using plastic for a chasis to hold a mic pre

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versuviusx

Well-known member
Joined
Sep 12, 2004
Messages
227
Location
Wilmington,NC
hi i was wondering has anyone ever tried using plastic as the material of choice for making a chasis for a mic pre?
generally chasis are made out of alluminum. but couldn't plastic be used as well? the only problem would be finding a way to ground stuff and take out the hum and EMI. maybe i could spray on the plastic anti EMI paint or use tape or use aluminum foil. any ideas? i'd love to know what potential problems i may run into using plasitc.
 
Hey, i´be been lazy and tried with just the alu front panel, wooden sides and so forth. And I´m pretty sure that you need at least some metal or ground material between the circuit and where you put your fingers.. i once tried to build a 438 amp on my workingdesk and it did not stop to oscillate until it was mounted nice in box with metal front. I think PRR wrote once that space is actually the best way to keep hum and buzz away, but the surroundings makes the difference.If You put another 19" inch under/above with net TX then you are in trouble with just normal plastic(I meen no composit) material. If you read oyher articles about Millenia and so forth, they put lot of effort to make the boards and boxes right. But if you dont need that xtra +10 dB of hum and buzz cancellation, try plastic. Cheers Bo
 
Is there any reason why you would use plastic?
I sure would not. You just make it more difficult to shield the unit from interference.
 
well i was thinking that maybe if i could make a plastic box with a metal faceplate i could experiment with something and maybe offer DIY people a way to have an affordable chasis. just an idea.
 
not a good idea. shielding is important, specially in a studio, where many units work together one very close to the other in a rack, with many power transformers and audio transformers and all...
 
Hammond makes conductive plastic boxes. Series 1594RFI and 1599RFI.
Mouser stocks them.
I have used one for a small pre-amp.
They are pricey.
I can't see any advantage over metal boxes.
 
You can use plastic boxes. Then you can use aluminum foil. Or you can use "Magnetic Paint" if you are on the cheap. It is a paint designed for kids. So you can make boards for them that magnets will stick to. It is basically paint with metal filings.

Anyway, the only time I would use plastic is when you using a non-traditional enclosure. I only use it for stomp boxes. That way I can use PVC pipe caps, or old sewing machine pedals and such.

But since it is such a PITA. I lately have been buying $2 electrical boxes.

As far as buying plastic chassis, they are just as expensive as metal. Sometimes more expensive depending on the form factor. Even if they are a little cheaper, add the time and pain of using aluminum foil and super 77, or having to paint the inside.

If you are looking for really cheap boxes, you might want to consider getting metal studs for home building. Typically they are only a couple bucks a piece. With some cutting and bending, you can make almost any size chassis you need.

http://www.geofex.com/Article_Folders/steelstud/steelstud.htm
 
[quote author="aortizjr"] you can use "Magnetic Paint" if you are on the cheap. It is a paint designed for kids. So you can make boards for them that magnets will stick to. It is basically paint with metal filings.[/quote]

Have you actually tried the magnetic paint? I'd be surprised if you got any decent conductivity from it, since ferrous metals are not very well suited to making conductive inks/pastes. Most shielding paints still use silver as the conductive phase. If we could get away with ferrous metals and still get decent performance, we would.

spray shielding

-Chris
 
I made a little preamp for my parents to use with a USB interface I gave them to record their folk band. It needed to be small and light, but durable for gig bag traveling. I used a small plastic box with a metal lid from RatShack (left over from a high school project) and lined it with aluminum foil. It works fine, no radio pickup or anything. I think the step up input transformer encased in mu metal helps a lot.
 
You can use plastic if you shield it like you would a guitar cavity--with either copper foil or conductive paint. Stew-Mac has those supplies, so does Guitar Electronics and I think Allparts. The conductive paint is much lower in conductivity, but it seems to work OK. It's kinda messy though, it scrapes off easily and gets graphite on everything.

Also, Digikey carries a few Serpac plastic EMI boxes, if you are looking for something small and portable.

I have done circuits that are fully balanced throughout in plastic, but that is probably not what you had in mind, and still not really ideal.
 

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