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Old 28th March 2010, 03:18 AM   #1
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Default LM3886 - DC offset problem

Ok, I have the amp completed and no smoke this time (I fried the rectifier earlier; had a short between one of the connections and the heatsink. Point to Point on crowded spaces is not my forte). I test for DC offset with no load and I have 3 mv on one channel (at 0 vol) but have 320 mv on the other channel. With volume up, I have 32 mv on the 'good' channel but the other channel (the one with the short earlier) goes to
(minus) -220 mv. What could cause that?

Thanks,

Luis
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Old 28th March 2010, 04:25 AM   #2
sangram is offline sangram  India
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Check connections around the input resistor. Do you have an input capacitor? Feedback capacitor? What values of resistors have you used? Can you post a schematic/pictures?
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Old 28th March 2010, 05:02 AM   #3
wwenze is offline wwenze  Singapore
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Quote:
Point to Point on crowded spaces is not my forte).
Buy teflon tubing - the insulator tubing that one uses for DIY wires, and use that to cover up all exposed metallic leads leaving enough just for soldered connections.
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Old 28th March 2010, 08:49 AM   #4
tomchr is offline tomchr  United States
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3 mV sounds normal. 320 mV sounds high. A few 10's of mV probably won't hurt any. But if it gets beyond that, I'd deal with it.

~Tom
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Old 28th March 2010, 01:11 PM   #5
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Originally Posted by sangram View Post
Check connections around the input resistor. Do you have an input capacitor? Feedback capacitor? What values of resistors have you used? Can you post a schematic/pictures?
Sorry, no input resistor - will connect a 22k one today. no input capacitor nor any feedback capacitor.
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Old 28th March 2010, 01:12 PM   #6
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3 mV sounds normal. 320 mV sounds high. A few 10's of mV probably won't hurt any. But if it gets beyond that, I'd deal with it.

~Tom
Sorry, I meant 32mv instead of 320mv. It does go negative with volume full up. Thanks.
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Old 28th March 2010, 01:15 PM   #7
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Buy teflon tubing - the insulator tubing that one uses for DIY wires, and use that to cover up all exposed metallic leads leaving enough just for soldered connections.
Wwenze, thanks for the tip; I should've used it on the capacitors, but there isn't a lot of exposed wire. Will try to post a picture if you guys promise not to laugh...
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Old 28th March 2010, 01:54 PM   #8
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Default Bad hum, bad hum..

This morning I was getting ~50mv offset on the amp so decided to connect some Polk Audio bookshelf speakers. With the source (CD plyer) off, I could hear a hum, and also some motorboating from the amp. I realized that my RCAs were not isolated so I moved them off the case and connected the source again. This time the hum was really LOUD at 0 volume. Any help would be appreciated.

Cheers, Luis
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Old 28th March 2010, 03:07 PM   #9
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The inputs have to be shorted to measure offset.
No capacitor in the feedback leg leads to an offset.
No input capacitor can lead to an offset if DC is present at the output of the source.
How many uf ( filtering)in your PSU?
Your schematic, including PSU would be welcome.
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Old 28th March 2010, 03:59 PM   #10
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Originally Posted by bobodioulasso View Post
The inputs have to be shorted to measure offset.
No capacitor in the feedback leg leads to an offset.
No input capacitor can lead to an offset if DC is present at the output of the source.
How many uf ( filtering)in your PSU?
Your schematic, including PSU would be welcome.
bobodioulasso, I thought volume at 0 shunuted the input to ground.
PS is Toroid, 22v+22v, single rectifier, snubbed.
Here's the ckt schematic. One star ground on the chassis.

Click the image to open in full size.

Capacitors are 10,000 uf, snubbed with .1uF.

Question: where do I ground the (-) input? Right now it's not grounded - the input ground(s) go to the volume pot and nowhere else. I tried to ground them to the star ground but it made no difference.
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Last edited by luis.jimenez; 28th March 2010 at 04:08 PM.
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