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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
almarg's avatar
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almarg
8,673 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
bifwynne
2,069 posts
 
Al, ... any thoughts about the tin foil hat idea? :)

@Matt, Al's suggestion to check the ground socket hole is a cheap and quick way to confirm that your socket is properly grounded. If Al's idea is negative, I'd move on to his other suggestions. But if still negative, move on to the next least expensive test.

You said that "[w]ith only the amp plugged into power and the speakers connected," you still get the hum. Does that mean the other source components were also disconnected from the amp? If not, disconnect them.

What Al said about the hum not changing with volume probably eliminates my thought about shorting the inputs, but what the hay, I'd give it a whirl. All you have to do is buy some cheapy shorting caps from Radio Shack.

If still no solution, try borrowing another amp from a friend, or maybe a friendly local dealer. If you're still stuck, try the tin foil hat idea.

Please report back.

Regards,

BIF

Have a great weekend.

BIF
dweller's avatar
dweller
2,152 posts
 
I had a problem with my Oppo 105 shedding EMR and being picked up by my turntable's RCA ICs.
This happened with Nordost Blue Heaven (unshielded) and Harmonic Technology Magic (shielded).
The ICs were below the Oppo resulting in 60 Hz hum.
When the Oppo was moved to the side (away from ICs) the
hum stopped completely.
mwsl OP
26 posts
 
Thanks for all your thoughts, suggestions and quips :-)

So, first, yes, the amp makes this hum with ONLY it plugged in with the speakers connected. All other equipment was disconnected for this experiment. This same hum occurs in two different outlets (which, to be fair, are likely on the same "line" in my house). Each outlet has a different high quality audiophile outlet.

Second, didn't mention this, but prior to purchasing this amp, I auditioned a tube amp, and it too created a hum through the speakers....it, however, got louder when volume was increased. For the Coda amp, the hum "volume" stays the same regardless of output volume on the amp.

Third, I have tried different power cords, and have plugged different power cords into: the power outlet (wall), a high quality power strip, and/or a line conditioner. Nothing has impacted the hum.

Fourth, I am aware of possibility that speaker cables over power cords might create hum and have moved all cables/cords around to no effect (doesn't seem to change anything).

I have not tried shorting the inputs.....thought about it, but not done it yet. Maybe that'll be tomorrow's experiment if i can get to RS before it closes.

Any other ideas would be appreciated.

Maybe I just have exceptionally "dirty" power? If so, would that explain this symptom? And then, what the heck would I do about it?

Thanks again!

matt
bifwynne
2,069 posts
 
Please report back tomorrow after you short the inputs. There's a couple of other switch-outs you can try, but I caution you that if everything else fails ... its the tin foil hat for you.
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