BR-100 Bridged LM3886 pcb - diyAudio
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Old 9th April 2017, 10:29 PM   #1
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Default BR-100 LM3886 pcb

Hi all,

This is the first time I've attempted to design a pcb; any helpful critiques would be much appreciated.

https://easyeda.com/Rabenvald/Gaincl...c5c5702145f766

https://easyeda.com/Rabenvald/Gaincl...ddca0cd6aa66ab
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Old 10th April 2017, 01:35 AM   #2
jaycee is offline jaycee  United Kingdom
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I presume those BIG capacitors C11-C13 are polypropylenes used for speaker crossovers. DONT do this. Any advantage of them being polyprop's is cancelled out by the huge antenna they form.

Wima MKS2's are film caps if you must use film, polyester but they have 5mm pin pitch.

Power supply board - dont waste one side on being a largely useless ground plane. Either make it a single sided board, or put the same tracks on both sides to double current capacity. I wouldnt bother with the bleed resistors R1 and R3 either. I also wouldnt bother with so much capacitance, 20,000uF per rail is crazy for a chip that will only give you 50W at most.
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Old 10th April 2017, 01:36 AM   #3
jaycee is offline jaycee  United Kingdom
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Also put some smaller power supply decoupling caps eg 470uF on the amplifier PCB, near the chip. Dont rely on the capacitance on the PSU board alone, as you have wires connecting one to the other. The PSU board's capacitance should be for energy delivery, and the amp board's capacitance should be for supply decoupling.
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Old 10th April 2017, 01:41 AM   #4
jaycee is offline jaycee  United Kingdom
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Im also not a fan of ground planes on the amp pcb, especially not large ones like you have. Just asking to pick up noise. Keep the ground plane for power ground, or dont use one at all. Keep signal grounds short.
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Old 10th April 2017, 07:32 PM   #5
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Don't forget that the output impedance of the preceding stage will change the gain of the inverting LM3886 and attenuate slightly the input to the non-inverting LM3886. Also R5 and C11 are now parts of the input impedance seen by the inverting LM3886. You probably need an op-amp buffer or better two (one for each LM3886) on board to make sure that no matter what is connected to the input it does not alter the behavior of your amp.
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Old 10th April 2017, 07:36 PM   #6
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Also don't forget the mute pins of the LM3886's...
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Old 12th April 2017, 09:29 AM   #7
AndrewT is offline AndrewT  Scotland
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Don't attempt a dual PCB when you don't know how your amplifier works.
learn how to build a mono single chipamp first. Then you can use that learning to design a single chipamp PCB and only after doing that are you ready to attempt a dual chipamp.
Give yourself achievable targets.
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Old 12th April 2017, 03:16 PM   #8
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The amplifier board, especially before I reverted it to a previous state, is a bit of a mess; I know. I've only touched it in my free moments after work.
Anyway, I really should have made a separate copy for revisions earlier, but here we go:
https://easyeda.com/Rabenvald/Gaincl...f9cecec184a42f
Click the image to open in full size.

@jaycee: Regarding those large caps acting as antennae: I suppose that's probably not something that can be overcome with shielding? Bleh, that's a shame...

@OlegSh: I didn't need the mute functionality, so I omitted that part of the datasheet. Does that pin need to be connected somewhere?
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Old 12th April 2017, 05:28 PM   #9
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Did you actually read the datasheet? The mute state is the default state of the LM3886. Figure 1 of the datasheet shows how to connect the mute pin to unmute the IC. At the top of page 17 there is more info on the mute function.
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Old 12th April 2017, 07:13 PM   #10
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You are missing the connection between power ground and audio/input ground. Or do you have a solution that I can't see?
Why not build two non-inverting lm3886 and invert the input to one of them?

Missing mute and other important parts to guaranty stability.
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