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Old 10th May 2004, 06:53 AM   #1
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Default Gainclone sucking back Current

Hey all hopefully you guys can help me here..
I made a PCB for a non-inverting gainclone, the circuit is exactly off the datasheet with zobel and 10ohm/inductor on output and 3.3uF dc blocking cap on the input. When I power it up off the bench supply, +/- 20v with no input or output connected, nothing happens. Once I connect the speaker (still no input) the current on the negative rail jumps to .75 Amps and continues slowing crawling up and up and up. (like around .1 amps per second) same thing happens if i have the input connected. Music does come outta the speaker if i put a signal in, and doesn't sound too bad, until the protection kicks in of course.

Any ideas?

thanks muchly guys / girls
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Old 10th May 2004, 06:54 AM   #2
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And the PCB
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Old 10th May 2004, 07:21 AM   #3
boholm is offline boholm  Denmark
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One of the inputs of the opamp must have a DC connection to ground. This could be achieved by moving R5 (value?) from the input side of cap C3 to +input of opamp.
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Old 10th May 2004, 07:53 AM   #4
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Yes I agree. Your + input is floating. measure the outp[ut, you will see a large DC. Grounding the + input with 22k (ideally the same as the feedback resistor) will cure it.

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Old 10th May 2004, 09:39 PM   #5
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Thanks muchly guys, I replaced the cap with a wire and the amp works properly now. got pretty hot tho, i'll have to make sure that it is bolted properly to the heatsink. I am using a 4x8" piece of aluminum with 1/2" high fins every 1/2", should be more than adequate....
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Old 11th May 2004, 09:14 PM   #6
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Ok now I don't know whats up but lets see if you do

I fixed one amp, it has a 10k linear pot in place of R5 (no log pots in stock at the shop in town) and i replaced the input cap with a wire link. This amp works ok at low volume, but when i turn the volume up it sounds pretty terrible. I'm gonna try replacing it with a 100k log pot that I have handy, just to see if it works better.

I also have another amp, same schematic with R5 as a 22k resistor and I was just gonna use the input volume control (computer, cd player....)
This one I replaced the input cap with a wire link, but it still draws 250mA on idle (both +/- rails), and when I connect the speaker I get the same sort of behavior as when the + input was floating.

Any ideas??

Thanks again.
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Old 11th May 2004, 09:50 PM   #7
boholm is offline boholm  Denmark
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First . . . you go out and buy a multimeter. Then you can check for errors . . . BEFORE connecting anything to equipment in question.

When you say "input cap" you mean the C3 one?

When nothing is connected to the input and you connect the speaker does it fail then?
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Old 11th May 2004, 09:59 PM   #8
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I have a multimeter, and I measure about 120mV on the output when i power it up, and this drops to about 60 mV after about 5 seconds. What other errors should I be checking for?

Yes, C3 has been replaced with a wire link as this was the easiest way to fix the problem with PCB.

This test was completed with nothing connected to the input.

no input/output, the bench supply shows 250mA at +/- 20V . still no input and I connect the speaker and the negative rail jumps to 1A.
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Old 11th May 2004, 10:19 PM   #9
boholm is offline boholm  Denmark
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Quote:
Originally posted by rayban68
I have a multimeter,
Oh, sorry. I apparently got the wrong impression reading your posts
Quote:
and I measure about 120mV on the output when i power it up, and this drops to about 60 mV after about 5 seconds. What other errors should I be checking for?
All this sounds fine.

Quote:
Yes, C3 has been replaced with a wire link as this was the easiest way to fix the problem with PCB.
OK, just making sure . . .

Quote:
This test was completed with nothing connected to the input.
Fine.

Quote:
no input/output, the bench supply shows 250mA at +/- 20V ..
Sounds right, . . .
Quote:
still no input and I connect the speaker and the negative rail jumps to 1A.
Now it gets spooky . . . BTW, which IC are you using?
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Old 11th May 2004, 10:27 PM   #10
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Ya its pretty strange, I can't understand it.

I am using the LM3886TF . I removed the chip from the pcb, stripped off the caps and put new resistors and the caps on a new board, with the old chip. Same problem. So i ripped off the chip, destroying it in the process of course, and put a new chip in (my last one ) and same problem. Next I guess I could pull off the R5 and replace it with a 10k pot, but that doesn't really answer the question.


The 250mA that it draws with no connections is not very good, it seems to me that it should be like 30mA or something... The other amp that I have draws hardly anything at all when idle.
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