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In Reply to: Speaker still hums with amp shut down: cause? cure? posted by Bruce from DC on March 23, 2004 at 07:03:13:
I think you're on the right track. My first thought was the amp storing up power. An easy way to test it would be to turn off the amp with the source still running. I'll bet you'll hear the music continue for a fair number of seconds. Once it stops, then check for hum.
Yeah, I'm going to shut everything down overnight -- then see if the hum is present the next morning. All the PS caps should be fully discharged by then.
with the amp powered down all night (to let the PS caps fully discharge), it's still there.Somehow, the speaker cable is picking up powerline hum. (When you turn lights on and off, the hum changes.) The cable passes fairly close to a large cast-iron radiator (not it's not electrically "hot"). Re-arranging the position of the cable slightly eliminated the hum.
weird.
I know, I know..keep my day job...:-)Two possibilities exist..Either e-field pickup, or m field.
E field would require very large fields to couple enough to make it audible..So I would rule that out.
So, where is the M field?
In the wall wires are usually parallel, non twisted, so they radiate a dipole field...strongest nearer the wire.
The speaker has internal loops, like the woofer litz wires. But you said moving the wire changed it.
The only way m field can couple to produce hum is if the amp impedance at the output terminals is not too high...that completes a loop...
You're speakers must be pretty efficient..
Do you have more than one speaker connected to the terminal?
With the wire in the place position to produce the most hum, get an old speaker magnet or a good piece of magnetic iron...or even a good horshoe type magnet, like the ones to pick boat motors from the lake..
Use the iron or whatever, to enhance the field near the wire...run the iron along the wire, listening for louder hum...If that happens, the source of the field is there.
If you twist the speaker run tightly, it should stop hum pickup.
I'm dyin to know what you find.
BTW, I'd have seen this interesting problem earlier if ya posted on prophead..I don't normally look here..
Cheers, John
There's no house wiring in the wall -- this is an outside wall. There's a wire under the floor that connects to a floor outlet. But it's gotta be 12 to 18 inches below the speaker wire, maybe more.Stupid question: if there's no current flowing through that house wire, would it still produce a magnetic field? I thought it wouldn't. The wire I'm thinking of terminates in this outlet; connected to the outlet is an incandescent lamp that is usually off.
Amp is a little Krell integrated, probably pretty low output impedence.
Speakers are Joseph RM-7 si's, not particularly efficient -- 86dB IIRC.
I'll check for any voltage potential to ground from the radiator. Seems odd that there could be any; everything is tied together in the system to a water inlet pipe.
Stay tuned . . .
Does changing cables reslove it?This is crazy, but what if you ground the radiator to the system. Also, what if you mive the speakers out in the middle of the room so that the cables aren't near anything. And then try suspending them. Get the girls to hold them up. Is there power in the floor?
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