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Old 28th November 2004, 06:53 PM   #1
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Question loud hum w/ 1st Gainclone

Well, I finished my 1st GC using BrianGT's boards/parts. I hooked it up to the computer output & turned it on. The output of the rectifier board was right on the money. I hooked up a single free air speaker (8 Ohm) to the right chanel and got a really loud hum--I was quite a bit put off, but had read about hums & thought I'd see if the hum would be present with a signal running through it. The output was ~2.2 VAC at the speaker. I opened up iTunes & turned on some music with no output & then increased volume. The net result was a blown fuse (2A) and an acrid smell from the speaker.

Well, before I replaced the 2A with another or larger fuse, I thought I question the collective wisdom about this. I traced lines & everything looks according to BrianGT's diagrams. To get a real clean look, however, I have to pull out the frame that the amp/rectifier board are mounted on. The box I built is nice looking, but a pain to work with. Any thoughts?

TIA,
Bret Morrow

PS I have 2 pictures of the guts, but I'm not sure of the value as the boards are difficult to see below the wire "cloud".
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Old 28th November 2004, 07:01 PM   #2
boholm is offline boholm  Denmark
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If you don't have anything connected to the input, does it hum then?
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Old 28th November 2004, 08:01 PM   #3
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Is it the LM3875 or the LM4780?

a friend of mine has some problems with the lm4780 board, though his are just plain dead. all four of them..

regards
marius
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Old 28th November 2004, 10:45 PM   #4
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Quote:
Originally posted by boholm
If you don't have anything connected to the input, does it hum then?
Yep, just as loudly. I haven't tried with music playing yet. I switched to a slo-blow so I'm ready to give it a try soon.

Quote:
Originally posted by demogorgon
Is it the LM3875 or the LM4780?

a friend of mine has some problems with the lm4780 board, though his are just plain dead. all four of them..

regards
marius
Sorry, the LM3875 premium package.

Thanks & still looking for the answer!

Cheers,
Bret
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Old 29th November 2004, 06:20 AM   #5
boholm is offline boholm  Denmark
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Quote:
Originally posted by JazzzSpazzz
Yep, just as loudly. I haven't tried with music playing yet. I switched to a slo-blow so I'm ready to give it a try soon.
Then there is no need to try with music for now. Not before the hum is gone. I do not know the boards, and I certainly do not know, how you have assembled it all. But there is definitely an error in the assembly - a loose wire, bad soldering, wrong wiring, whatever.

If you could make a sketch of how you have assembled it, it would help. And maybe even some pictures.
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Old 29th November 2004, 08:22 AM   #6
Nuuk is offline Nuuk  United Kingdom
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And measure the DC offset on the speaker terminals!
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Old 29th November 2004, 01:49 PM   #7
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Originally posted by Nuuk
And measure the DC offset on the speaker terminals!
Sorry, a bit of confusion on this side of the Atlantic. Isn't the signal from the amp to speakers AC? I did measure VAC at a steady 2.2 VAC--it fluxed up to 16VAC briefly when turning on. The "instrument" used to measure was a cheap digital multimeter. A standing 2.2 VAC differential across the + & - terminals of a speaker doesn't seem very health. I'm pulling the frame out tonight so I can carefully verify the wiring--this is likely where I screwed up. But my question is--should I be measuring VDC? javascript:smilie('')

Thanks,
Bret Morrow
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Old 29th November 2004, 01:56 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Bret, the signal to the speaker is AC but that's what is supposed to go to the speaker and won't do any harm at 'proper' levels.

DC is what destroys speakers and we need to keep it to a minimum to protect the speakers.

So measure the DC offset on the output of each channel with a 10 ohm resistor connected across them. You should be aiming for sub 100mV and the closer to 0mV the better.
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Old 29th November 2004, 10:13 PM   #9
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Default Ohhhhhhh

Quote:
Originally posted by Nuuk
Bret, (SNIP) DC is what destroys speakers and we need to keep it to a minimum to protect the speakers.

So measure the DC offset on the output of each channel with a 10 ohm resistor connected across them. You should be aiming for sub 100mV and the closer to 0mV the better.

Then DCV on the humming channel is 35V--I think that this may not be good ;-) The other channel comes in at 13.4mV--I have not listen to anything through this channel. I looked for misplaced wires, but everything looks good. Nothing seems loose either. I am going to take the amp board out of the box later tonight to look at the bottom side of the board to see if there is a short there. I have to pretend that I never read your site--esp. where you advise buillding the amp on a piece of wood before placing it into the box--I wish I would have done that first. Does the symptom (35VDC) indicate anything specific to you? (other than something is wrong ;-)

Thanks,
Bret Morrow
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Old 29th November 2004, 10:36 PM   #10
boholm is offline boholm  Denmark
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It can be, that the positive supply is shorted to the input or the output. Or the opamp itself is dead.
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