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Old 21st April 2012, 04:41 PM   #1
epilot is offline epilot  United States
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Default LM3886 volume pot

Hello guys,

I have amde a power amplifier based up on LM3886.

Unfortunetely the input signal decrases when I put a 10K pot for the level control at the input of the chip per datsheet.
Whats wrong here? Is that due to impedance of the input signal?

the other question is that The sound decrases/incrases in level just by small turns of the volum. How can I correct this problem?

Thanks a lot
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Last edited by epilot; 21st April 2012 at 04:45 PM.
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Old 21st April 2012, 05:25 PM   #2
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Increase the pot value to 100K . Best would be 100K Log Pot of Philips.
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Old 22nd April 2012, 08:05 AM   #3
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Check the connection. If you look at the pot from the knob's side with the pins facing downward, the left pin goes to ground, the right to the source and the one in the middle to the amp.

You should also make sure that you use a log pot. A lin pot will give you a very slow response at low level and a very fast response at high level.
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Old 22nd April 2012, 11:36 AM   #4
AndrewT is offline AndrewT  Scotland
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Don't change to a 100k vol pot.

Add RF interference attenuation. i.e. add a low pass filter.

Add back in the missing part of the Thiele Network. i.e. R+C from output PIN to PCB Power Ground

Add back in the input offset current grounding resistor. ~100k from PIN10 to PCB Signal Ground
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Last edited by AndrewT; 22nd April 2012 at 11:39 AM.
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Old 22nd April 2012, 12:27 PM   #5
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Dear epilot,
How did you confirm that the signal decreases? I mean how did you confirm that signal gets loaded by 10K. did you listen it without pot ?
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Old 22nd April 2012, 01:08 PM   #6
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I reiterate PACIFICBLUE's comment. Check your connections.

Even a high resistance connection across R3 to the input of the IC could cause this problem.
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Old 22nd April 2012, 01:21 PM   #7
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10K across the input should not reduce the volume unless the output impedance of your preceding equipment is ridiculously high.
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Old 27th April 2012, 05:19 AM   #8
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Input Load 10k
R1 to 27K
R2 to 820R
R3 to 470R
R4 to 15K
C3 to 470uF or larger

-or-

Input Load 50k (anything in the range of 20k to 100k)
R1 to 82k
R2 to 2.7k
R3 to 680R
R4 to 15K
C3 to 150uF or larger


There's somewhat of a guess in these examples, but please observe that these examples have R4 and C3 are at values that will work. . . and then compare with the schematic in post 1 that has frail mute current plus a bass blocker.
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