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Old 23rd December 2004, 09:34 AM   #1
sl_1800 is offline sl_1800  United States
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Default First GC Amp .... something wrong?

I have built my first GC amp using Brian's boards. I measured for DC offset with not load resistor and measured a negative 110mv. How can I be getting a negative reading? And yes I'm sure I had the meater hooked up correctly and the outputs on the board are correct. Is there really a problem or should I check with a load resistor? All other voltages are correct.

I was pleased when I pluged it in and got no smoke and no flames but I really am confused on the negative reading on the DC offset.
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Old 23rd December 2004, 09:43 AM   #2
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what are the measurement for supply rails. check solder joints
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Old 23rd December 2004, 09:52 AM   #3
kafka is offline kafka  Poland
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I really don't understand Your confusion... If You can have positive DC offset You can have negative as well, cos' the power supply is symetrical.
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Old 23rd December 2004, 10:07 AM   #4
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kafka right but 110mv is kinda high
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Old 23rd December 2004, 10:21 AM   #5
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If you calculate what you _can_ get this is not extremely high.

Let's say that you have between offset voltage (according to datasheet) 1-10 mV and assume that the input bias currents are small in relation with chosen resistor values.

The gain is 30 => offset 30-300 mV!
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Old 23rd December 2004, 02:53 PM   #6
sl_1800 is offline sl_1800  United States
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Ok, it this negative DC offset is not out of the question then how do I go about reducing it from the 110mV to something a little closer to 0 ???

I have heard about a Ci cap which would be between Ri and ground, correct? If so then I'm assuming I would have to pull the ground end of Ri and insert a non polar cap inbetween ??

Also will a different value resistor for Ri have a effect ?

Thanks for the help.
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Old 23rd December 2004, 07:14 PM   #7
boholm is offline boholm  Denmark
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It can have something to do with your (?) choice of values of the resistors. Can you please post a schematic with resistor values in it?
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Old 23rd December 2004, 07:20 PM   #8
sl_1800 is offline sl_1800  United States
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The values used are the ones that come with the BrianGT standard LM 3875 kit.
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Old 23rd December 2004, 07:37 PM   #9
boholm is offline boholm  Denmark
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If I am looking at the right schematic - and I am not very familiar with BrianGTs products - there is a 220 ohm resistor between input and pin 7 of the LM3875.

This is the resistor to experiment with. In theory it should be 601 ohms ( 620 // 20k ). You may try with a trimmer (1 kohm) if you got one.
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Old 23rd December 2004, 08:44 PM   #10
sl_1800 is offline sl_1800  United States
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I can give that a try, but won't that change the gain of the amp or not?
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