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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
9,670 posts
 
Quite a mystery. At this point I can think of only two possibilities, both of which seem very unlikely:

1)The electricians who wired the house when it was built messed up big time, perhaps leaving a safety ground run unconnected, or even reversing the hot and neutral runs, such that all of the outlets you've tried were affected.

2)Or (as Bill/Grannyring suggested) the amp has a bad filter capacitor or other issue, which Coda was negligent in not detecting when you sent it back to them.

Regarding your question about an isolation transformer, I see no reason to expect that to help.

Regards,
-- Al
zd542
5,339 posts
 
"I then tried changing the inputs on the amp (not that I have anything plugged in aside from my XLR and speaker cables), and ran through them all, with each remote outlet (with extension cord attached), and then again with amp plugged into the power strip I've been using. NO CHANGE on any input."

Just to be clear, when you were swapping out components, did you leave the IC's plugged into the amp? Because if you left them in the amp but not connected to anything on the other end, that can produce a hum like you are describing. Especially if open ends happen to be touching something. Also, does the amp have a way to go back and forth between balanced and single ended operation? Having that on the wrong setting can also cause a hum if its not in the right mode. Last, if the amp allows you to adjust the level of each input individually, along with a global volume control, try reducing the level on the individual input.
lak's avatar
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lak
4,623 posts
 
If you look at my two systems listed on Audiogon you will notice that my power filtration system uses two Isolation Transformers 5 KVA/each into my subpanal which feeds all my audio outlets. Although you could set an isolation transformer different then I did, and plug the isolation transformer into you audio outlet and then components into the isolation transformer I can almost garentee (99% sure) that it will not help your situation as Al mentioned. It can however clean most of the grung from your AC line but I don't think that's creating you problem. After reading all that has been posted in this forum, I think your amp is the culprit.
bifwynne
2,206 posts
 
Mwsl, picking up on Zd542's suggestion. If your integrated amp has both XLR and SE input capability, can you use a shorting shunt on unused XLR inputs?

I had a hum problem with my former ARC power amp, which had both XLR and SE inputs. I'd get a hum if I used the SE inputs and forgot to shunt the XLR inputs.

Other than that, I think Zd542 and Al are leaving you with the best straws to grasp at for now. I gotta say that if your amp was checked out by Coda and they missed bad power supply or coupling caps, I'd be extremely surprised. But, as Forrest Gump said, "sh*t happens."

Please let us know how you make out, especially if the dealer comes out.

Cheers,

BIF
tls49's avatar
tls49
1,351 posts
 
I was able to do the extension cord test. Used a (probably) 14 gauge 25 foot Home Depot type extension cord. Tried it in two separate outlets: one across the living room from the outlets I've mentioned, one down the hall. NO CHANGE!
This test is not exactly like Al recommended,
an extension cord that is long enough to allow you to connect the amp to a different outlet on a different AC run.
The important part is "different AC run." "One across the living room" is likely the same run as any outlet in the living room. "One down the hall" may also be on the same run as the living room. The only way to know is to turn off the breaker for the living room, and see if the hall outlet still has power.

I don't think there is any doubt that this is either an amp issue or house wiring issue(just living room or maybe whole house).

IMHO, the best plan of action would be to take the amp to the hi-fi shop you mentioned, then if it works OK there, you know it is your house. Then try the amp in another room of your house, preferably as far from the living room as you can and if it works OK, it's the living room, or if it hums, then the whole house.

You could try a different room in your house first, but if it hums, then you must take the amp to the hi-fi shop for testing.

Good Luck
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