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Old 8th October 2012, 09:00 PM   #41
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Hi Marc, this is more about learning to control - adjust offset (as with your case).

This is my original schematic.
I have remove Cin, Ci & Cc
Rin =10K
Rb=330 R
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Old 8th October 2012, 10:03 PM   #42
Marc Vi is offline Marc Vi  Netherlands
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Hallo Zakman35,

Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.

And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!

Don't remove Cc because it prevents noise and - if necessarely - oscillation.

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Old 9th October 2012, 02:57 AM   #43
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Quote:
Originally Posted by zakman35 View Post
I measure dc voltage directly on the pins of the chip, both rails are exactly the same (+36.2V, -36.2V)
Good. Now, what is the AC (noise content) voltage of both DC rails?
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Old 9th October 2012, 08:17 PM   #44
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Ac noise = 0V on both dc rails.
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Old 9th October 2012, 08:23 PM   #45
Marc Vi is offline Marc Vi  Netherlands
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Can we return to the original subject: LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage.

Zakman35 can you reply on that and on my last post?

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Old 9th October 2012, 08:41 PM   #46
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Quote:
Don't remove Ci because the input resistance will be dependent on the output resistance of the pre-amplifier and that may change the output offset voltage of the LM3886 quite a lot.

And, if there is an output offset voltage on the pre-amplifier, that output offset voltage will be multiplied by 33 !!

Don't remove Cc because it prevents noise and - if necessarely - oscillation.
I install Ci back again (no changes at offset).
My pre amp (minidsp) is AC coupled at the output, so no dc from there.

I will try tomorrow to install back the Cc cap and report back.
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Old 10th October 2012, 02:53 AM   #47
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Having eliminated so many other possibilities, I think there's 6mv worth of ground loop.
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Old 10th October 2012, 08:06 AM   #48
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Anyway, back to the topic.

What about adjusting - reducing positive's rails voltage to trim output offset to approximate 0mv. Is that an acceptable practice in power amps?
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Old 10th October 2012, 10:11 AM   #49
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Sure. You can run some led's on that rail, and you can add two of 4k7 1/4w bleeder resistors. It will make a very mild difference.
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Old 10th October 2012, 11:50 AM   #50
Marc Vi is offline Marc Vi  Netherlands
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No, No, No, this isn't a solution at all!! (Or I don't understand it)

To trim out a offset voltage of let's say 6 mV, you have adjust the positive rail for a multiple of that. And that reduces your output power and induces distortion much earlier on one flank.

Why not use the way I proposed?

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