Ground loop issues

Hi. I have a problem with what I believe is a ground loop issue. The background is that I have a portable stereo which is dual battery-AC powered. A boom box, if you will.

This is a custom project of mine. I fitted a new amp module and the input signals was taken first from the volume control (via shielded cables) directly to the amp taking the original amp out of the circuit. But then I got this hum through the speakers. It is a steady and noticeable hum that can be drowned out if the volume is cranked up but when quiet, the hum is very noticeable. So I swapped the location where I took the signal directly from where the original amplifier chip was and there was no difference. As long as the signal wires are connected, there is the hum. If I disconnect all the signal wires, the hum from the speakers is gone.

The reason why I think it's a ground loop issue is because with the amp connected and powered by the boom box power supply -- if I feed an external signal to the amp from a portable CD player or ipod, basically anything not sharing the power source with the amp, then the audio is clear without the annoying hum.

Thinking it might be easier to avoid this problem by using the high-level (speaker level outputs) and converting to low level signal just before the new amp, that maybe I could correct this. But the issue persists.

Wondering if anyone has any thoughts or suggestions on this and what steps could be taken to correct this issue. It's driving me crazy!

 

Does it still hum when running off the batteries and not connected to AC? If so, that's not a loop, it's pick-up. Same noise, but it is getting in thru an amp's input.

Also, by connecting another signal source you may be grounding the device and thus decreasing it's sensitivity to pick-up. A loop would make it louder, not softer, when you connect more stuff.

 

No hum on DC and I'm pretty confident it's not "pickup" since the signal wires used are pretty expensive teflon shielded cables and I tried grounding both ends of the shield, one or the other end, or no end. The other sources I'm talking about where I get no hum are portable battery operated devices. The new amp only hums when connected with the boom box which shares the same power source.

I should also say that even if non-shield signal wires are used (plain old alligator clipped jumper cables) on the external battery-operated source, the audio is still quiet and hum free which makes me even more confident that it's not pickup.

 

OK, it sure sounds like a loop as you originally said. <grin>

I'm having trouble visualizing your set up. Can you make a sketch or take some pictures? What is this new amp doing, it it a replacement for the power amp so you can turn it up to 11?

 

Thanks for the replies Ernie.

I don't know how best to describe what I'm doing but I've drawn these block diagrams to sort of show how I did it. I described the old amp as HA1392, which is the amp chip module but it's mounted into a main PCB of course. And yes, you are correct, I want the volume control to go to 11 with better fidelity. As you know, boom box amps are rated at 10% thd at rated power. The amp I built is based on the TA8210AH chip and is essentially based on the sample and recommended schematic as provided by the manufacturer. It works great powered by a bench power supply connected to a portable CD, MP3 or iPod. I've also tested it with the onboard PS of the boombox and it works fine too. As the new amp is a bridged amp, it provides higher power at lower distortion and it appears to accomplish this admirably. All I need to do is figure out why it has this downright annoying hum.

Just a note: I did build and stuffed this exact same project into other such projects and did not have the problem. ONLY on this one.