What is the best way to shield my workbench from EMI - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: January 14, 2017, 08:47:37 pm
content: Hi,With all the possible interference from my own house and outside. What can I do to shield amps I am working on?Thanks,Billy

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2017, 08:48:43 pm
content: What sort of amps?

author: Planobilly
author_rank: Regular Contributor
timestamp: January 14, 2017, 09:02:35 pm
content: Tube guitar amps for the most part. I don't work on to much other stuff.

author: Spuddevans
author_rank: Unknown
timestamp: January 14, 2017, 09:24:04 pm
content: Quote from: Planobilly on January 14, 2017, 09:02:35 pmHave you seen how most tube/valve guitar amps are shielded? I don't know about newer models, but all the older ones I've come across have not what I would call "stringent" shielding.Guitar signals are reasonably large when compared to Mic level, so the circuitry is not so susceptible to household levels of EMI.I'd just keep any strong signal sources away from the Amp Under Test (ie mobile phone, wifi router)Tim

author: KL27x
author_rank: Super Contributor
timestamp: January 16, 2017, 08:40:14 pm
content: Two words. Tinfoil wallpaper.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 16, 2017, 09:42:29 pm
content: How much of the time does it need to be shielded?If you just want it for testing, mount your board in a grounded metal housing and you should have plenty. If you want some breadboard-phase shielding or an option to do testing without putting it in a case, a cheap option could be a piece of cardboard covered with tin foil, then a big metal bowl to put over top, with the tin foil sheet as the surface on the bench and grounded. Would do a pretty decent job, but wouldn't really let you work on it while shielded.That all said, there's probably no need. As mentioned, guitar and line levels are way over mic levels so they are less susceptible, plus the audio band is so low, most of the ambient EMI from wifi and radio and whatnot is way, way above the frequency you're looking at. If your high voltage supply for your tubes is shielded or just far away, and you don't have powered up instruments with high power switching stuff going on nearby, you probably don't need any sort of shielding.If you're just looking for shielding for the chassis, either a metal box or foil tape or something... but the same deal applies, you may find that after going through all the effort to shield it, you see virtually no better performance.

author: bson
author_rank: Unknown
timestamp: January 16, 2017, 09:44:38 pm
content: Just get a suitable metal enclosure and ground it.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 16, 2017, 09:47:36 pm
content: Why do you want to shield them? A guitar amp has to work wherever you put it. If it only works right inside a shielded enclosure, it's worthless for its intended purpose.

author: Planobilly
author_rank: Regular Contributor
timestamp: January 18, 2017, 12:55:43 am
content: Thanks for all the feedback.I have had some issues with interference from a wifi signal. That is easy to deal with as I just turn it off. Some guitar amps are better shielded than others.That got me to thinking what other signals were getting into the amps.As I thought about trying to build some sort of shield it became apparent that it would not be at all easy to do. The EMI could inter through the guitar and cables and through the speaker and the power cable.Whatever one could build would have to be pretty well thought out. Open amp chassis with 700 volts and a metal contraption could get pretty exciting. LOLCheers,Billy

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2017, 03:35:44 am
content: Tube amps should not even understand the concept of "2.45GHz", even at point blank range...Is there actually SS in there? Or is something else wrong?Shielding the bench is not the correct approach. A wifi router is an FCC approved Part 15 intentional radiator -- it shouldn't interfere with anything; conversely, nothing should be so susceptible as to be upset by one.If it's interfering with device operation, it's the device's problem -- add ferrite beads and such until it behaves.If this is a repair cost issue, it should be easy to convince your clients/customers to add the fixes -- just hold a GSM cellphone near the susceptible device and they should cringe appropriately. Or if they refuse, that's their problem.Tim

author: DaJMasta
author_rank: Super Contributor
timestamp: January 18, 2017, 03:47:44 am
content: Yep, 2.4Ghz shouldn't even manifest in a tube amp because the tube can't react that quickly (and if it could, you couldn't hear it). Even if you could, you could put a simple low pass filter on the guitar input and filter out everything over audio levels and take care of any amount of antenna that your guitar cable becomes for those high frequency ranges.It is entirely possible to shield a guitar amp, but maybe things are pointing towards that not being the problem. What makes you think it's the wifi? Is your wifi router and the amp being tested on the same power circuit?