Frustrating and Confusing Amp/speaker Connectivity Problem - Your Expertise Requested - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 3 Old 02-18-2017, 09:29 PM - Thread Starter
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Frustrating and Confusing Amp/speaker Connectivity Problem - Your Expertise Requested

I just purchased a replacement 5 channel amplifier to handle part of the needs of my home theater (after the almost twenty year old one developed issues). The amplifier I bought was used, but had just been sent to it's manufacturer for some updates and to ensure a clean bill of health. In fact, the manufacturer shipped it to me, for the seller.

When I first connected it up last night and turned it on, the speaker connected to channel four (of the five discrete amplifiers inside it) produced a loud hum and some distortion. I shut everything down and triple checked the connections. The only thing I found that "could" have impacted things was the rca connector of the cable coming from the receiver that I use as a pre-pro was of the locking type, and I loosened it up to ensure maximum contact. When I turned everything back on, there was only the slightest noise from the speaker and it was not audible while watching programming. I sat back and enjoyed a long movie. When I went to shut down the amplifier at the end of the night, as I reached to press the off button, my other finger touched the faceplate of the amplifier about an nch from the switch. That suddenly resulted in the speaker that originally had the problem suddenly making the loud humming and distortion sounds. It was late, at that point, so I put off troubleshooting until today, certain that I had some kind of ground issue.

Today, I rechecked all of the connections, reseated the speaker cable wires, and the rca cable, with no change, still the loud hum and noise. I then moved the matching speaker from the "right" side over to channel four (it had been connected to channel five) of the amplifier certain that I would experience the same hum and noise from that speaker, clearly indicating that channel of the amplifier must have a problem. There was, however, not a sound out of the speaker that I had just connected.

In the end, because we had friends coming over to watch a movie, I simply took the right and left speakers involved and moved them to my other existing five channel home theater amplifier and everything worked perfectly. I am now at a complete loss as to how to explain what's happened, and have no idea where to go with additional troubleshooting.

When I moved the second speaker and it's connectivity over to channel four of the amp, it too should have exhibited the same problems if it was the amplifier that had a problem. When I moved the original problem speaker and it's connectivity over to the other amp, that should have indicated a problem if there was one. Any assistance, suggestions or insight at this point would be greatly appreciated. Thanks.
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post #2 of 3 Old 02-19-2017, 05:03 AM
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This should be pretty easy to troubleshoot. Typically you aren’t going to get noise of that type caused by the speaker itself or the speaker cable. So it sounds like an interconnect issue, or the amplifier channel itself.

So, first check the cable. From your description, it sounds like it’s intermittently making and breaking connection. Typically cables fail just past the barrel of the connector, where the wire flexes and bends a lot. So with the cable connected to both pre/pro and amp, wriggle the cable around just past the connector barrel, on first one end then the other. If you get the noise, then you’ve found your problem. If not, go ahead and swap out the cable for a different one, and move the suspicious cable to a different channel to see if the problem follows. If the problem doesn’t move with the cable, then it’s the amp.

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post #3 of 3 Old 02-19-2017, 08:46 AM
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Sounds like a ground loop somewhere in the system. I have a big old Heathkit amp at the stereo end of my room and my Yamaha AV receiver at the theater end of the room (different circuit breaker). I tried using the Heath to power a subwoofer and always got a REAL loud hum. My nephew who set up a lot of pa systems on the road had me remove the 3 pin power line of the Heath amp and stick on a 2 pin cheater. Like magic the hum was gone. The audio cable between the Heathkit amp input and the receiver output was part of the ground scheme. Maybe I could have used an RCA cable with only the one end of the shield connected to eliminate the problem, not sure.
Do you have a lot of 3 pin (110+ safety ground) components? Do you have two separately powered component amplifiers involved like I did. Try unplugging the connecting rca cables one at a time and see if the hum goes away. If it does, stick in a cheater plug (one that lets you plug in a 3 pin component to a 2 pin socket) for that component and see if that solves your problem.

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