Peavey Mark III Class D conversion - Noisy, maybe ground loop

[glebert]

I've been wanting to do a class D conversion for a while. Mostly don't want to spend $40 on transistors for a $50 amp only to have it still be underpowered. I have a 200 watt IRS2092 from Ebay (like this one: https://www.ebay.com/itm/IRS2092-Cla...frcectupt=true) and an 80's (?) Peavey Mark III 400BH head that was given to me as a project. Module has +/- power rails and ground, speaker output, and signal/ground input. I disconnected all the shorted transistors from the Mark III and attached wires to pull the +/- 52V rails to the module. Attached the module power ground and signal ground to the ground on the power supply board and took the input signal from where the signal enters into the power amp section, across R35 on page 3 of the schematic. I removed R73 to isolate from the rest of the power amp.

In this configuration the amp worked, but there was a nasty buzzing noise when signal is connected, dead quiet when signal is disconnected. Made up a cable so I could get the input from the preamp output jacks. Same noise. When I go to probe with my scope I don't see noise except on the output of the power amp. I get an essentially identical noise when I run cable from preamp out jack to another amp which is plugged into different outlet, noise goes away when I plug them both into same outlet, which makes it seem like a ground loop.

Tried putting an inductor in the ground to the class D module power connections. With that the speaker output oscillated wildly as soon as I connected the input to the preamp. I have not tried floating the ground of the input signal, one out of control speaker thrashing is enough for today.

Any thoughts?
Greg
MarkIII.pdf


MarkIII.zip

[Enzo]

Speaker has one side grounded in teh PV, but not for your class D, make sure you did not ground one side of th speaker with the jack

"ground" is not generic, WHERE did you ground the power connections and the signal to the amp chassis? The power ground should come from the point where the PT center tap and the main filter cap commons join. Signal ground should come from the preamp.

[glebert]

Speaker has one side grounded in teh PV, but not for your class D, make sure you did not ground one side of th speaker with the jack

"ground" is not generic, WHERE did you ground the power connections and the signal to the amp chassis? The power ground should come from the point where the PT center tap and the main filter cap commons join. Signal ground should come from the preamp.

Speaker coming directly off class D module for now. Currently using a shielded cable for input plugged into front panel jacks, so that should be getting ground from preamp board. Power ground coming off corner of board about 1" from PT center tap, but only doing metal on metal contact right now, should maybe solder it down.

[glebert]

Looking at the power rails with scope there is some noise there, even with module disconnected. I would totally think that needs new main caps, but can't figure out how the class D module could be totally silent when no input hooked up and then start letting the noise out once anything is connected to the input. There is a little trim pot there, I wonder if it is a sort of squelch circuit. Turning it all the way to one extreme did increase the noise, don't know that I did it with no input connection.