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An open source layout for LM3886?
An open source layout for LM3886?
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Old 26th April 2018, 05:30 PM   #1
00940 is offline 00940  Belgium
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Default An open source layout for LM3886?

There must be a hundred threads about the lm3886 basic implementation. Some good, some awful. Wouldn't it be nice to have a fool proof pcb-schematic-BOM set we can refer beginners to ? Maybe not the ultimate fidelity or the most tricked up design ever but just a solid, decent performer ? The pcb would provided as tested gerber files.

Here's a first suggestion up for criticisms/improvements. It's based on a smaller smd design which works quite well in practice but for a simple board I moved everything back to through hole (except for the muting resistor).

Nothing extravagant. A few options are provided for the input cap and the output/PS connectors can either be screw terminals or faston. The board is 65*50mm. All electrolytic caps are sized for 50Vdc caps. The resistors at the input are 125mW, the ones in the feedback network 500mW.

Final version:
I leave this first post pretty much untouched but the layout attached isn't perfect yet of course. The final version can be found here: An open source layout for the lm3886 ? - Page 6 - diyAudio

Eagle and gerbers files:
The PSU is at post #63 of the thread, the protection board at post #121 and the amp board at post #80.


Acknowledgement:
- This layout for the lm3886 has been made possible by the wealth of information given by Tom Christiansen, on his website neurochrome.com and on this forum (see especially this seminal thread, among many others).
- In this thread, I'm particularly indebted to the contributions of Mark Whitney and bozoc who pushed things in the right direction and corrected me when needed.
Attached Images
File Type: png lm3886sch.png (13.4 KB, 2795 views)
File Type: png lm3886th.png (60.6 KB, 2823 views)
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Last edited by 00940; 2nd May 2019 at 04:02 PM.
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Old 26th April 2018, 06:37 PM   #2
DPH is offline DPH  United States
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Just going off the schematic, I'd throw 100-330uF electrolytic in parallel with a tight-to-pin 10uf/100v x7r for psu bypass (on each rail). If the layout keeps the inductance low, then the 3886 will enjoy low impedance all the way out to its ugbw.

Edit: looks like c15,c16 accomplish that.
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Old 26th April 2018, 06:45 PM   #3
bullittstang is offline bullittstang  United States
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An open source layout for LM3886?
speaking as a newbie - great idea 00940. Can't offer assistance in the design, but commend you on getting this thread started.
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Old 26th April 2018, 06:59 PM   #4
Lingwendil is offline Lingwendil  United States
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Any chance on an inverting configuration layout too?
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Old 26th April 2018, 07:44 PM   #5
adason is offline adason  United States
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...inverting with good jfet buffer would be even better!
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Old 26th April 2018, 07:57 PM   #6
DPH is offline DPH  United States
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Beginner board, lets minimize scope creep and focus on the fundamentals, guys!
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Old 26th April 2018, 09:51 PM   #7
00940 is offline 00940  Belgium
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Yes, I'd rather keep it simple at first. Non-inverting has the advantage of a highish input impedance, so the input cap can be kept small and a buffer isn't really required to drive it. A 10k log pot (providing a 2.5K output impedance at worst) would be fine in most uses. I'd also rather like to test only one version if possible.

Still, I'm open to suggestions to options that make life easier (such as the different footprints for the input caps or the like).

@DPH: yes, C15-C16 are X7R 4.7uF/100V (a compromise in between physical size and value). C13-C14 can be polymer up to 39uF/50V (biggest I've found in 8mm wide caps). C9 to C12 can be up to 470uF/50V electrolytics, twice over the minimum value suggested by the datasheet.
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Old 27th April 2018, 12:15 AM   #8
DPH is offline DPH  United States
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Good deal, Ben. I was looking at the top part of the circuit and didn't realize you called out the bypass on the PSU side, my apologies. That all looks good.

Couple things of note (read: ramblings of someone whos not actually that good at layout): any reason not to have the entire ground plane flooded? Are you worried about the high current bounce on the input filter? And then just drop C6 wherever it lands? It's not a high current path, and may lower the overall plane impedance.

R5, R6, R7 could be turned 90 degrees and then you might be able to shorten the critical path of R5 (it should go first as it has the greatest effect on HF stability, even over the compensation network). Any reason to *not* flood the -input pin?
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Old 27th April 2018, 12:29 AM   #9
bozoc is offline bozoc  Croatia
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I would use one footprint size of the input cap.
4.7uF is one dollar on mosuer.
I used that footprint on my layout.
Board istself is similar(75x38mm), uses more SMD components.

There is a one detail people tend to overlook, and that is a access to LM3886 screw hole, I would spread out L and C6 for easier mounting.

Here is my board. Top layer is GND.

Click the image to open in full size.
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Old 27th April 2018, 01:16 AM   #10
00940 is offline 00940  Belgium
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Thx for the comments.

@DPH:

- feedback resistors: Tom explained once why he suggests to take the feedback point from that point and why it matters more than the absolute shortest feedback track provided you have a Thiele network in place.
- on the split groundplane: yes on the bounce, less for the filter than the input resistor. But I could take a hint from bozoc's layout and use a top groundplane from low level signal and flood the bottom. I'd rather give C6 its own track to the output terminal too.

@bozoc:

- I like smd too. I've got a working board which is just under 5*5cm. See here under. But smd seems to scare many. I just slipped one smd part in, hoping it goes unnoticed.
- The multiple footprints don't cost anything. They allow to use either mkt or mkp. Decent MKT is good enough in many cases if you ask me.
- good call on accessing the screw. In my experience with the smd pcb, it hasn't been a big problem, if the feedback cap is sufficiently away from the IC.

Click the image to open in full size.
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