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Old 5th November 2004, 08:54 PM   #1
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Question Noob ? BrianGT Lm 3875 with 10k pot?

Sorry for the noob question but I have built the amps Lm 3875 basic kit witch work good with a 100k "radioshack pot". I have some 10k pots I would like to try but do not get much volume. What do I need to change? I did try to search for it but all the formulas concerning gain I am lost. Any help would be great.
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Old 6th November 2004, 01:34 AM   #2
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You don't get much volume with a 10k pot because you have a 22k resistor on the PCB, from NI input to ground.
Change that resistor for 100k.

You will have better results with a 10k pot than with a 100k pot IMHO.
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Old 6th November 2004, 03:25 AM   #3
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Thanks for the response . The 22k is where I have the pot (R2) no resistor just the pot. R1 is a 220ohm. Do I need the pot wired in different?
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Old 6th November 2004, 03:27 PM   #4
mcs is offline mcs  Denmark
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How is it wired now?

You should have one end to the input, the other end to ground and the wiper to the chip input pin.

Best regards,

Mikkel C. Simonsen
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Old 7th November 2004, 05:58 PM   #5
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Mikkel , that is how I have it wired in. I still havn't figured it out.
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Old 7th November 2004, 06:08 PM   #6
Vikash is offline Vikash  United Kingdom
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I'm using a 10k pot and I've left the 22k from NI to ground as is without any volume issues.

Link to pot wiring just in case.
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Old 8th November 2004, 02:11 AM   #7
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I suspect there may be a mistake with the gain setting resistors on the chip.
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Old 8th November 2004, 08:36 PM   #8
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I'm using the pot as a preamp, with my cd player connected directly. If I leave the 22k to ground I get no control with the pot. VERY loud (like full volume), works fine with my preamp with the 22k but not with the pot alone. If one of you could post a diagram that would be great "pics are worth a thousand words espacialy for newbies like me. Thanks again guys.
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Old 8th November 2004, 09:23 PM   #9
Vikash is offline Vikash  United Kingdom
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have you connected something in place of the removed 22k resistor?
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Old 8th November 2004, 10:15 PM   #10
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There's definitely (or so it seams) a mistake with the way you wired the pot.
Can someone make a sketch, as I don't have the time now?
A new amp is calling me, it's sounding good.
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