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Denon 3808 receiver
5.1 set up with in ceiling polk speakers
Cablevision set top box.
Using hdmi in/out all sounds great.
Using rca out from cable box to denon there is an annoying humming sound coming from front speakers. Tried several inputs on denon.
Tried monster rca cables.
No luck.
Any ideas?
 

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You have removed the cable box from the setup? It could be a bad ground on the incoming cable line.
 

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Try disconnecting the coax cable and see if the hum goes away.
 

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Thanks 4 response. Cable rca audio out to denon.
Cable box component to tv
Bad ground is the most probable reason. Problem gets probably worse because you created a ground loop as follows: Box ground to AVR ground via rca cable, AVR to TV ground via HDMI shield, Box to TV via component.
Why don't you send the box's video signal via component through the AVR, too? The TV gets Video via the HDMI-cable as usual.
It seems you do want to hear Audio from the box via your AVR when watching it on TV, don't you? Tehn, this would make sense AND remove the ground loop. Maybe the noise vanishes then, too, though I'm not sure.
 

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Getting rid of hum can be a trial and error extravaganza.

One thing to check is to ensure that your electrical outlet ground really is going to ground (I've seen it where it wasn't hooked up correctly before...).

Change one thing at a time until the hum goes away. Reinstall whatever it is you changed last and if the hum comes back you've got it nailed. It is an exercise in patience, something I don't have an excess supply of so these are always trying problems. Good luck!
 

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I agree that the cable box could be the problem. You can get isolation transformers that will break the ground connection. However, you can make your own if you have two 75 ohm to 300 ohm matching transformers like those used on TV antennas. Connect the 300 ohm sides together and then insert the combo into the coax line - easy way to find out if that is the problem. Then go buy the isolation transformer if you want something that looks neater. Otherwise, stay with the home made setup.
 

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I just saw this thread and thought I'd offer my experience on this issue as it happened to me too. Dont know if you go it cured, but here's a link to my problem thread where I had the same problem, only with a Directv DVR. Some of the steps may work on your problem too.
http://www.hometheatershack.com/forums/av-home-theater/69519-mystery-c-hum.html

With Directv, you cant use an isolation transformer, but you can with cable.
Hope it helps you, ground loops are a real pain to find and cure.
 
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