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Old 18th April 2004, 02:16 PM   #1
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Unhappy GC oscillation ?

Hi there,

I'm using Brian's boards and the LM3875 heats up
very rapidly when power is applied. I assume it's
oscillating. Now I notice that if I disconnect
the RCA jack wires from the PCB, then the chip
stays cool.

Any ideas what this may mean?

Thanks,
Dennis
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Old 18th April 2004, 02:38 PM   #2
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The minimal GCs are quite prone to oscillating if they have no input signal or the input is not connected to ground. Try testing yours with an input signal, anything will do, signal or sinewave, and see if the problem continues.
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Old 18th April 2004, 02:53 PM   #3
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Quote:
Originally posted by pinkmouse
The minimal GCs are quite prone to oscillating if they have no input signal or the input is not connected to ground. Try testing yours with an input signal, anything will do, signal or sinewave, and see if the problem continues.

Does that mean when the last track of a CD is finished I should turn the GC off in case there is no input signal that it may get into oscillation? Wow I need to remember not fall to sleep when listen to my GC then it may start up a house fire after the last track, this reminds me of the mission impossible TV series...not good

Regards,
Chris
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Old 18th April 2004, 02:57 PM   #4
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No, this is not the case

You just need to have your interconnects attached, so the amp sees some impedance at the input, that's all.

It's only when you leave it with no inputs connected, it may start oscilating and getting warm. You might try to disconnect the cables and see what happens, usually some noise appears at the speakers.

As a side note, I had my output cables shorted at least for an hour one day. The amp got pretty warm, but nothing happned.
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Old 18th April 2004, 06:49 PM   #5
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Quote:
Originally posted by Peter Daniel

As a side note, I had my output cables shorted at least for an hour one day. The amp got pretty warm, but nothing happned.
That's because the chips have short circuit protection. Probably they were switching off, cooling down, then back on, heating off, protection kicking in, off, cool, etc.

Just a guess though

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Old 22nd April 2004, 12:46 PM   #6
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I'm kind of having a similar problem, so in the interest of not starting another thread, I thought I'd post it here.

After listening to my IGC for a while, I have the tendency to fall asleep and leave it on. There's never a problem when there's a signal, but after I've been asleep for a couple of hours, a sort of low frequency oscillating humming can be heard from the speakers. Its not very loud, but its not soft either and it sounds like this:

...wubwubwubwubwubwubwubwubwubwubwubwub....

The input cables are plugged in and I haven't changed anything else. Also, its worth noting that it does not happen right away-- it usually takes about two hours or so before the noise begins and once I turn it off and back on it goes away.

Any ideas on this one? Is it oscillation? Drives Mrs. Annan crazy. Crazier, anyway.

Kofi
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Old 22nd April 2004, 02:34 PM   #7
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Are the heatsinks warm? Could be noise coming from your transformer which can expand slightly as it gets warm. If your transformer is bolted to the amp chassis, try putting a layer of rubber, or bits of rubber, between it and the case and tighten it reasonably firmly (but don't over tighten). I use three little rubber feet meant for speaker isolation, or chunks of an ordinary eraser would work.

If that doesn't work, try an alarm clock.
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Old 22nd April 2004, 07:19 PM   #8
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Ropie-- good to hear from you and thanks for all the great advice in the past!

Well, the amp chassis is made of wood (a breadbox, actually) and its insulated with rubber on the top and bottom. Also, the noise stops when I switch it off and then back on almost immeidately, so I'm not sure that the trafo is what's causing it, but what the hell do I know.

I'll dry to duplicate the wubwubwubwubwubwubwubwubwub this weekend and see if I can poke around and find the problem.

Thanks for the advice!

Kofi
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Old 22nd April 2004, 07:27 PM   #9
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Why not solder a 100k or so resistor across the input? Or if this doesnt work, try an LPF with 1k or 2.2k and a 100pF capacitor at the input. No flames that it will "ruin the sound" etc, please.
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Old 22nd April 2004, 08:11 PM   #10
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Won't that ruin the sound?

Kidding, of course.

I suppose you're talking about a Zobel network, maybe? I haven't tried that and I think its a good suggestion.

I'll give it a whirl and see if that clears things up.

Kofi
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