help! power supply producing a hum.

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sproutrecording

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I just upgraded from a pair of Behringer MX-8000's to a Mackie 24x8 with the 24E expander. Before the upgrade, I got no noise. Now, I am getting a hum that I can't for the life of me figure out. The power supplies and the power amp (samson servo) are racked in the same rack with nothing else. If the power amp is on and the power supplies for the consoles are off, I get no humming, but if I turn on the power supply to the consoles, then i get a lower-mid humming, even if all the levels on the console are at negative infinity and everything that can be muted is muted. When I racked the components I put electrical tape on the rack rails and covered the components sides in electrical tape, so I am fairly positive that I am not experiencing the side-effects of a ground-loop. Plus, I had used no electrical tape when I racked the power amp and the power supplies for the Behringers and I got no humming whatsoever with that. They are all plugged into the same circuit like they were before the upgrade. The only thing that has changed is that I am using Mackies and their power supplies instead of behringers and electrical tape on the rails.

Any ideas on what could be causing this and/or what I could do to fix it or find out what is causing it?
 
Try moving the power supplies away from the power amp... sounds like the transformers in the power supplies may be inducing hum. If this is the case should should not hear any hum monitoring through the phone jack of the Mackie.

Seperating them by a single rack space may cure your problem... but I'd move them out of the "AUDIO" rack completely.
 
no luck. i took the power amp out of the rack and moved it 4-5 feet further away from the power supplies and i am getting the same hum.

i have noticed, however that if if turn on the power supply to the 24 channel expander, then i don't get the hum. it is only when i turn on the power to the mackie 8-bus that i get the hum. i am thinking that something in the board is generating the hum itself and it is showing up at the control room outputs. the thing is is that i will have everything muted and all pots and faders all the way down, and i will still get the hum.

any ideas?
 
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