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Old 25th April 2008, 11:39 AM   #1
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Default DC offset problem

Hi!

I have a problem that almost cost me my speakers. I have a power supply of 10000uF per rail, the transformer is 2X18AC 150VA and also I have blue alps pot. When the pot is on minimum the DC offset is around 10mV but when I turn it up the DC offset goes to the 140mV. Also i have gain around 40. What would solve my problem.



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Old 25th April 2008, 11:50 AM   #2
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short out both your inputs to ground.

put a load (8ohm) resistor on the output

measure again.

140mV wil NOT destroy you speakers. believe me.
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Old 25th April 2008, 12:02 PM   #3
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I measured as you told me and it was 139mV. Also the speaker membrane moves out. Oh and before this one of the speaker got really hot and these speakers are like 5months old so I don't think that speakers are problem.

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Old 25th April 2008, 12:23 PM   #4
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Quote:
What would solve my problem.
probably a cap in the feedback network, so DC is not amplified anymore.
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Old 25th April 2008, 12:33 PM   #5
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Ok! So how big should be this capacitor(voltage and capacitance). I have a feedback resistor 22k and 560E for gain 40.

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Old 25th April 2008, 12:52 PM   #6
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47-100uF, the voltage rating is not important
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Old 25th April 2008, 01:05 PM   #7
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Thank you! While you were writing this post I made some search and found that some projects have 22uF. So I used 22uF. It works and the DC is now at any volume around 5mV or even less. Thanks again guys!

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Old 25th April 2008, 01:22 PM   #8
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Default Re: DC offset problem

Quote:
Originally posted by mravlcax
When the pot is on minimum the DC offset is around 10mV but when I turn it up the DC offset goes to the 140mV.
Picture this: You have a constant current into the input of the IC (input bias current). If you have the input shorted the voltage drop will become zero + the voltage offset. If you have 10 kohms times the current = 2-10 mV amplied 40 times. You have a varying resistance from the input down to signal ground, that's why you'll the change when you turn up the volume. You may also have a small DC voltage from your signal source. Why do you have so much gain? 20-30 is normal.
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Old 25th April 2008, 01:31 PM   #9
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As this thread is in the chipamp section ... Did you all talk about a Gainclone ? Is the chip in single or parallel or even bridged parallel operation ?

Interesting answers up to now, but I could never give any suggestions before i wouldn't have known these facts or even a closer look into a small part of schematic ...

But if it is really a sort of gainclone chip like LM3875/3886, why don't you use a DC-Offset regulator circuit to fix this problem ? Adding a cap in the Feedback would not be my favorite.

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Old 25th April 2008, 01:37 PM   #10
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Quote:
Interesting answers up to now, but I could never give any suggestions before i wouldn't have known these facts or even a closer look into a small part of schematic ...
of course seeing the schematic first, would be preferred.
But reading a while in the chipamp section, gives one a good picture of the usual suspects.
It seems that beginners often make the same mistakes and rely on questionable implementations circuling around the net.
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