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Old 21st November 2005, 05:44 PM   #1
sumacSK is offline sumacSK  Slovakia
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Default Lm3875 Rev C

Hi all,

I know that it has been discussed many times before but I still have some obscurities. I ordered Non-Inverting LM3875 rev.3 Stereo Kit. My Q's are: What are those optional rezistor and capacitor for? Whether should I use them? Next thing, there are only doides and capacitors for PSB, is it all what I need for PSB?
Maybe if somebody has a picture of asembled circuit boards it could be very usefull.

Thats all for now, thanks Martin
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Old 21st November 2005, 06:27 PM   #2
javven is offline javven  United States
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It sounds like you have one of Brian's kits.

The optional resistor and cap are discussed somewhat on Brian's page (look for the LM3875 construction PDF's).

The recifier PCB only does have some recifier diodes and caps. That's all it seems to need. There is the 'snubberized psu' option, though. If you were looking for 3000uF + filter caps on the PSU, the snubberized version is where you'll find them. Read up on the 3886 construction and the snubberized PSU, same site.

I was surprised the snubberized kit helps as much as some folks seem to be saying - but it may be worth the build. I'm building both and comparing.
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Old 21st November 2005, 06:59 PM   #3
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Brian's LM3875 board is similar to what I'm using. This schematic may be helpful: http://www.audiosector.com/lm4780%20psu.pdf
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Old 22nd November 2005, 12:04 AM   #4
BrianGT is offline BrianGT  United States
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Default Re: Lm3875 Rev C

Quote:
Originally posted by sumacSK
Hi all,

I know that it has been discussed many times before but I still have some obscurities. I ordered Non-Inverting LM3875 rev.3 Stereo Kit. My Q's are: What are those optional rezistor and capacitor for? Whether should I use them? Next thing, there are only doides and capacitors for PSB, is it all what I need for PSB?
Maybe if somebody has a picture of asembled circuit boards it could be very usefull.

Thats all for now, thanks Martin

Martin,

Peter's schematic should explain the extra component placements on the pcb. The standard configuration is the one in the users guide for the LM3875. I am guessing that you bought it from him, as my kits have only been shipping out for US orders.

As for assembled pictures of the boards, if you are to use the extra components for the snubberized supply, it should look something like this:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

I found it best to solder the large caps in last (including input/output wiring), as they are the only component mounted on the bottom of the pcb.

Best of luck with your project,

--
Brian
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Old 22nd November 2005, 12:29 AM   #5
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Default Re: Lm3875 Rev C

Quote:
Originally posted by sumacSK
Hi all,

..... Next thing, there are only doides and capacitors for PSB, is it all what I need for PSB?
Maybe if somebody has a picture of asembled circuit boards it could be very usefull......

You need a transformer too.
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Old 22nd November 2005, 09:28 AM   #6
sumacSK is offline sumacSK  Slovakia
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Hi all,

THANKS! everyone for your help.
Brian, I bought it from you- I've been to US couple of monhts ago.
So if I understand it correctly I can build recifier PCB with only those diodes and caps (maybe I'll add those components to make it snubberized later). But I'm still unsure about those optional Cz and Rz on amplifier PCB. They should be used only when snubberized supply is used or is it a zobel network, or... ?? I'm confused..
And the last Q is: do I have to (also on Rev.3 board) solder the NFB resistor directly to the pins of the chip? Or it is that Rf resistor?

Thanks again Martin
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Old 22nd November 2005, 11:29 AM   #7
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by sumacSK
Hi all,

THANKS! everyone for your help.
Brian, I bought it from you- I've been to US couple of monhts ago.
So if I understand it correctly I can build recifier PCB with only those diodes and caps (maybe I'll add those components to make it snubberized later). But I'm still unsure about those optional Cz and Rz on amplifier PCB. They should be used only when snubberized supply is used or is it a zobel network, or... ?? I'm confused..
And the last Q is: do I have to (also on Rev.3 board) solder the NFB resistor directly to the pins of the chip? Or it is that Rf resistor?

Thanks again Martin
For the power supply board, you can always build it with the standard configuration and upgrade later.

As for the the feedback resistor, Rf, you can mount it on the pcb, or mount it on the chip directly. It is your choice.

As for the Rz and Cz, there are the optional zobel components, a 2.7ohm 2w resistor and a 0.1uf cap.

--
Brian
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Old 22nd November 2005, 11:35 AM   #8
sumacSK is offline sumacSK  Slovakia
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Thanks Brian, now everything seems to be clear..
Thank you. Martin
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Old 22nd November 2005, 11:45 AM   #9
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by sumacSK
Thanks Brian, now everything seems to be clear..
Thank you. Martin
Great! Best of luck on your project.

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Brian
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Old 22nd November 2005, 05:37 PM   #10
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maybe offtopic;

Why is my GC mini buzzing(not loud)?
I connected all GNDs to one point(speaker,pc out,chip...-aka star connection),I have 2x1000uF caps per a chip-directly mounted,2x28V on secondary...

See below for schematic
NOTE:I didnt add 1,5K optional resistor
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File Type: jpg gcmini.jpg (23.2 KB, 934 views)
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