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LM3886 Layout ok?
LM3886 Layout ok?
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Old 26th September 2009, 12:51 PM   #1
Maxxtr0 is offline Maxxtr0  Germany
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Default LM3886 Layout ok?

Hello,

I´ve just finished my first PCB layout for my LM3886 Amp and i want to ask you if it´s ok?

The schematics I´ve used are more or less exactly the same as those from chipamp.com

The 0,1µF Caps in parallel to the two 2200µF Caps will get soldered directly to the pins of these; Rfeedback will also geht soldered directly to the LM3886s pins!

Click the image to open in full size.
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Old 26th September 2009, 01:38 PM   #2
Salas is offline Salas  Greece
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LM3886 Layout ok?
Welcome to the forums. Don't use the .th.jpg in your pic link. It only posts a thumbnail. Check it out when you post and delete the .th part. Did it for you in that one.
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Old 26th September 2009, 02:06 PM   #3
Maxxtr0 is offline Maxxtr0  Germany
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Thank you Salas!

So hereīs a second Layout... completely different from the first one:

(without .th )
Click the image to open in full size.
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Old 26th September 2009, 03:23 PM   #4
jaycee is offline jaycee  United Kingdom
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Second one is better, but I would not put the output pin where it is. You need to use a thicker track too, go for at least 2.54mm. You probably want to disable thermals on the power planes as well.
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Old 26th September 2009, 03:43 PM   #5
Maxxtr0 is offline Maxxtr0  Germany
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Why wouldnīt you place the ouput where Iīve placed it?
You mean a thicker track for the output, yes? I thought about soldering an solid copperwire right onte the track...
And the thermals are already "disabled" - more or less
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Old 26th September 2009, 06:44 PM   #6
jaycee is offline jaycee  United Kingdom
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I never like the speaker output to be anywhere near power tracks.Think that with only a small solder bridge, stray bit of wire etc, your speaker can end up directly connected to +V = bye bye speaker. Try to isolate it. I'd suggest skipping the track to pin 5, and instead put a wire jumper between the two.

I also find that the Zobel components RZ and CZ work just as well if you put them across your speaker terminals instead of on the PCB. That will give you a little more room. My own LM3886 is stable without the Zobel though, if you wish to try that.

edit: I just noticed there is a component missing - there is no feeback resistor between the output and the inverting input. I guess you are planning to add this by hand between the pins?

Last edited by jaycee; 26th September 2009 at 06:46 PM.
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Old 26th September 2009, 08:50 PM   #7
Maxxtr0 is offline Maxxtr0  Germany
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So... now I´ve done the following things:

- correctly dissabled the thermals
- removed the connection from Pin1 to Pin5; will solder it on the LMs Pins, as suggested
- Output is now directly under Pin3
- thickend the "track" from Pin3 to Output

Click the image to open in full size.

I want to use the Zobel-Network, but I´ve got to ask, if the tracks to Rz an Cz have to be as thick as the one from Pin3 to Out?

And yes, Rf will get soldered directly to the Chip

Last edited by Maxxtr0; 26th September 2009 at 09:06 PM.
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Old 26th September 2009, 09:32 PM   #8
akis is offline akis  United Kingdom
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Why is the board so small? The LM3886 requires a sizeable heatsink which implies there should be available space in your enclosure. I notice the +/- 18V which implies less heat generated, but still...
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Old 27th September 2009, 10:55 AM   #9
Maxxtr0 is offline Maxxtr0  Germany
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It can have itīs "sizeable" heatsink, even if the Boardīs that small
If thereīs no need to make it bigger - then why should I make it bigger? Of course making it that small makes it much more complicated - but thatīs why I am here, requesting your help
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Old 27th September 2009, 04:57 PM   #10
jaycee is offline jaycee  United Kingdom
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A picture is worth a thousand words

Here is how I did it

Click the image to open in full size.

edit: Technically, this is not a gainclone. The onboard capacitors are small, only 330uF. This is designed to be connected to a seperate PSU board which has 2x4700uF per rail

Last edited by jaycee; 27th September 2009 at 05:04 PM.
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