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Dirty-South Blues Harp forum: wail on! >
Metal Bodied Mic Wiring.
Metal Bodied Mic Wiring.
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tookatooka
2907 posts
Apr 12, 2012
8:18 AM
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Is it OK to wire the shield part of the coax to the mic body?
Building a harp from a light fitting and if I use 1/4 jack socket, the shield wire will be in direct contact with the body.
It will only ever be used in a battery amp so there is no risk of electric shock.
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isaacullah
1910 posts
Apr 12, 2012
10:34 AM
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Yeah, it will be fine... Typically it's good to ground metal chassis' to help shield from radio interferences... Just be careful not to create a short from the chassis to the hot lead anywhere...
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STME58
126 posts
Apr 12, 2012
11:49 AM
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Isaac,
If this shild were connected to the mic shell and grounded to the mains via the ground on a 3 prong plug (asumung an AC power supply), would this reduce the possibility of an electric shock to the mic user in event of a fault, as well as cut any noise the signal cables might pick up?
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tookatooka
2908 posts
Apr 12, 2012
1:00 PM
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Whoa! Careful @STME my use is only for a battery powered amp. Please get professional advice.
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STME58
127 posts
Apr 12, 2012
1:22 PM
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Don't worry tooka, I am not about to build an amp. If I were, I would consult with the power supply experts I work with.
It is just that in my woking with them a bit it seems that having ground planes connected to a good solid ground helps with both safty and noise on the signal lines. When we use a 2 prong plug for the power supply we have to add extra insulation to insure saftey (details worked out by experts, not me). This is why you should never use one of those 3 prong to two prong converters without making sure you have provided a substantial ground.
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isaacullah
1912 posts
Apr 12, 2012
1:43 PM
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Typically, the third prong is a totally neutral "earth" wire, connected directly to the ground (usually by the plumbing in a building), and is there to provide a route to the dirt for electricity in case of a catastrophic short. This will not prevent you from being shocked, but it will make it a little less deadly. Typically, the metal chassis of any amplifier is connected to this third prong (for safety reasons), and thus, because the signal ground is usually also tied to the amp chassis, if your mic's chassis is grounded, then it is already connected to the third prong. Typically, this is okay, but it does mean that if something gets shorted, you WILL get shocked.
It is important to note the difference between the third prong and the normal "ground" (negative) lug of of an AC outlet. I would not connect anything directly to that mains ground, other than the negative input of your amp's power transformer (and actually, a slow-blo fuse before that). In almost all modern amps, the "ground" of the circuitry is separated from the mains ground by the power transformer. In some older tube amps, they designed to work at 120v DC power supplies, so the AC power gets converted without a transformer and then goes straight into the circuit. If you are using a grounded shell with that kind of amp, there is a real shock hazard. You can get *some* protection if you use an impedance matching transformer inside your mic, provided that the signal ground on the outgoing side of the transformer is NOT connected to the mic shell. However, the only safe way to use one of those old transformerless amps is to use an isolation transformer between the power plug and the wall socket.
Probably, the safest bet is to use modern amplifier with a good transformer (and fuses!) with the chassis grounded to the third prong. Then, ground your metal mic chassis to signal ground, then coat the outside of you mic chassis with something non-conductive (like several coats of spraypaint).
I am NOT an electrician, or a really qualified electrical person.... I'm a hobbiest who has read too many websites... So get some professional advice before believing me!
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