Same old story LM3886 and DC offset - diyAudio
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Old 22nd June 2004, 06:19 AM   #1
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Default Same old story LM3886 and DC offset

I have this sub amp Iím building for well for a sub .
You can see the progression of the crossover here. I Donít think itís the problem.
Sub plate amp design

Tonight I hooked up the power end of the amp and fired it up.
I had each of the op-amps driving a crappy 6x9 both of em just go buzzing away. -215mv offset for each chip.

I have about 10mv DC offset from the crossover, it then goes into a 100K pot, and then into the 10uf cap shown. After that I get a variable amount of dc around .5-2mv (I think it's just the meter has nothing to read, since it acts the same disconnected)

You can see things as they are laid out here.
http://home.comcast.net/~0fficeboy/s...s/photo_6.html

The power pin 1 jumpers to pin5, V- is connected right to mute, pin7 gnd is connected to signal ground.

Any ideas?
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Old 22nd June 2004, 06:20 AM   #2
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Here is an LTspice file that gives the layout of the parts.
And Iíll have more pictures of the assembled amp tomorrow.
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Old 22nd June 2004, 06:48 AM   #3
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It's a classic by now. You have no path from the non-inverting inputs to ground. You'll need to connect something like 10kohm from the connection point of C1,R1,R2 to ground.

Rune
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Old 22nd June 2004, 07:34 AM   #4
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Default Re: Same old story LM3886 and DC offset

Quote:
Originally posted by officeboy
Any ideas?
Rune has the answer to this ..... but..... why can't you download the datasheet and look for typcial connection. The datasheet has always the very basics about the IC it bescribes.

If I am going to use an unknown part, transistors, IC's, whatever, I always try to hold of the datasheet.
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Old 22nd June 2004, 02:23 PM   #5
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Dosn't the 100K pot before the cap give me a path to ground?
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Old 22nd June 2004, 02:26 PM   #6
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Quote:
Dosn't the 100K pot before the cap give me a path to ground?
The secret lies behind the word BEFORE.

Regards

Charles
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Old 22nd June 2004, 02:36 PM   #7
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I don't see any pin of the LM3886 (GND) connected to the PSU caps (GND).
And from Mute to V- should go a resistor, 10k is fine.
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Old 22nd June 2004, 02:57 PM   #8
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Quote:
Originally posted by carlosfm
I don't see any pin of the LM3886 (GND) connected to the PSU caps (GND).
And from Mute to V- should go a resistor, 10k is fine.
Ground goes from caps back to power ground on PSU board, Ground from 3886 pin goes to crossover board ground, they are connected from these star points.
There is a 22K resistor from V- to mute pin.

And phase_accurate, that does seem to be the rub. I didn't think it would matter, but this morning i got some alligator clips and hooked up a 22K resistor to ground from non-inverting input and offset dropped to -100mv, and paralleling 2 of gave me 50mv.
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Old 22nd June 2004, 03:21 PM   #9
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Quote:
Originally posted by officeboy

Ground goes from caps back to power ground on PSU board, Ground from 3886 pin goes to crossover board ground, they are connected from these star points.

Connect the GND pin of the LM3886 to the PSU caps, and from there to the crossover board.
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Old 22nd June 2004, 03:52 PM   #10
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Officeboy, you just have to remember what the purpose of your input cap is. This a "DC blocking cap" which prevents any dc from your source getting into your amplifier. As this blocks all dc, it also prevents your non-inverting input from sensing ground through a dc path. That is why you need a resistor from your non-inverting input to ground, typically this resistor is the same value as your feedback resistor.


I learned this the hard way too
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