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Old 7th March 2012, 05:49 AM   #151
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Location: El Triunfo
Question Audio Amplifier with LM1875

Hi all!
Here are two seeds on the internet where different feedback: Av=(22k/1k)+1 and Av=(180k/10k)+1 !?
Which schematic better?
And here's how I made audio amplifier ​​with the LM1875 .
thank you!
Attached Images
File Type: jpg Shematic-1.jpg (55.6 KB, 518 views)
File Type: jpg Figure-F.JPG (98.3 KB, 522 views)
Attached Files
File Type: pdf Shematic and texst.pdf (20.7 KB, 122 views)

Last edited by formula 22; 7th March 2012 at 05:58 AM.
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Old 7th March 2012, 09:27 AM   #152
bcmbob is offline bcmbob  United States
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JohnnyPOO, Please take a look at this thread. A new approach.
Bob M.
"Arrange Whatever Pieces Come Your Way."
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Old 7th March 2012, 12:01 PM   #153
AndrewT is offline AndrewT  Scotland
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There is much to commend post151 schematic.
It shows:
Input filters
AC coupled.
Local decoupling
Output Zobel
Fused supply rails
Separate Signal and Power Grounds.

All that's left is ON/OFF muting and selecting component values.
regards Andrew T.
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Old 7th March 2012, 05:42 PM   #154
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Originally Posted by JohnnyP00 View Post
Would it be possible to replace the 22k resistors on BrianGT's amp boards with 1/4 watt instead of 1/2 watt. Having a hard time sourcing 1/2 watts that small in Canada. Thanks for your help.. JohnnyP
That is a low current area and thus 1/4w will do fine.
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Old 4th March 2013, 12:14 AM   #155
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Originally Posted by BrianGT View Post
I haven't had a chance to work on the manual yet, but here are some board pictures, and info:

Amp board:
Click the image to open in full size.Click the image to open in full size.
R1 - not used (optional resistor to ground before C1)
R2 - 22k (input to ground resistor)
R3 - 1k (gain resistor , gain = 1 + R4/R
R4 - 22k (nfb resistor - on bottom of pcb)
R5 - 2.7 2w (optional - zobel network)
R6 - jumper (optional resistor between input ground and output ground)
C1 - 2.2µF (optional input capacitor)
C2 - 22µF bipolar (optional)
C3 - 0.1µF (power supply decoupling)
C4 - 0.1µF (power supply decoupling)
C5 - 0.1µF (power supply decoupling)
C6 - 47µF (power supply decoupling)
C7 - 0.1µF (optional - zobel network)

PSU board:
Click the image to open in full size.Click the image to open in full size.
R0 - 10k (series resistor for LED)
R1,R2 - 1 2w (optional "snubber" network capacitors)
C1,C2 - 1500µF (main power supply caps)
C3,C4 - 0.1µf (power supply decoupling)
C5,C6 - 0.1µf (optional "snubber" network capacitors)
D0 - blue LED
D1-D4 - MUR860

Transformer wiring:
For the PSU transformer connections, AC1 denotes the first secondary winding of the transformer, and AC2 denotes the second secondary winding of the transformer. It is also possible to use a center tapped transformer with this PSU board by wiring the center-tap to either AC1(bar over top) or AC2.

As for size, the LM1875 amp boards are almost half the size of the original LM3875/LM3886 amp boards.

Basic Assembly Info:
Solder components on board in order from lowest profile to highest profile, with the PSU diodes and LM1875 chip being soldered last.

As the pics show, the boards were made on the 11th week of 2007... over a year ago. I believe I mailed 10-15 boards out to various people, so let me know if you have any additional questions,

Thanks to Allan for transcribing his component notes and taking pictures!

Does anyone have the original pictures for this? Was a guide ver made?
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Old 4th March 2013, 11:12 AM   #156
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Originally Posted by magomago View Post
Does anyone have the original pictures for this? Was a guide ver made?
Brian's kit is here: LM1875 Amplifier Kit |
It seems that there's no published manual nor pictures on the site at this time. That site needs some maintenance. However, the boards may still be available.
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