S

surroundguy

Audiophyte
I just had an interesting issue with a surprising fix. I've been setting up a Dolby Atmos 7.2.4 system little by little as items come in. My pre-pro is feeding a 7-channel power amp with XLR cables which feed all the "floor" speakers (7). This has been working fine with no hums or buzzes at all.

Yesterday, I hooked up the front left and right Dolby Atmos speakers for the first time (2 of the .4) using the power amp section of a NAD receiver. The pre-pro is feeding the NAD with unbalanced RCA connectors. I fired it up and all I got was a LOUD 60 cycle hum. What?? I am using high grade cables and everything is hooked up properly so I was quite stumped!

I swapped out the two RCA cables feeding the inputs for the left/right Atmos speakers on the NAD power amp thinking I may have had a short in one of them. This made no difference. I ran grounding/bonding cables from both amps to various points on the system trying to figure out where the ground loop was coming from. All to no avail!

My equipment is sequentially powered up and I am using a few DC trigger cables to power up the power amps just before the 55" OLED screen, which is the last unit to come on. The two DC trigger outs for the power amps were both fed from the pre-pro. Frustrated, I unplugged the DC trigger cable running to the NAD from the pre/pro and VOILA, SILENCE!! Very strange, but that fixed it. One wouldn't think of that as a probable cause of hum, yet there it was. I'm wondering if the polarities are flipped on the two trigger cables/inputs, or did the connection just cause a nasty ground loop for some reason. I've changed things so that the first power amp now triggers the second and am using the same 1/8" mini cables with no problems. When both amps are triggered from the pre-pro though, you get a nasty hum in ONE of the power amps!

Any thoughts on this? Has anyone experienced a similar situation? Anyway, it's something to try if you are ever in a head-scratching situation.
 
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