Non-Inverted Gainclone PCB layout (LM3875) - diyAudio
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Old 22nd February 2004, 12:17 AM   #1
BrianGT is offline BrianGT  United States
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Default Non-Inverted Gainclone PCB layout (LM3875)

Well, after making a few gainclone gainclone amps for myself, I have a bunch of friends who want me to build them one, so I decided to make a pcb to simplify the build.

I am going to place the NFB resistor on the device pins to get the shortest signal path. For the capacitors, I want to try using some SMD Panasonic FK caps, in addition to the FC caps I have used before. I also added spacing for the BG caps.

Let me know if you have any comments/suggestions for the pcb.

--
Brian
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Old 22nd February 2004, 12:18 AM   #2
BrianGT is offline BrianGT  United States
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Schematic, using the recommended components by PD.

22k MK132V resistor on the device pins.

--
Brian
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Old 22nd February 2004, 12:35 AM   #3
sek is offline sek  Germany
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Hi Brian,

Star Grounding?
I'd recommend not to implement an output gnd connector on the board at all. Just wire the speaker terminal "-" to the supply star ground.

Sebastian.

PS: And get yourself a better eCAD soft
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Old 22nd February 2004, 12:38 AM   #4
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Quote:
Originally posted by sek
Hi Brian,

Negative Feedback?
Star Grounding?

Sebastian.
The NFB resistor is mounted off the pcb, directly to the LM3875 pins, as the schematic and pcb picture denotes.

As for the grounding, I had seperate ground planes for the signal ground, and for the power ground, and I join them together at the chassis ground pad on the pcb (CGND).

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Brian
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Old 22nd February 2004, 12:42 AM   #5
sek is offline sek  Germany
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Sorry, Brian, I was editing my post while you replied!

Quote:
I join them together at the chassis ground pad on the pcb (CGND).
That effectively means you've got your supply star ground on the board? Remember to ground everything on the board then. Of course you wouldn't be able to do stereo/multichannel applications with a single supply then...

Sebastian.
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Old 22nd February 2004, 12:49 AM   #6
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by sek
Sorry, Brian, I was editing my post while you replied!



That effectively means you've got your supply star ground on the board? Remember to ground everything on the board then. Of course you wouldn't be able to do stereo/multichannel applications with a single supply then...

Sebastian.
I have my current setup with a single supply for two channels with no ground hum. What about this setup will cause problems?
Both channels will be going to the chassis ground.

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Brian
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Old 22nd February 2004, 12:54 AM   #7
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Default count me in

If you decide to make a lot of these and sell some, I would like to get in on the final version. I would probably take 10 to 20 depending on the cost per item.

Thanks

Jeff
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Old 22nd February 2004, 01:01 AM   #8
sek is offline sek  Germany
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If it works this way, then everything's fine.

Another one: I think your thermals are too narrow. You could easily implement some fuses here (e.g. at the V- pin of the chip).

Just another question: The blue lines are jumpered wires (e.g. underneath)? Can't you just route them on the board? There would be a possibility to keep proper ground-planing.

I'm just thinking about it... If you put the OUT pad right near the OUT pin of the chip and concentrated the grounding thing a little better (e.g. "real" star ground near the rectifier), you could move the caps together a little (e.g. in front of the chip). By doing so you could shorten the distance between the caps and the chip (it's stated to be important) and save board space (more prints per board sheet, more modules per heatsink, etc... At least Jeff wants 20, so efficiency is an issue ).

Sebastian.
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Old 22nd February 2004, 01:08 AM   #9
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Default I just here to help out.

My friends want some as well.
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Old 22nd February 2004, 01:19 AM   #10
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by sek
If it works this way, then everything's fine.

Another one: I think your thermals are too narrow. You could easily implement some fuses here (e.g. at the V- pin of the chip).

Just another question: The blue lines are jumpered wires (e.g. underneath)? Can't you just route them on the board? There would be a possibility to keep proper ground-planing.

I'm just thinking about it... If you put the OUT pad right near the OUT pin of the chip and concentrated the grounding thing a little better (e.g. "real" star ground near the rectifier), you could move the caps together a little (e.g. in front of the chip). By doing so you could shorten the distance between the caps and the chip (it's stated to be important) and save board space (more prints per board sheet, more modules per heatsink, etc... At least Jeff wants 20, so efficiency is an issue ).

Sebastian.
I made the heat relief traces going to the pads larger, increasing to 30mils, and making a bigger one on the one going to the chip.

The blue lines are the traces on the bottom layer of the pcb.

I thought about putting the caps in front of the chip, but settled on this arrangement. The size of this board is already quite small... size the image to 2.5" x 1" on your screen (6.35cm x 2.54cm).

Thanks for your input. Here is the modified picture with thicker heat relief traces, and the bottom layer traces not shown.

--
Brian
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