problem. -35v at the output.

atenolol50 · 1st February 2005, 01:54 AM

hi,

i'm having a bit of a situation over here.

-35v DC at the output.

i'm using the schematic from brianGT for LM3875.
i'm able to get +/-36V at the V+ and V- resp. measured at pins 1 and 4.
the NFB resistor is also in place between pins 3 and 8.

need troubleshooting tips.

thanks.

Sheldon · 1st February 2005, 02:47 AM

You can probably find your answer here: speaker sits fully forward (or backward)

Sheldon

scone · 1st February 2005, 02:51 AM

It sounds like you have a short between -V and your output, I would check the solder connections. Check the voltage on your chassis too...

Mattyo5 · 1st February 2005, 03:09 AM

I had that, turned out my feedback resistor's paint had chipped off and was bridging my -V rail and the output. yikes!

i'd check on something like that in this case

-Matthew K. Olson

Enrico · 1st February 2005, 10:56 AM

Hello Atenolol50,

During testing of my LM 3886 gainclone (gainclone with smd parts by Per-Anders), the voltage at the speaker output is 36V. But after replacing the LM3886 IC, the voltage at the output becomes 0V. Now my gainclone is already playing music. :-)

By the way, I am also from Singapore. If your are free, you can contact me.

cheers,

Enrico

Address:
Blk 231 Ang Mo Kio Ave.3, #11-1226, Singapore
Tel. No.: 64514239

demogorgon · 1st February 2005, 02:24 PM

I had a simelar problem a while back, turned out i had missplaced the feedback resistor. i was regulating at 25v..
lm3886 +25V out with hizz and hum!
heres the thread..

atenolol50 · 2nd February 2005, 03:49 AM

its kinda strange.

when i got -36v i thought my chip was broken so desoldered and replaced it.
i got a -15mv reading, was happy and went on redo the other channel and hooked both of them up.
the 2nd channel still gave me -36v, went back to check the first, but its back to -36v too.

are my chips damaged?

Reklats · 2nd February 2005, 06:49 AM

I was building a p2p lm3875 amp a while ago, and the first indication that I'd done something wrong was -36 volts on the output. It turned out I'd forgotten to connect the inv. input. (I was rushing)

Double check your p2p wiring. A lot.

Sheldon · 2nd February 2005, 11:24 PM

Quote:

Originally posted by atenolol50
its kinda strange.

when i got -36v i thought my chip was broken so desoldered and replaced it.
i got a -15mv reading, was happy and went on redo the other channel and hooked both of them up.
the 2nd channel still gave me -36v, went back to check the first, but its back to -36v too.

are my chips damaged?

The chips are pretty tough, so most likely its a wiring error /bad joint somewhere. It is possible to cook them by reversing the polarity of the power supply, but I'm not revealing how I know this.

pinkmouse · 2nd February 2005, 11:49 PM

With the LMs, if they look ok, they usually are, the internal protection is pretty good.

It's when they look like this, you know you might need to replace them.

1st February 2005, 02:51 AM	#3
scone diyAudio Member Join Date: Nov 2004 Location: Austin, TX	It sounds like you have a short between -V and your output, I would check the solder connections. Check the voltage on your chassis too... __________________ audio projects

2nd February 2005, 11:49 PM	#10
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	With the LMs, if they look ok, they usually are, the internal protection is pretty good. It's when they look like this, you know you might need to replace them. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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1st February 2005, 01:54 AM	#1
atenolol50 diyAudio Member Join Date: Nov 2004 Location: Singapore	problem. -35v at the output. hi, i'm having a bit of a situation over here. -35v DC at the output. i'm using the schematic from brianGT for LM3875. i'm able to get +/-36V at the V+ and V- resp. measured at pins 1 and 4. the NFB resistor is also in place between pins 3 and 8. need troubleshooting tips. thanks.

1st February 2005, 02:47 AM	#2
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	You can probably find your answer here: speaker sits fully forward (or backward) Sheldon

1st February 2005, 03:09 AM	#4
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	I had that, turned out my feedback resistor's paint had chipped off and was bridging my -V rail and the output. yikes! i'd check on something like that in this case -Matthew K. Olson

1st February 2005, 10:56 AM	#5
Enrico diyAudio Member Join Date: Oct 2004	Hello Atenolol50, During testing of my LM 3886 gainclone (gainclone with smd parts by Per-Anders), the voltage at the speaker output is 36V. But after replacing the LM3886 IC, the voltage at the output becomes 0V. Now my gainclone is already playing music. :-) By the way, I am also from Singapore. If your are free, you can contact me. cheers, Enrico Address: Blk 231 Ang Mo Kio Ave.3, #11-1226, Singapore Tel. No.: 64514239

1st February 2005, 02:24 PM	#6
demogorgon diyAudio Member Join Date: Jul 2004 Location: Behind you.	I had a simelar problem a while back, turned out i had missplaced the feedback resistor. i was regulating at 25v.. lm3886 +25V out with hizz and hum! heres the thread..

2nd February 2005, 03:49 AM	#7
atenolol50 diyAudio Member Join Date: Nov 2004 Location: Singapore	its kinda strange. when i got -36v i thought my chip was broken so desoldered and replaced it. i got a -15mv reading, was happy and went on redo the other channel and hooked both of them up. the 2nd channel still gave me -36v, went back to check the first, but its back to -36v too. are my chips damaged?

2nd February 2005, 06:49 AM	#8
Reklats diyAudio Member Join Date: May 2004 Location: Ohio	I was building a p2p lm3875 amp a while ago, and the first indication that I'd done something wrong was -36 volts on the output. It turned out I'd forgotten to connect the inv. input. (I was rushing) Double check your p2p wiring. A lot.

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