PCB Review: BT Audio Receiver - Page 1
Forum section: electronics / projects

author: shuchir
author_rank: Newbie
timestamp: August 17, 2025, 02:53:51 am
content: Hi everyone,I'm new here so please tell me if I'm doing something wrong!I'm trying to design a bluetooth audio receiver. I'm still a beginner with the vague idea I don't know what I'm doing, so I thought I'd ask for someone to look over my board before I order. I have an ESP32-WROOM-32E, PCM5102 (DAC), and TPA6138 (Headphone Amp). I've thrown together this board with the help of a couple of guides and datasheets and I'm not sure if it'll work. I think my main concern is the very long D+/D- lines from the USB and the long traces for line-level output, but I'm not sure how to shorten them. Thanks!

author: AaronD
author_rank: Frequent Contributor
timestamp: August 17, 2025, 05:02:33 am
content: I often end up with wacky component placement just to shorten the traces. Probably don't need to - most of what I do is theatrical, and most of that is pretty slow: certainly analog audio, but digital usually is too, like DMX... - but it satisfies my paranoia.So my (very rough) order is:Power planesBypass capacitors right up next to each chip, using the ratsnest to minimize the trace length in the next stepTraces between the bypass caps and the power pins, and viasfrom the capsto the supply planesSignal processing and other supporting passives, packed tightly around the chips that they support (see Note)Traces within each module: a single chip and its supporting parts (practically negligible length)Move each module as a group, to untangle the remaining ratsnest between modules and pack them togetherDraw those tracesIt rarely works outexactlylike that - I might find that my "tunnel vision" layout of a particular module has buried an external connection, so I remove all of its traces and rearrange it; or I decide that my 4 different power planes on a 4-layer board need to be in a different order; or I've "pinched off" an important section of a plane: not a complete disconnection, but just hanging on by a thread; or whatever - but that's the rough idea.It's definitely an iterative process! By the time you can't poke any holes in it that you haven't already rationalized, you're likely to have done about 5-10x the work that you initially thought, from ripping things up and trying something else, etc.If you're not married to the positions of the connectors and controls, then you can include them in a larger module and keep those traces short too.If you're ordering from a fab house that doesn't care (mass manufacturing does, a lot of prototype services don't), don't be afraid to put parts on both sides. I have yet to encounter a situation that actually prevented me from doing that. For example, a through-hole active part (TOSLINK transmitter, for example) might have a surface-mount bypass cap on the other side, using some 2-dimensional space that would otherwise be empty. Likewise for LED resistors or even a transceiver chip under an RJ-45 jack. Or you might even put an entire module on the backside and slide things together even more.Don't worry too much about dropping parts off the first side of the board while you solder the second. As long as they're small or supported, surface tension will hold them on...if their solder even melts again, which it may not. I put stuff on both sides of almost everything now, and never had a problem with hand-applied paste (no stencil) and hot air. No hotplate either. In fact, it's more like a "coldplate": the metal tray from a toaster oven, so I don't burn my wood bench.In choosing what to put on which side, try to avoid vias on fast signals. Sometimes it's unavoidable, but theyarea discontinuity in the trace, and that can cause problems for fast signals if you're not careful.And for troubleshooting reasons, don't put signals on inner layers. Sometimes that's unavoidable too, but when something "just disappears" over here and "just pops out" over there, it makes things a lot harder to measure and modify.I've probably been outside of my wheelhouse more than I've realized, but it's served me well to just keep everything as absolutely short as I can get, regardless of the layout that it produces. If something would normally have been a problem (wrong impedance, accidental antenna, etc.), it's short enough that I got away with it. (if I were mass producing things commercially and have to actually follow some regulatory standards, I'd have a problem, but these are all one-offs)It also helps to follow the recommended layout in the datasheet for those that have them, and call that a module....and a byproduct of all that is I can order a smaller prototype PCB, which sometimes makes a difference too.---Note: I've seen boards that have 0402's packed so tightly that there really aren't any traces. Just slightly overlapping pads, and that's it. Of course there's no room for silkscreen either, which I've seen solved on other boards by putting a block of silkscreen off to the side that has all of the relevant information laid out the same way as the parts are.Or, since those were assembled by machine, they really don't need silkscreen anyway. That's just for humans.

author: shuchir
author_rank: Newbie
timestamp: August 18, 2025, 01:31:50 am
content: Thank you! That's definitely a helpful process. I'm ordering from JLC so double-sided is a little more expensive- going to try and keep it on one side for now.

author: AaronD
author_rank: Frequent Contributor
timestamp: August 18, 2025, 04:16:11 am
content: I like JLC too!But I don't see any difference in price, not logged in, between 1, 2, or 4 layers on a 50mm x 50mm board. (probably doable with your parts, but I'd have to spend a good solid day or two to confirm)Watch the options of course! Going from 4 layers to 2 keeps an expensive option for via coverings that becomes free at higher layer counts, but when you turn that option off, or go from 1 layer to 2, it becomes $2.00 again for 5 of those boards. (plus shipping)It's not nearly as common, but sometimes you do have important design decisions applied on the options page when you order and not in the gerbers. For example, you might need a specific thickness for mechanical reasons (the cheap default has always been fine for me), or maybe you need to re-enter it there to get a specific impedance that KiCAD may actually know but the gerber format does not. Or an LED lighting board might use white soldermask and black silkscreen instead of the standard green and white. Etc.And, if you put a Text object on your silkscreen that says "JLCJLCJLCJLC" (JLC four times, all caps, without quotes or spaces), then there's a free option to replace it with their order number. There's also an option to not have the order number on there at all, but that makes it harder for them to manage their process and so they charge extra. Your specified placement is free, if you put that text on and then choose that option. So I put it under a connector or somewhere else that will be hidden in the final product.Other fab houses do that too - it's not unique to JLC - but of course they all have their own replacement text. So you have to change it before you upload what is otherwise the same thing to one of them, if you want the same result.