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Old 7th August 2003, 03:54 PM   #1
JoeBob is offline JoeBob  Canada
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Default DC offset... On turnoff?

I've got a 4 channel LM3875 (preferred it over the LM3886) amp, and while I only connected two channels so far, I noticed this on both of them, when I'm playing into a speaker there is the normal 50mV of offset, but as soon as I switch it off I get around -5V DC on the output, and it slowly declines. I don't know why this is so, it's like the ground is live, with 5V or so, could this be because I've not connected the ground to the wall ground? Or maybe it is because my test speaker has a high-pass filter (just a single capacitor) at a relatively high frequency (in the mid-band, so DC doesn't kill it while I do tests such as this one...)?
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Old 7th August 2003, 04:05 PM   #2
JoeBob is offline JoeBob  Canada
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I just ran the amp into a 12 ohm load (a resistor, first value I found, close enough) and there was no DC offset on turnoff, so I'm gathering it was the cap on the test speaker. Makes sense, I guess. Time to try it on a real speaker now...
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Old 7th August 2003, 05:23 PM   #3
Nuuk is offline Nuuk  United Kingdom
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Joebob, I had a similar situation when I (presumably) physically damaged the 10R resistor across my speaker terminals and found the the DC offset rose to a few volts when powering down. As soon as the resistor was replaced, everything was OK.
The truth need not be veiled, for it veils itself from the eyes of the ignorant.
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