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Old 13th February 2013, 12:29 AM   #1
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Default Help! Diagnosing a problem

Hello everyone,

I just finished rewiring my amp with new wire. It uses a LM3886 x2 board for the left and right channel and an LM4780 paralleled for the subwoofer. Also replaced the LM3886 x2 board because I wasn't happy with my job before.

Before doing this, turning the subwoofer volume knob half way almost made the sub xmax so that it would sound like a motorcycle. Now I can't even get it that loud if I tried. Full volume now would be like 1/4volume before.

So I took some measurements. Somehow they don't seem right to me. But I'm not smart enough to know what they mean.

Between the Left + and the Right +, I measured 20kOhm.
Between the Left + and the Left -, I measured 38kOhm.
Between the Right + and the Right -, I measured 41kOhm.

First off, shouldn't Left + and Right + be open circuit?

But the amp works and the left and right channels sound very good (as does the sub; it's just quiet).

Any ideas?
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Old 13th February 2013, 01:02 AM   #2
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I'd check the solder joints on your feedback resistors on the LM4870. also check that you have put the resistors in the right place. These resistors set the gain of the amp so if the lower one is too high in value your gain will be reduced.

Tony.
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Old 13th February 2013, 01:15 AM   #3
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Thank you very much. However, I have had the amplifier working for over a year now and only this week did I decide to rewire it. I did not touch the sub amplifier at all, so I don't see how that is possible, but perhaps.
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Old 13th February 2013, 01:42 AM   #4
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Reporting back to say that I think I've fixed the problem. A piece of solder splash appeared to be shorting two pins.

Be careful when you solder!

Thanks.
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Old 13th February 2013, 08:48 AM   #5
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Good to hear. Lucky it was just reducing output and didn't blow anything!!

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