60Hz hums...

I built a preamp and it worked fine and sounded nice and all, but when I went a built a secound (with the only difference being the use of LM7171s instead of LM6172) I get a 60Hz tone humming constantly. My questions is, where would this come from, the mains? So it'd probably be a grounding problem, right?

Usually I love a hummer, but this is just annoying...
 
Lately, there has been discussion of unbalanced ac mains. The result is a DC component. Also, an unsymetrical mains waveform may cause an imbalance in the rectified power inside the unit. In the past, when I have had a humming problem, I had learned from others to simply unplug the power cord and turn it over in the outlet. That way, the phasing of any irregularities becomes equal and the result is cancellation It did work for me, as simple as that. This is easier if the power cord you used is non-polarized. Otherwise, unsolder the internal power line connections inside the preamp and switch them, considering you built the unit yourself.
 
Take an AC voltmeter and hook it up between each supply rail and ground, and then between the rails. Do this at every point on the board when an op-amp is recieving power and ground, and at the power supply. You should read zero volts, the whole time.

This helps determing if your supply has ripple, or if the ground is weak anywhere. If you read zero between rails and a small reading when connected between ground and rails, then your grounding is bad. If you get a non-zero reading every time, you probably have major supply ripple... check the regulator wiring. Make sure you have a common return point for everything, including the power supply filtering capacitors.

If you tap ground from the capacitors, this can also cause hum very easily. One time I moved the ground return an inch on a power supply (away from the capacitors), and solved a hum problem completely.

Good luck...

JoeBob said:
I built a preamp and it worked fine and sounded nice and all, but when I went a built a secound (with the only difference being the use of LM7171s instead of LM6172) I get a 60Hz tone humming constantly. My questions is, where would this come from, the mains? So it'd probably be a grounding problem, right?

Usually I love a hummer, but this is just annoying...
 
Well, I tried probing for AC and I got nohting at all (it didn't even read any mV). It's rather puzzling because when I have the output connected to the power amp I hear very little hum (although it's still there when I turn it up) but when I connect the input and change nothing else the hum gets much worse. Also if I change the ground to the ground pin on the main power connector the hum is very bad compared to connecting the ground to the neutral pin (it's was originally hooked up to the neutral, but it's a multi-tapped transformer and when I hook up the other tap to the power-amp portion the ground and the 15v neutral pin are connected and the transformer goes nutty, could that have anything to do with it?)...
 
First of all, you shoudn't tie audio ground to the mains ground or nuetral. This could be a big source of hum -- which is what sound like your problem is since connecting the input worsens it (completing another ground loop). If you want to earth the enclosure, make sure no audio (or circuit) ground touches the case anywhere.

Try messing with the RCA's to find a possible ground loop. Pull left and right RCA halfway out so that the middle pin is still connected but the ground is not. If all of your equipment is sharing the same ground through the mains line everything should still work except with less hum. Individually push one at a time all the way in then half way out again to see when the hum starts. Does the hum stay even when the volume is all the way down?

Do this for both input and output sides of the pre-amp and let me know what happened. Do you have any schematics and/or pictures of your equipment?
 
JoeBob said:
Usually I love a hummer, but this is just annoying...
:p He he he.

JoeBob, I'm curious why there is a difference between your first and second preamps. If the wiring is absolutely identical, there must be a bad connection somewhere in the second. If they're not quite wired identically, what is the difference? Could you have something like a phono jack accidentally grounding to the chassis or something like that?
It really does sound like a ground loop with the experimental results you've posted. Hang in there, you'll get it solved.