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Old 6th November 2007, 01:57 AM   #1
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Question Question about LM3886 Designs

I have a question. the Muting pin of the LM3886 has a 100UF capacitor on it to the ground point in most designs that i see.

some questions:

Is the capacitor required? if so why? What is the point of this cap?

is the value of this capacitor critical?

what is the typical voltage rating of the capacitors and would a 100uf at 35V work? (all i have on hand. +/-35v rails).
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Old 6th November 2007, 02:07 AM   #2
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Have you looked at the ap notes from National Semiconductor to see what their recommendations are?

I_F
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Old 6th November 2007, 02:08 AM   #3
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no all i have is a datasheet. the datsheet doesnt really say. all it says, is its used for a large time constant.

but that doesnt tell me anything.
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Old 6th November 2007, 06:03 AM   #4
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The resistor un-mutes by pulling a small amount of current from the mute pin when connected to the -ve rail. I think that's right. I just woke up, but it certainly involves a small amount of current either way

A capacitor will very briefly act as a short while it is uncharged, so when powering on it will tie the mute pin to ground for a short period of time while it charges (through the resistor to the -ve rail), and thus the chip will be muted briefly upon power up, and will mute upon power down as the cap discharges.

This can be used to help remove any power up / down thumps or strange noises.

It's not strictly required. Just the resistor will un-mute the chip, but for the extra couple of pence this capacitor will likely cost, you might as-well have it there really. If you leave it out and experience thumps and such when turning on / off the amp it might annoy you. Though saying that, in the past I have left out the cap and it was fine.

I believe it will see the voltage of the rails, so a 50v cap would be a safer option.
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Old 6th November 2007, 07:28 AM   #5
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Default Re: Question about LM3886 Designs

Quote:
Originally posted by mbates14
I have a question. the Muting pin of the LM3886 has a 100UF capacitor on it to the ground point in most designs that i see.

some questions:

1 Is the capacitor required? if so why? What is the point of this cap?

2 is the value of this capacitor critical?

3 what is the typical voltage rating of the capacitors and would a 100uf at 35V work? (all i have on hand. +/-35v rails).
1 No but it is very nice to have an absolutely pop free amp!

2 The value sets the time.

3 35 V is OK since it's a low duty location, nothing much is required in terms of performance.

I have chosen (C13)100 uF + (R10)33 k. Check the datasheet for determine the resistor value. You have it in a diagram.
Schematic here

Here you can see how the pop free action looks like.
http://www.keld.eu/gainclone/test.htm
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Old 6th November 2007, 12:03 PM   #6
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since im going to use a 12 to 35v SMPS to run the chip, im probably going to put the cap in there. so the thump at powerup dont draw enormous currents to cause the supply to fail.
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Old 6th November 2007, 01:36 PM   #7
CJ900RR is offline CJ900RR  Sweden
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Default Re: Re: Question about LM3886 Designs

Quote:
Originally posted by peranders

1 No but it is very nice to have an absolutely pop free amp!
OT: I would be sad if I couldnt play POP-music in my amp
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Old 6th November 2007, 02:33 PM   #8
sangram is offline sangram  India
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FWIW I've never used the muting cap - 3886, 4780 or 4766, and never have I had to suffer switch-on or switch-off pops. A very slight click at switch-off is the worst I've encountered, and that cap doesn't prevent that in any case.

There is no chance of the amp failing at switch on due to absence of that cap, IMO. The built in pop protection of the chips (under-voltage protection) ensures that the on and off thumps are minimal.
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Old 6th November 2007, 03:33 PM   #9
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Quote:
Originally posted by sangram
FWIW I've never used the muting cap - 3886, 4780 or 4766, and never have I had to suffer switch-on or switch-off pops. A very slight click at switch-off is the worst I've encountered, and that cap doesn't prevent that in any case.
Same thing I have experienced
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Old 6th November 2007, 05:59 PM   #10
jaycee is offline jaycee  United Kingdom
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I use only 22K to -V in my circuit, with no capacitor... cant hear any switch on pop. IMO it is better to add this sort of thing in your DC protector circuit, if you have one.
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