Resistor from Pin + (7) to ground in inverted GC - diyAudio
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Old 31st August 2006, 08:57 AM   #1
100db is offline 100db  Thailand
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Question Resistor from Pin + (7) to ground in inverted GC

Hello all,

For the inverted LM-3875, I need your advice on the different between having ...
1. Resistor between Pin + (Pin7) to ground
2. Resistor in parallel with 0.1uF cap to ground and...
3. Connect pin + directly to ground ?

How is it work, and which one should give the best sounding ?

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Old 31st August 2006, 09:31 AM   #2
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I would go for a direct connection, but it's simple enough to test all three and see which you prefer.
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Old 31st August 2006, 11:12 AM   #3
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Its a nice story, really:

Part of the amp DC output offset is caused by the difference in DC bias at the two inputs.

Those two inputs need a small DC bias current to work (base current of input transistors). So if the DC impedance seen by those bias currents at the two inputs is different, the DC voltage resulting from current times impedance is also different and will cause output offset DC (the input DC difference will be amplified by the amp closed loop gain of course).

So, very often you see the pin 7 resistor as being equal to the DC resistance of the - input to balance out the offset. The DC resistance seen at the - input is the parallel value of the two resistors in the feedback network (if it is DC coupled, else it is just the value of the main feedback resistor).

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Old 1st September 2006, 12:28 PM   #4
100db is offline 100db  Thailand
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Thanks Al, Thanks Jan

So, if I don't have any problem with DC offset... then it's O.K. (better) to connect pin + directly to ground.

Am I understand this correctly ?

Will try it out this weekend.

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