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LM3886 RC filter + DC offset questions.
LM3886 RC filter + DC offset questions.
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Old 27th April 2007, 05:38 PM   #1
blackboy is offline blackboy  Hong Kong
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Question LM3886 RC filter + DC offset questions.

Hi all, new comer here. I finished a lm3886 chip amp 3 weeks ago. Recently, I start changing the input cap (I used a very crap pair for testing at the very beginning). I tried BG PK 4.7uf which sounds quite muddy to me. At that time, the DC offset was arround 10mv and - 30mv. Then I tried changing the RC resister to 100k and the capacitor to Wima mkp 0.47uf, because I saw the formular of "f = 1/(2 * £k * 100000 * 0.0000001)" from tangentsoft.net... and then the problem came. The DC offset got up to -50mv and -95mv which really scared me like hell. I tried playing it, it sound alot better, but I was really worry about my speakers, therefore, I change the resistor back to 15k and use a BC mkp 4.7uf. But the DC offset are still very high... -50 and -90... I wonder if I have damaged the chips... or if there any solution for my amp... Thx guys!
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Old 27th April 2007, 06:08 PM   #2
FastEddy is offline FastEddy  United States
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Location: North Californie
Muddy output, ah ...

INOP: Advise adding polystyrene (or other plastic) snubbing caps to the power rails, close coupled to the power pins on the chip. This little trick always "brightens up" my op-amps and seems to help power amps. (This is what most of the op-amp and power amp chip makers do to get their great specs. SOP at TI and Analog Devices.) Values should be between 1/10 and 1/100 of the electrolytics on the power rails. From http://www.national.com/pf/LM/LM3886.html and your info below, I make this to be about 0.047 uF (47 nF) ... 2 places, parallel across Cs on the +/- rails to local ground. (You might also change C1 (2.2 uF) to an MKT type (plastic) ... which may improve response a little)

DC offset on any chip amp or op-amp is dependant on the power supply rail differences between + / - and local ground (which is amplified by the amp at the output in proportion to the over all gain ... but you already knew this). IMOP: Advise making adjustments to the voltage regulation ... adjusting either the plus (+) or minus (-) regulation, but not to both at once. Measure the DC voltage on each rail to several decimal places and adjust the one that is low. ... there are a whole bunch of ways to do this, like Andrew_T's suggestions below ... =

Also from Andrew_T: " ... Once this is offset issue settled, go back and select a larger plastic film capacitor to block DC at the input. ..." IMOP this blocking cap should be 4.7 uF or larger and also should be plastic MKT type or other poly type. ... But IMOP, if the DC offset can be done without a DC blocker, "Mr. Sulu, make it so .... "
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Old 27th April 2007, 06:16 PM   #3
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
I don't think there will be any damage.

The chip likes to see the same resistance on both it's input pins.
But even when this is done there will still be a smallish output offset.
If you care to experiment you can either reduce or increase the grounding resistor on the non-inverting input in small increments (5% or so at a time). You have find that going bigger will increase the output offset in the negative direction. You need to reduce the value slightly to reduce that -30mV.

As you have already found the offset varies with temperature and particularly from cold at start up.
I try to arrange for the offset to straddle the 0mV, slightly bigger when completely cold moving through zero when warming up and going to slightly less but opposite polarity when fully warmed up.

I would go back to the optimum resistance to minimise offset over a range of temperatures. Do you have a low impedance source? A passive pot throws all your trimming out the window.

Once this is offset issue settled, go back and select a larger plastic film capacitor to block DC at the input. Buy the best quality you think is good value. Aim for an RC time constant of about 70% of the RC on the NFB loop. I try to get 80 to 90mS on the input and 120 to 140mS on the NFB. You may have to use 4.7uF//4.7uF or 4.7uF//3.3uF to achieve this.
Remember that any DC block at the source will be in series with the dc block at the amplifier and this reduces the time constant markedly. It can completely ruin the bass peformance if ignored.

Following on from Fasteddy's advice try film cap from +Vpin to -Vpin as well as to ground.
regards Andrew T.
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