Yet another LM3886 layout... is this any good? - diyAudio
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Old 3rd July 2014, 02:03 AM   #1
hjf is offline hjf  Argentina
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Default Yet another LM3886 layout... is this any good?

Hi. I'm finally ready (took me 4 years!) to build my proper 2.1 chip amp. This is the layout i came up with, I was wondering if it's OK (don't worry about component sizing as I need to find what's locally available and work from that).

I tried to make a star ground (I use Eagle's POLYGON and lines in the bRestrict layer, this effectively "cuts" the polygon in the places i want, making a star ground and carefully cutting off all loops).

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Old 3rd July 2014, 06:15 AM   #2
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First make a list of design goals, star ground can be one.
How do you want to place the bypass capacitors or route the input? Should you closely route supplies and returns? Where do you need low inductance routing? What is the best power supply configuration? If its a multi channel amp how will you connect the amps to the power supply? Ground loops? Feedback loop? ..........................
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Old 3rd July 2014, 07:12 AM   #3
tomchr is offline tomchr  United States
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Why limit yourself to one layer? You'll get better performance with a ground plane on one side of the board, supply pours on the other. See example here: LM3886 PCB vs Point-to-Point (with data)

~Tom
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Old 3rd July 2014, 09:33 AM   #4
Mihkus is offline Mihkus  Estonia
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Double sided, supply rails as far from signal rails, supply rails +- as close together as possible. HF filters as close to supply pins as possible, keep your supply rails wide and similar to eachother as possible. Round curves are always better than sharp 90 corners.
On both signal and supply side. Ground planeitsquite good idea...
No speaker GND on PCB. Id rather use .125 or .250mW resistors because theres no need for high wattage ones. Im not an expert on this anyway....
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Old 3rd July 2014, 01:52 PM   #5
hjf is offline hjf  Argentina
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I have to use single-layer because it's what i can make at home. This is a weekend project (I'm looking to finish it this weekend). I can't have boards made, or buy online, since I'm in Argentina and it takes 2-3 MONTHS to deliver here (customs delays). I can make double-sided at home but I don't have the ability to make vias, and alignment is always hard to get 100% right so I prefer to avoid it.

I like the idea of no speaker GND on PCB, didn't think if that.

This is a multi-channel amp (stereo + sub), so I'll have a power supply with big capacitors, and then point-to-point power distribution to each amp.

Bypass caps are C1/C2 (big ones) and C4/C5 (ceramics). I placed them as close as possible to the supply pins of the IC.

The only issue i can think of is input signal ground: since each amp is independent, connecting a stereo output will effectively make a loop in the signal ground cable. I think this could be solved by "ground lift" (connect the shielding of the cable to ground in one end only).
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Old 4th July 2014, 02:08 AM   #6
macky is offline macky  Philippines
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Old 4th July 2014, 06:15 AM   #7
tomchr is offline tomchr  United States
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I make double-sided PCBs in my garage using the toner transfer method. As you mention the alignment is tricky. I cut the printout of the board layout leaving a 1 cm margin. Then I tape the top and bottom layouts to a piece of scrap PCB material. If you place them on a light table (or hold them up against a window in daylight) you can align top and bottom layers to within +/-0.2 mm without too much trouble. I preheat the board and sandwich it between the top and bottom layout printout feeding everything through the laminator at once.

Vias can be made by soldering a piece of wire in a hole. Generally, I try to make connections between top and bottom layers at the component pins rather than using vias. You don't need thru-plated holes. Just solder on top and bottom sides.

~Tom
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Old 4th July 2014, 02:41 PM   #8
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i can say that your PCB layout is not good...

too many angles, unnecessary one... current do like 90 degree connections.
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Old 4th July 2014, 03:21 PM   #9
hjf is offline hjf  Argentina
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Here's an updated version of the board. Highlighted is the ground pour, as you can see there is no star ground in this version, but I tried to keep all "high power" ground separated from signal ground. The ground planes are split but they connect right at the left of R2.
I made a version of this board last night and I'll be testing it today.

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Old 4th July 2014, 06:13 PM   #10
frank1 is offline frank1  United Kingdom
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Quote:
Originally Posted by bryanwizard View Post
too many angles, unnecessary one... current do like 90 degree connections.

RUBBISH!

This is AUDIO not UHF/Microwave.
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