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Old 25th May 2009, 08:38 PM   #1
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Default Paralllel configuartion LM3886 - adjusting variable resistors

I am looking for some assistance with adjusting the variable resistors on a LM3886 parallel configuration. I recent purchased a prebuilt amp PCB from Nemo-e-store on eBay and I was very impressed with the sound from this configuration. I subsequently purchased the PCB's to DIY the design but, although the PCB came with a schematic it did not explain how to balance the 3 chips by adjusting the 3 variable resistors. I have come up empty handed searching the web and asking the guy I bought the PCB from.

The PCB I bought is here:
http://cgi.ebay.com/lm3886-3-Paralle...3A2%7C294%3A30

Can anybody help me with this problem?
Thanks for all y'alls help.
Sincerely,
Jeff M.
Lawton, USA
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Old 25th May 2009, 09:07 PM   #2
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Without a schematic it is guesswork, but they are probably there to adjust the DC offset to zero.
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Old 26th May 2009, 02:09 AM   #3
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I purfed up a 3*3886 non inverting and balanced it by lifting the three output resistors and adjusting each chip individually to the lowest dc offset. Then I adjust the two lowest to match the highest (8 mv).


Frank
http://www7.taosnet.com/f10/
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Old 26th May 2009, 02:39 AM   #4
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Ooops, meant to write 'inverting'
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Old 26th May 2009, 03:08 AM   #5
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Where exactly do you take the measurements? Across pin 3 and GND on each chip? Or is there another way to measure the DC offset? Also, how do you put a load across the output and GND of each individual pin or is it even neccessary?
Jeff
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Old 26th May 2009, 03:14 AM   #6
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A load is not necessary and yes, from 3 to ground but the resistors must be lifted so the outputs are isolated.
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Old 26th May 2009, 05:44 AM   #7
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i am sorry but this is a first for me....which resistors must be lifted? What is the general value of the resistor you are speaking of? There are several resistors, I am not certain which you are speaking of.
Thanks,
Jeff
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Old 26th May 2009, 07:17 AM   #8
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The big ones R2, R5, R11 on the PCB you linked to.
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Old 26th May 2009, 11:21 AM   #9
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I got it, thanks! Anybody know of a source to buy similar PCB's. I have tried to email www.Shine7.com several times with no response. I would love to get a few of the PCB's found at their website!

Thanks again for all of your assistance!
Sincerely,
Jeff
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Old 1st June 2009, 10:02 PM   #10
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Quote:
Originally posted by deepthrob
I purfed up a 3*3886 non inverting and balanced it by lifting the three output resistors and adjusting each chip individually to the lowest dc offset. Then I adjust the two lowest to match the highest (8 mv).


Frank
http://www7.taosnet.com/f10/

Sorry, but I can't understand. Could you explain how exactly you adjusted each chip individually to the lowest dc offset.
Thanks.
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