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Speaker hum problem - need help

I've searched the forums and have seen past posts for this topic, however, I've yet to find an answer that fits my situation.

First, here's my equipment:
Coda CSib integrated amp
Ayre QB-9 DSD DAC
Auraliti PK-90 USB server
Dynaudio Focus 380 speakers
Audience Au-24 speaker cables
Cardas Clear XLR interconnects
Cardas Clear & Grover Huffman power cords
Wireworld Platinum Starlight USB

Here's the problem:

With only the amp plugged into power and the speakers connected, I get a 38 db invariant hum coming out of the speakers. The hum does not change no matter what the volume is on the amp. The hum does not change regardless of whether I have the amp plugged into the: wall, a power strip, or a power conditioner. The hum does not change regardless of power cord connected to it. The hum does not change when I plug in the remainder of components and cords into my system.....it is invariant.

I have tried to "lift the ground" with a 3/2 cheater plug, but this had no impact. I sent the amp back to the factory to be checked out and slightly upgraded; they could not reproduce the hum at the factory. A previous amp (tube) produced the same issue, however, with the tube amp, the hum got worse as the volume was raised.

I love the overall sound of this system, but this hum is definitely getting in the way of my enjoyment. Any thoughts out there about what is causing this problem and how to correct this issue?

Many thanks,

Matt
mwsl
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drubin
3,274 posts
 
Nice system.

Have you experimented with the inputs on the amp? How many are there and how many of them are in use? Does the hum change at all when you choose a different source?
bifwynne
2,206 posts
 
Good point Drubin. I wonder if the OP's problem might be helped if he used shorting caps on all of the amp's inputs, except for source inputs that are being used. Almost seems like the amp, through ambient inductance, is picking up a 60 cycle signal at the inputs and the signal is getting amplified. Heck ... maybe he should short all the inputs to see if the problem is ameliorated.

The other alternative is that maybe the OP should fashion a tin foil hat when he listens to his rig. LOL :)

Please report back.

Best.
almarg's avatar
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almarg
9,670 posts
 
Dan & Bruce, fair points, but the probability that effects at the inputs are causing the problem would seem to be low given that the hum is insensitive to the volume control setting.

Matt, given your thorough (and excellent) description of the issue, I'm mostly stumped. But to grasp at some straws:

1)Perhaps for some reason the AC safety ground pin on the outlet isn't really grounded. It seems conceivable to me that plugging into an ungrounded outlet could cause these symptoms, especially in the case of a high powered amp such as this one.

If you have a multimeter, see if ~120 volts AC can be measured between the smaller of the two vertical slots on the outlet and the metal in the outlet's safety ground opening.

2)Make sure that the speaker cables are not routed close to and in parallel with any power cords. If they are, 60 Hz EMI could conceivably be coupled into the speaker cables, and from there into the amp's feedback loop.

3)Disconnect power from any other components or electrical devices that are close to the amp, although it sounds like you've already tried that.

4)Try turning off any fluorescent lights, compact fluorescent lights, and dimmer switches that are anywhere in the vicinity.

Hope that helps. Regards,
-- Al
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