
I have performed a few more tests this morning and I can replicate the issue Amir sees. It does appear to be related to the measurement system screen connection.
So, whilst the amp input is balanced, the output is ground referenced. So the modules speaker out negative is connected to the whole system screen, xlr pin 1 and the chassis. Being powered by an SMPS I have no doubt there are mains leakage currents through Y caps into the ground, chassis and therefore the speaker negative.
In Amirs test set-up my understanding is that the AP input balanced screen was not connected. If I do the same I get a very similar problem.
Excuse my lash up test system.
measurement screen disconnected - note its 50Hz mains here in Oz as opposed to 60Hz in the US.
screen connected to speaker negative (system ground reference) - problem gone!
Note that there is no mains safety earth connection, the Hypex modules are class II double insulated and amp construction follows relevant guidelines. I went this way to minimise the potential for ground loops from single ended source connections. Note that a mains safety earth connection may reduce the issue but is not a correct and potentially problematic solution. See below.
If I release the measurement screen but connect my scope ground clip (which as you know will be connected to mains safety earth) to the amp ground (in this case the screened cable chassis connection) the problem also disappears. Interesting to see some 8kHz usb packet noise spring up
This will be due to the screen connection going a torturous route through the mains wiring, back to the PC and probably through the USB screen to the measurement system.
So on the face of it this seems a similar to the problem you can get connecting single ended audio sources into balanced inputs. Its really a measurement anomaly and not an amp fault or design issue. It's really the age old "ground loop" problem. It won't manifest in the speaker.
Hypex talk about this where the screen (in this amplifier audio input instance) should be taken to the far source end and terminated with the balanced low so the ground noise currents dont flow in the signal low. In our example the measurement system is the "amp" and the P502 is the single ended audio source
BTW it should be noted that unless you know the amp design you need to be cautious connecting screen to an output. I gather that some class D designs are floating and can go bang! So sensible starting point for Amirs testing.