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Confusing problem with hum

Posted: Sat Jun 28, 2008 5:00 pm
by nismo
I have a QSC RMX850 amp power my sub. I have an HK AVR325 receiver as well as a Comcrap cable box. I had this setup in my old apartment and had no issues...but the RMX was powering my mains before. I had everything hooked up at the new place with no problems, but the moment the cable is plugged into the cable box I get horrific ~60 Hz hum. I thought maybe I was getting ground loop because I switched the cable box to feed composite video + stereo audio to the receiver. I unhooked both of them and it made absolutely no difference. I also tried changing the power cord from being bundled with everything to being out of the bundle and plugged straight into the wall (same outlet), and the hum didn't change.

The only thing that makes the difference is unplugging the cable box from the cable. First off, any ideas WTF is causing this, and second, any ideas on how to fix it?


Posted: Sat Jun 28, 2008 5:27 pm
by Derek
are you using the same wall outlet for everything? i had a ground loop issue with my qsc too....switched it to another outlet...bada bing. or make everything 3 prong..

Posted: Sat Jun 28, 2008 10:12 pm
by nismo
Can't 3 prong the Comcrap. I had everything in the same power strip, but switched the amp to the outlet directly and had nothing change. I don't have an extension cord handy to try another outlet, but I'll give it a shot.

Thanks D,

Posted: Sun Jun 29, 2008 4:08 am
by 95Honda
Cable TV outlets are notorious for introducing hum. Usually it is because there is AC that is on them from all kinds of places up the line.

Try this test, where you plug in the cable (or unplug it) that creates/kills the noise, take the end off the cable and strip it. Leave only the center conductor exposed (cut the shield back a 1/2" or so. Now stick the center conductor into whatever socket you were using before, just make sure the shield DOES NOT make contact with anything. You should still see a picture pretty much fine. Now, if the noise is gone or much lower by doing this, you know you have some 60Hz AC riding on your cable and it is wreaking havoc. You can put a capacitor(s) in series with the center conductor and shield and this will effectively block the 60Hz for the most part (use like 1uf or smaller). Or you could try an isolation transformer.

Try this and see if you notice any difference..

Posted: Sun Jun 29, 2008 4:03 pm
by KU40

Fixed my problem a couple years ago. It only costs like 4 dollars in parts so it's worth trying.

Posted: Mon Jun 30, 2008 12:31 am
by 95Honda
That would be an isolation transformer... :)

Posted: Tue Jul 01, 2008 4:44 pm
by KU40
ah, right, well, there you go!

Posted: Sun Aug 17, 2008 9:16 am
by eezip
Where is the audio source for the QSC coming from - the receiver?

You can also check the audio connections. Does the hum go away when you tie the chassis of the cable box/receiver and QSC amp together? Can you use the balanced inputs of the QSC amp and run it pseudo-balanced from the (presumably) unbalanced jacks on it's source (RCAs?).

Is the hum affected by the gain of the QSC amp?

Do ground lifts (3-prong-to-2-prong adapters) on anything eliminate the hum? Not exactly safe, but worth a shot.

Posted: Thu Aug 21, 2008 6:20 pm
by nismo
The audio signal runs from the Comcrap box to the receiver and from the receiver's sub output to the unbalanced inputs of the amp. Everything was hooked up and sounded great...then I plugged the RG-6 coax into the back of the Comcrap box and I instantly got a hum from the sub. The noise is gain-dependent...but only because when the gains match it is really bad, and when they're adjusted independently it gets quieter--but never fully stops.

The receiver has only a 2 prong plug, the QSC has a 3 prong. I haven't tried to ground them together. Would it tell me anything if I put a multimeter on resistance and metered chassis-to-chassis?