ground loop hum in diy amp

[Tala Mink]

I had a Pioneer SX-770 that had way too many issues to be worth repairing (only the amp, power supply, and phono were somewhat functional) so I pulled the parts and made a power amp from them.

After getting it functioning there was a noticeable hum in the speakers that sounded like a ground loop, the strange thing is it is only present when both channels have inputs, if only one rca is connected the amp is completely silent.

This is my first diy project so it is a little janky but I like how it turned out at least cosmetically.

 

[swechsler]

Check for good contact to all inputs jacks on the amp, and output jacks on the preamp. What you're describing is a classic ground loop symptom. Your patch cable has two grounds. There's a good chance there's some resistance on the shield in one channel, either within the cable or the jacks. You might also try a different cable.

 

[cademan]

I would just disconnect the grounds from the coax's and just use both black wires to your universal grounding wire only.

 

[Lavane]

What about adding an earth ground to the end of the ground buss?

 

[gadget73]

what does your grounding setup look like and what does the amp itself use for grounds? Some of those boards had power and signal grounds that were not meant to tie together. Looks like you have a shielded wire from the board to input jack and a ground back to the buss. Might need to lose the wire to the buss and just ground the coax to the signal ground on the amp board if its got one.

 

[rothwellaudio]

You have mains in there that isn't insulated. Fix that first before you kill yourself.

 

[swechsler]

That's a good point. Plus you have an uninsulated, unsupported high voltage fuse sitting right above a low voltage circuit board. That's a recipe for disaster.

 

[Tala Mink]

I figured it out, I put a 4.7 ohm resistor from the input ground to the amplifier ground.
I'm not sure what it does but I assume it stops the grounds of each channel from interfering with each other as its 5ohms to ground and 10 to the other channels ground.
if someone has an explanation for how it works id love to learn.

also i shielded the power input, i hadn't done that before because I wasn't sure if i wanted to keep it that way but now its somewhat safe.

 

[gadget73]

It breaks the ground loop.

 

[Stephen300]

Looks to me like you have the wrong wire fused on your IEC jack. The fuse always goes on the Hot (Live) wire and not the Neutral.