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Old 1st January 2015, 09:43 PM   #1
mozarti is offline mozarti
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Default dc offset of 180mv.

I have an audiosector LM3875 kit with high offset numbers: One channel is as high of 186 mv. I can lower the DC to 95 by loosening the screw that attaches the amplifier to the sink, but in this case the amplifier hardly touches the heatsink. In fact, by merely putting the amplifier next to the heatsink the DC jumps up to 186. I have tried placing an 47 uf electrolytic between the one end of 680 ohm resistor and the chassis ground, but this had no effect on the DC.

Do you think there is something with my amplifier (the other amplifier has a DC of 96mv.)? Is there anything else I can do to lower the DC figures?

Thanks in advance for any help you can give.
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Old 2nd January 2015, 04:07 PM   #2
Peter Daniel is offline Peter Daniel  Canada
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dc offset of 180mv. Send a message via AIM to Peter Daniel
The chip needs to be attached firmly to the heatsink for proper heat dissipation. The force with which chip is attached shouldn't affect DC offset reading.

The only explanation would be that with loosening the screw, the chip gets excessively hot because it's not in contact with heatsink and that reduces the offset.

What if the offset reading with source connected?
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Old 2nd January 2015, 07:32 PM   #3
mozarti is offline mozarti
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Thank you, Peter. Your explanation makes sense since the DC reading increases simply when with my hand I put the chip next to the heat sink.

As to the DC reading when it is connected to the source, It fluctuates between 167 and 188 mvolts in one channel and it is about 80 in the other.
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Old 2nd January 2015, 07:44 PM   #4
Peter Daniel is offline Peter Daniel  Canada
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dc offset of 180mv. Send a message via AIM to Peter Daniel
So it means that your source is coupled through a capacitor, as with DC coupled outputs the offset will normally go down.

More info on offset here: Commercial Gainclone kit- building instructions

You could change shunt resistor value (R2) from 22k to 10k and this should reduce offset.
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Old 2nd January 2015, 08:25 PM   #5
tauro0221 is offline tauro0221  United States
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Hi,
Read the rails voltages with the voltmeter in AC. It is take awhile to the read to settle. Sometime any noisy in the rails voltage it will show in the output. This is a test I always do when built a LM3886. Just a thought.
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Old 2nd January 2015, 11:18 PM   #6
mozarti is offline mozarti
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Thank you Peter and Tauro. I did check the AC on the rails and they all read 0. So I will try to change the resistors.
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Old 2nd February 2015, 02:51 PM   #7
r100 is offline r100
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Hi, I've just completed my Audiosector LM3875 build. The proto build sounds really good and there is no hiss or funny noises coming from the loudspeakers.

DC offset is around 4 mV on the right side and around 20-25mV on the left side. I've visually checked the soldering and all seems ok.

Although 20-25mV is not much, what could the reason for the difference between the left and right channel be ? Is it a concern ?

Thank's

Rupert
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Old 2nd February 2015, 02:59 PM   #8
tauro0221 is offline tauro0221  United States
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Hi,
Another test you can do is to short the input and read the output. This will eliminated the noise it is coming from the input.
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Old 2nd February 2015, 03:31 PM   #9
AndrewT is offline AndrewT  Scotland
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post4 gives a link to reducing offset in the Audiosector amplifiers.
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Old 2nd February 2015, 03:51 PM   #10
r100 is offline r100
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ah, ok ... thank's Andrew,

from what I see here it looks like each LM3875 has its own DC offset values (I think ?)

So, in my case if I understand correctly there is nothing to worry about ?

I have the amp playing in my main system without a hiccup so I'm guessing all is OK.

Tauro, I did the measurements with / without the inputs shorted and in both cases the values where more or less identical.

Thanks for all your input.

All the best
Rupert
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