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TiGeRGuY 4th October 2012 12:46 PM

what do you think about my LM3886 PCB design?
Hi! this is my design, its not final, i guess the real thing would be a little more aesthetic, but it shows the idea.

- the supply caps on the board will be changed to 220uF
- i know the V- cap is far from the chip but i thought it would be better to sacrifice it for good grounding.
- the speaker ground won't be connected from this board, it will be connected from the star ground of the case, this trace is an optional connection, do you think it may cause a problem?
- 0.1 caps will be soldered directly at the suply pins and will be connected to the pin 7 (ground pin of the IC), is it ok?

tnks guys :D

Boscoe 4th October 2012 05:37 PM

There's no pic...

yagoolar 4th October 2012 05:51 PM


Originally Posted by Boscoe (
There's no pic...

There is. I can see.

Boscoe 4th October 2012 05:53 PM

I can see it now.

The layout looks okay but you could get that coupling cap closer to the chip just out it in the other side of the other cap and join their GNDs together with a link to GND - I'm sure the LM won't mind!

Why do you have a 10r output resistor? You should connect your speaker GND to the board then connect the board to PSU GND. The PSU connection to chassis (if you choose to use one) should be a branch off the PSU star GND.

TiGeRGuY 4th October 2012 08:16 PM

Tnks Boscoe! You realy opened my eyes! I can't think of a way to ground this cap if it will be where you ment (if i figured right its above the CS2),
But your comment made me think of another way to get it closer, to lower the ground trace to the connector, and make another tarce of V- (for the cap) from between the connector's V+ and ground pins.
But I would be happy if you'll try to explain me your suggestion, it may be way better.

The 10R resistor will be paralleled to an indactor, if i won't find a nice choke it will be a magnet wire wraped around the resistor.

About the grounding for the speakers, I saw on the web several ways to do it, but the way that convinced me the most was to run the PSU ground to a star on the case and from their run it to the boards and to the speakers directly.
I don't know what way is the best so i left the option you said and i will check what's best.

And another thing, I cant decide what resistor to use for the zobel (RSN) 2.7R or 10R.

Boscoe 5th October 2012 06:48 AM

Work out the roll off of the filter and see what you think. Regarding the output inductor/resistor, leave the possibility to add it later, I have built quite a few LM3886/3875 without the zobel even so you might get away without it but this needs to be decided once its built and tested. However it might be fine with one setup and unstable with another depending on the cable and speaker load/crossover.

TiGeRGuY 5th October 2012 12:58 PM

Tnks Boscoe, I prefer the amp to be stable for every setup so I will keep the zobel. :)
I will always have the posibility to wire the output to the connector directly (without the zobel).

So here is what i did, very messy but only to present the idea:

you think it is better like this? maybe i should use your idea (witch i didn't understand yet)?

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