Ground planes and audio - Page 1
Forum section: electronics / projects

author: Guitar42
author_rank: Contributor
timestamp: March 11, 2015, 03:31:11 pm
content: Hey guys. I've been building small analog guitar effects for a few years now and I've been getting more ambitious lately. My latest project is finally working after a few PCB layout issues, but it's still a bit noisier than I'd like. I'm using a 4 layer board with one internal layer as ground and another running power. I'm getting some background hum that gets quieter when I touch the chassis (add myself as ground). I'm wondering if adding more ground fill on the top and bottom layers may help reduce noise. Any thoughts? Is there a reason I should not do this? This is strictly analog running voltages from -9 to +18 volts DC. Thanks.

author: tszaboo
author_rank: Super Contributor
timestamp: March 11, 2015, 04:35:09 pm
content: No, that is probably system level grounding issue, so check the connections. Where is the ground coming from, where are the grounds connected together, which cable is going where.

author: Guitar42
author_rank: Contributor
timestamp: March 11, 2015, 05:05:37 pm
content: I have an entire layer of ground that all of the ground pins on components are connected to which is also connected to the sleeves of both 1/4" jacks using wires coming off the two bottom corners of the board. The jacks are open frame Switchcraft jacks so they connect the chassis to ground where they bolt to the enclosure. Could adding more ground fill make things worse or is it the sort of thing that is probably not a bad idea in most situations? BTW, I'm having a terrible time finding this sort of PCB dos and don'ts info. I'm not a complete noob, but I have no formal training and I'd love to know where to look for this sort of stuff.

author: ajb
author_rank: Super Contributor
timestamp: March 11, 2015, 05:38:14 pm
content: What I think NANDBlog was getting at is what those 1/4" jacks are connected to. Presumably, since you said guitar effects, one end goes towards the guitar and one towards the amp? Possibly through other effects boxes? And unless they all run from batteries, each one of those devices has a power input. The result is a fairly complex power and signal network with multiple opportunities for noise to be coupled in or ground potentials to shift, causing current to flow through signal grounds. That's the system you need to look at. An isolation transformer or two would be a good debugging tool here, so you can isolate each signal connection in turn and see what effect that has on noise.For most audio circuits you wouldn't expect the ground plane to be THAT critical. Look at all of the consumer gear that's out there built on double or even single layer boards and working fine.

author: Guitar42
author_rank: Contributor
timestamp: March 11, 2015, 05:49:16 pm
content: Gotcha. I've tried it with a 9 volt battery and with a 9v AC adapter (no ground pin so isolated?) with the same result. Compared to other pedals I've built it's just way noisy. A-B'ed using the same guitar, amp, and power adapter. Perhaps it's just the nature of this circuit, but I am also trying to cram a lot of parts into a very small space. I have a couple of small changes to make on the board anyway and was considering adding as much extra ground anywhere I could fit it to try and help the issue. If I use vias to connect pockets of ground on the 2 outside layers to the ground layer am I asking for trouble, or is this a reasonable thing to do?

author: tszaboo
author_rank: Super Contributor
timestamp: March 11, 2015, 06:00:36 pm
content: Quote from: Guitar42 on March 11, 2015, 05:49:16 pmWell for audio frequency circuits without power amplifier a solid ground plane should be good enough. Adding additional fillings probably will not have any effect on it. There might be a different issue with the circuit, if you can, post a schematic and or layout, so we could help you. it is possible that your topology is not correct, like you need differential amp on the input, etc.You will not find solid information about this because you can ask five engineers about the grounding, all might say different things and all might be correct. It takes different methods to do the grounding for precision analog, high speed digital, RF, power or a toy.

author: Guitar42
author_rank: Contributor
timestamp: March 11, 2015, 06:09:42 pm
content: I guess in that case I may as well just try it both ways and see how it goes. It's a clone of an existing circuit and the components are close to identical to the original, I'm just trying to make it a lot smaller, perhaps too small. Any idea where I can go to find out more about different methods of grounding? I'm not really in a position to go back to school, but I really want to learn more, especially the why behind the what.

author: DanielS
author_rank: Frequent Contributor
timestamp: March 11, 2015, 06:17:49 pm
content: If you are tying your analog grounds directly to the ground plane, your opamps might be picking up I*R noise across the ground plane. In that case, you would need to bring related ground pins as close together as possible or put them on a star ground to isolate them from ground plane currents.

author: MyElectronsFellOut
author_rank: Regular Contributor
timestamp: March 11, 2015, 09:32:56 pm
content: Is this a mixed signal board (digital and analog) if so, you could try isolating components by type if you haven't already done so. Also try tying similar (analog / digital) ground pins to a single point which in turn is connected to the main ground plane as close to the ground port as possible. Reducing the distance signals travel to the actual ground will help minimize noise.