Same old story LM3886 and DC offset

officeboy · 22nd June 2004, 06:19 AM

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.
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?

officeboy · 22nd June 2004, 06:20 AM

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

runebivrin · 22nd June 2004, 06:48 AM

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 · 22nd June 2004, 07:34 AM

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.

officeboy · 22nd June 2004, 02:23 PM

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

phase_accurate · 22nd June 2004, 02:26 PM

Quote:

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

The secret lies behind the word BEFORE.

Regards

Charles

carlosfm · 22nd June 2004, 02:36 PM

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 · 22nd June 2004, 02:57 PM

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.

carlosfm · 22nd June 2004, 03:21 PM

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.

rayban68 · 22nd June 2004, 03:52 PM

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

22nd June 2004, 06:48 AM	#3
runebivrin diyAudio Member Join Date: Jul 2003 Location: Huddinge, Sweden	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 __________________ Do wizards use spell checkers?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
dc offset question (regarding LM3886 Chipamp kit)	prospero21	Chip Amps	3	15th July 2008 11:19 AM
follow up: LM3886 RC filter + DC offset questions.	blackboy	Chip Amps	9	2nd May 2007 04:48 PM
LM3886 RC filter + DC offset questions.	blackboy	Chip Amps	2	27th April 2007 06:16 PM
LM3886, Jax how about offset	peranders	Chip Amps	11	21st January 2004 08:25 PM
My story	Dj BASS AMP	Introductions	14	16th March 2003 10:43 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

22nd June 2004, 02:23 PM	#5
officeboy diyAudio Member Join Date: Nov 2003 Location: NW Washington	Dosn't the 100K pot before the cap give me a path to ground?

22nd June 2004, 02:36 PM	#7
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	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.

22nd June 2004, 03:52 PM	#10
rayban68 diyAudio Member Join Date: Mar 2004 Location: Prince George, BC, Canada	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