Strange Audio PSU Problem - Page 1
Forum section: electronics / beginners

author: drdn0
author_rank: Newbie
timestamp: May 11, 2023, 05:35:27 am
content: Hi,Been putting together some basic PCB's for teaching beginners how to solder. I know...enough, but I'm not an expert by any means. It's all aimed at guitar effects. I have a small 12v SMPS that powers a pedal power unit (Mosky ISO10), and I'm assuming there's some sort of buck/boost situation going on inside it as I can get up to 18v out.I've made 5 small effect boards (two distortions, overdrive, fuzz, boost) + one universal switching/filtering board, and haven't had any issues with any of these boards at all. Everything works great, reasonably low noise and haven't had a single kit fail yet. The filtering is basic reverse polarity protection and a RC filter (47r/100uf) + 100nf decoupling. All the PCB layouts are reasonably sensible and straight forward, and I think pretty well designed for low noise (big ground plane, at least 0.5mm traces, nothing parallel, etc). I also have a second switching/filtering board which is much the same as the first, except it uses a 2n5088 capacitive multiplier + 100nf decoupling.I just got in my second batch of PCB's (6x more complicated boards, and another basic one), and things have gone... odd.Despite the 5 initial boards being fine and working without issues, the 6th was an absolute nightmare (despite it being much lower gain and much more basic than the others). As soon as I plugged it in, I get absolutely HORRENDOUS switching noise through my amp that is louder than the signal. Thinking I'd made a mistake I started digging in and seeing if I used a wrong component, but nope - that board was just horrendously noisy. Even running the board off of a 9v battery and having the pedal power unit plugged into the same outlet (even if it wasn't powering anything) made the effect unusable. I tried the capacitive multiplier boards, and had the same noise.I was doing some poking around, and as a test threw a 4.7nf cap in parallel to the pulldown resistor (1M to ground before coupling caps), and the noise instantly and fully disappeared. I even went to the cheap SMPS directly, and it's utterly noise free.I then moved onto the first of the more complicated boards - a very high gain, very wild fuzz (similar to a Zvex Woolly Mammoth). They are known for not playing along nicely with noisy SMPS boards, but it works perfectly - zero switching noise whatsoever even with everything dimed, and great success.Final board was a hot MOSFET boost into a LM386. High gain into higher gain. Built it, and there's almost no noise with everything dimed EXCEPT for if I roll my volume/tone knobs down on my guitar, in which case it becomes very obvious. Slapping that input cap in again as a test makes no difference. Without the guitar knobs rolled down, you can't hear it at all.So, to sum up - things that should be noisy aren't, things that shouldn't be noisy are, the noise can randomly be solved on an effect with a small cap to ground and it's made worse by guitar controls.I'm waiting on an oscilloscope in the mail, so it's all guess work currently.Assuming it is switching noise that some boards are just susceptible to, how would I approach filtering it if a simple RC filter or capacitive multiplier doesn't fix it? I've done some playing in LTSpice with LC filters, but for some reason they were never able to respond to noise reduction anywhere near as well as the RC filter.Thanks for all your thoughts!

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 11, 2023, 05:10:59 pm
content: It's hard to tell something without a circuit.

author: drdn0
author_rank: Newbie
timestamp: May 11, 2023, 10:06:46 pm
content: RC FilterCM FilterThese PCB's are universal for the effects in question.Fuzz that works with zero noise whatsoeverTreble boost that is horrendously noisy without the addition of the circled capacitorFuzz that -should- be noisy but is dead quietDistortion that should be noisy but is dead quietMy current thoughts are that it's not PSU noise (given I can get it on the treble booster without the ISO10 pedal power being used, but plugged in), but some sort of EMI that's being picked up. Even if this is the case, I want to make the universal switching/filtering board as good as it can get for filtering as people are going to be using these with many different types of power supply.

author: drdn0
author_rank: Newbie
timestamp: May 12, 2023, 05:23:16 am
content: Alright, day of experimentation. Given a 4.7nf cap from the signal to ground fixed the issue, I was right in thinking it's coming down the signal lines (probably through my cables) as it was still occurring with the pedal power in the vincinity even if it wasn't being used. Two things were able to reduce to the noise to it being non-existent (as well as removing a significant portion of other EMI).I fully isolated the enclosure from the signal earth (made minimal difference to noise), tried earthing the input jack directly to the enclosure (slight reduction in noise), then threw in a 100nf cap between ground and enclosure. Most EMI was gone, and only slightly noticeable if the pedal power unit was powering the effect, with the tone/volume rolled all the way downI put a 1nf cap between the signal/ground right at the input jack. Noise eliminated entirelyBoth are easy enough to apply on the switching/filtering board directly, so I might just do this as a precautionary thing.

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 06:57:17 pm
content: This sound like ground loops biting you. The power supply output some high frequency noise that is likely common mode or otherwise invisible until you introduce another ground. You now see the effect of that HF ground current. Try playing with how things are grounded, what's plugged in where etc.