Help finding hum problem

[wigginjs]

I recently added two circuits to my stereo leach amp (which I'm using as a subwoofer amp) and after installing them the amp creates a hum with no input (and with input). The two circuits were a Linkwitz Transform and a Graphic Equaliser, the hum didn't exist before I implemented them. I'm powering them from the DC rails of my leach amp (provides +58V, -58V) and it's being regulated down to +15V, -15V by a zener regulator (shown in the bottom right corner). At the bottom is a picture of how it is implemented. The ground for the zener regulator is the same AC safety ground used for the leach amp's supply, this is also used as the ground from both circuits. Is this a ground loop? How can I get rid of it?

[Mr Evil]

You may have a loop there, from the power ground of one circuit, through the signal ground connection to the second circuit, then back down the second circuit power ground, if power and signal ground are connected on the PCBs.

If that is the case, then break the signal ground connection between the two circuits (if the signal connection is made with shielded cable then disconnect only one end).

[wigginjs]

Geez, thanks a ton, it worked like a charm =). No more hum. I'm impressed =).

If you don't mind, could you explain why the hum developed?

[Mr Evil]

A ground loop, being literally a loop, is very effective at picking up the inevitable 50/60Hz radiation like an antenna. They can be mystifying, frustrating, and are unfortunately a subject not often taught in formal engineering courses, but there is a lot of helpful information around on the internet, which you can find easily enough by searching for "ground loop" on Google.

When you lay out a PCB then you should try to keep power and signal grounds seperate. This makes it a lot easier to avoid loops, and also helps avoid other methods of inducing hum and noise.

[wigginjs]

This may seem obvious, but I want to add it to the thread just to be more thorough. I moved the input signal wires going to and from the linkwitz/equaliser further away from the leach amp powersupply and the hum decreased even more. Clipping the ground between the two circuits made it go down dramatically, but I could still detect hum if I turned the volume way up and put my ear to the driver. Moving the signal wires got rid of that, now if there is hum it's barely a whisper with your ear against the driver and full volume. Thanks again Mr. Evil.