LM3875 problem: output < 0.35V - diyAudio
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Old 16th March 2012, 10:50 AM   #1
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Default LM3875 problem: output < 0.35V

Hello All,

hope someone may understand what's happening to my ChipAmp...

I just finished building some LM3875 Amps (AudioSector Premium Kit) and one is not working.

Problem:
Output is < 0.35V with an 1KHz sine wave input at max volume.
With a speaker attached (and max input) the tone can be heard ever so slightly but it sounds like I'm hearing overtones or some modulation, not a pure 1KHz tone.
The output voltage does attenuate to 0 if I turn the input volume down to nothing.

Build details:
-Power supply is a ChipAmp kit (not AudioSector kit)
-300VA Plitron transformer
-Rails verified to be running at +/- 28.7V
-Amp DC offset is 2mV (at max input)
-Zoble network NOT installed
-This amp is a mono block design

Notes:
-Also note, I have just built 4 amps (all mono blcoks) exactly the same.
Exactly the same... 3 work just fine, this one does not.

-The LM3875 has a proper heat sink and appears to be generating negligible amounts of heat.

I've spent sometime to understand if I've made a construction error, or if there's a fault somewhere, but for the moment I've not spotted anything irregular.

So now I'm turning to the forum for help.
Please help,
I'm down one amp

tangofish

Last edited by tangofish; 16th March 2012 at 11:18 AM.
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Old 16th March 2012, 11:07 AM   #2
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Is the DC offset very nearly the same as the others.........just looking for clues .
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Old 16th March 2012, 11:21 AM   #3
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I'm glad you asked me this - just measured them all...
-The DC offset on the non working Amp is +2mV.
-The 3 workings amps are -39,-55, & -61mV.
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Old 16th March 2012, 11:29 AM   #4
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Maybe youve put in an incorrect resistor value by mistake .
Or with the power off measure the resistance across each resistor and compare with a good amp.
Or failing that compare voltage readings at resistors with a good amp.
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Old 16th March 2012, 12:26 PM   #5
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Fust check all the resistor values, and verified the most obvious signal paths to the schematics. The good and bad amps are the same.

However, I did find one clue:
On the working amp, the resistance between OUT and OG or CHG is 22.5K
On the bad amp, the resistance between OUT and OG is 22.5K, but between OUT and CHG it is 13K.
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Old 16th March 2012, 04:56 PM   #6
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Ok, I pulled all the parts of the board leaving only the LM3875.
I tested and inspected all the parts. Everything seems fine.

Putting it back together, testing as I go I finding the resistance form OUT to CHG is around 10K. On the working amps it's ~22K Ohms.

Examining the traces on the board, I see that OUT goes to PIN 3 directly and PIN 8 Through a the 22K FB resistor. No were else.

I have a bad feeling this chip might be bad

Suggestions anyone?
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Old 16th March 2012, 05:25 PM   #7
Pixo is offline Pixo  India
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Are you able to post some pictures?
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Old 16th March 2012, 06:25 PM   #8
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Ok, been wrestling with this for the entire day...

Problem is now found and solved. The last thing I ever would have suspected... one the 1500uF caps was bad! I had hot glued all my caps to the board, so it's possible I damaged one, maybe, or maybe it was just bad

I've replaced them both. The amp is working now absolutely fine

I can still post photos if you would like Pixo?
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Old 16th March 2012, 06:42 PM   #9
Pixo is offline Pixo  India
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Nah, i'm glad you got that figured out. enjoy your amps
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Old 16th March 2012, 07:30 PM   #10
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Well found......
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