Confusing problem with hum

General discussion for all questions based in the home audio/video world.
User avatar
nismo
Posts: 6120
Joined: Tue Jan 16, 2001 1:00 pm
Location: Greenwood, IN
Contact:

Confusing problem with hum

Postby nismo » Sat Jun 28, 2008 5:00 pm

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?

Eric
User avatar
Derek
Posts: 20753
Joined: Fri Oct 22, 1999 12:00 pm
Location: in marshall's bunghole
Contact:

Postby Derek » Sat Jun 28, 2008 5:27 pm

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..
Here in death's dominion
Where even shadows die
I am the one almighty
You will cry my name
And when you seek forgiveness
You will see there is no god
And for all eternity
You will cry my name
User avatar
nismo
Posts: 6120
Joined: Tue Jan 16, 2001 1:00 pm
Location: Greenwood, IN
Contact:

Postby nismo » Sat Jun 28, 2008 10:12 pm

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,
Eric
User avatar
95Honda
Posts: 1691
Joined: Mon Jan 05, 2004 1:00 pm
Location: Germany
Contact:

Postby 95Honda » Sun Jun 29, 2008 4:08 am

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..
http://www.forceaudio.com

Marshall asked me how much it would be to fly to Idaho....
User avatar
KU40
Posts: 6461
Joined: Sun Mar 16, 2003 12:02 pm
Location: Lawrence, KS
Contact:

Postby KU40 » Sun Jun 29, 2008 4:03 pm

http://www.dplay.com/tutorial/cablehum.html

Fixed my problem a couple years ago. It only costs like 4 dollars in parts so it's worth trying.
-Nick aka NS1 aka mopey aka speedbump
Alpine CDA-9827
Crossfire CFQ5.2
JL Audio XR 650csi components, Planet Audio TT2300
Crossfire VR404 on factory rear 6x8's until I get them upgraded
SI 15D2 + 3.8 @ 30 hz + Crossfire BMF 1000D = :)
User avatar
95Honda
Posts: 1691
Joined: Mon Jan 05, 2004 1:00 pm
Location: Germany
Contact:

Postby 95Honda » Mon Jun 30, 2008 12:31 am

That would be an isolation transformer... :)
http://www.forceaudio.com



Marshall asked me how much it would be to fly to Idaho....
User avatar
KU40
Posts: 6461
Joined: Sun Mar 16, 2003 12:02 pm
Location: Lawrence, KS
Contact:

Postby KU40 » Tue Jul 01, 2008 4:44 pm

ah, right, well, there you go!
-Nick aka NS1 aka mopey aka speedbump

Alpine CDA-9827

Crossfire CFQ5.2

JL Audio XR 650csi components, Planet Audio TT2300

Crossfire VR404 on factory rear 6x8's until I get them upgraded

SI 15D2 + 3.8 @ 30 hz + Crossfire BMF 1000D = :)
User avatar
eezip
New Member
Posts: 1009
Joined: Mon Dec 17, 2001 12:43 am
Location: Orange County, CA
Contact:

Postby eezip » Sun Aug 17, 2008 9:16 am

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.
HT: Sony KV-27FS13 | Oppo DV-981HD | Denon AVR-2807 | 7x Cambridge MC100 | Acoustic Research ARPR1010 || Stereo: Dynaudio Audience 42 | Hales Concept 3 prototypes | wouldn't you like to know... :)

Home PC: Yamaha RP-U100 | B&W LM-1 | Mpyre 65x || Work PC: Yamaha RP-U100 | QSC AD-S32T | Sony MDR-V600
User avatar
nismo
Posts: 6120
Joined: Tue Jan 16, 2001 1:00 pm
Location: Greenwood, IN
Contact:

Postby nismo » Thu Aug 21, 2008 6:20 pm

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?

Eric

Return to “Home Audio/Video”

Who is online

Users browsing this forum: No registered users and 1 guest