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Old 28th February 2010, 12:17 AM   #1
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Default Popping sound after turn off?

I am experimenting with theLM3875 chip and currently have this set-up:

Click the image to open in full size.

The problem I have is that when I switch the amp off, about a second later a pop is heard from the speaker, like when the caps have discharged, what can I do to stop this?

Thanks in advance
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Old 28th February 2010, 01:20 AM   #2
Bill_P is offline Bill_P  United States
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Try these changes and see if it is better.
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Old 28th February 2010, 03:30 AM   #3
jaycee is offline jaycee  United Kingdom
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Make the 220 ohm resistor 1K. Put a 330pF after it to signal ground. 330uF between the input pins is wrong. Increase the 10uF in series with the 680 ohm resistor to 22uf

What is the 2uF in series with the 22K for? The input cap is also much too big - keep it to 10uF (or a 4.7uF polyester if you can get it).

Get some 100nF capacitors on the power supply rails, as close to the chip as you can.
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Old 28th February 2010, 09:53 AM   #4
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Hi Bill_P,

Sorry, my drawing has a mistake. There is a 330pF cap between + and - of the chip, I have written it wrong and the input cap is 4.7uF. My crappy drawing makes it look like a 47uF.

Anyway, I will change the other caps and resistor as you suggested and post back my findings

Thanks very much Bill_P
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Old 28th February 2010, 10:13 AM   #5
AndrewT is offline AndrewT  Scotland
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the 10uF in the NFB leg is sized so:

C >= 4.7uF * 22k / 680r * sqrt(2)
...>= 4.7 * 10^-6 * 22 * 10^3 / 680 * 1.414
...>= 0.000215F
...>= 215uF
use 220uF or 330uF.

The +IN pin and the -IN pin should each see similar DC resistance to ground to minimise output offset.
The +IN sees 22k+2uF // 4.7uF.
At DC both of these are effectively infinite resistance.

The -IN sees 680r+10uF // 22k. One route is infinite resistance, the other route is 22k.

The grounding capacitor, 2uF, on the +input must be removed. Then +IN sees 220r+22k
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Last edited by AndrewT; 28th February 2010 at 10:15 AM.
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Old 28th February 2010, 11:28 AM   #6
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Hi AndrewT,

There are so much to learn when starting out with amplifiers, and your information is very helpful.

Thanks again

Craig
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Old 28th February 2010, 02:36 PM   #7
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Right, so I now have this arrangement:

Click the image to open in full size.

But is the -ve speaker output grounded to the power ground or the audio ground?

Sorry for all the dumb questions... (12 years as an electrical engineer, a few years of PIC programming experience doesn't seem to be helping with this analogue stuff!)
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Old 28th February 2010, 02:44 PM   #8
sangram is offline sangram  India
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Power ground, as that leg carries the return signal from the speaker. It is a high current connection. Ideally you would want to return that to the junction of the power capacitors of the supply, or a suitable similar high-current path.
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Old 28th February 2010, 02:46 PM   #9
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That was what I was thinking, so wanted to ask so I didn't blow anything up.

Thanks
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Old 28th February 2010, 04:37 PM   #10
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I've been looking for a cheap 220uF capacitor on Farnell (just to experiment with) but don't seem to be able to find anything. Can you use an electrolytic type capacitor here? Or is this another silly question!
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