Same old story LM3886 and DC offset

[officeboy]

I have this sub amp I’m building for well for a sub .
You can see the progression of the crossover here. I Don’t think it’s the problem.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=33067&highlight=

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/shivaamp/photos/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?

 

[officeboy]

Here is an LTspice file that gives the layout of the parts.
And I’ll have more pictures of the assembled amp tomorrow.

 

[runebivrin]

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

 

[peranders]

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.

 

[officeboy]

Dosn't the 100K pot before the cap give me a path to ground?

 

[phase_accurate]

Dosn't the 100K pot before the cap give me a path to ground?

The secret lies behind the word BEFORE.

Regards

Charles

 

[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.

 

[officeboy]

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.

 

[carlosfm]

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.

 

[rayban68]

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

 

[officeboy]

Thanks rayban68 I think that was my confusion. I'll have some 22K resistors wired up to ground at lunch.

And carlosfm I'll try swapping the pin7 ground from the signal star to the power star. Not sure if it will make a difference for the offset, but it can't hurt for the sound.

Also I've got those extra pictures up now.
http://home.comcast.net/~0fficeboy/shivaamp/index.html

 

[officeboy]

Connect the GND pin of the LM3886 to the PSU caps, and from there to the crossover board.

You got this one right Carlos.
It’s funny I did quite a bit of searching on this topic and the best info i could find that that it was probably just a pin for control of the mute circuit and either ground point would be fine.
But i had a bit of a buzz, and about -40mv of dc offset that wouldn't go away. After redoing the ground I have that dead silence I’m used to, and my offset is down to -4mv

It does get bloody hot running my sine tests. Hope that my little heat sink will be ok for normal music and HT.