phantom powering non-phantom mics

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#3
Phantom powering puts equal voltage on both hot & cold leads of the mic (pins 2 & 3) with respect to earth. Most dynamic mics have no direct connection to earth (being shielding only) so phantom power applied does not effect the mic at all. However, any resistances in leads and connectors can upset the balance and cause hum and other crackling noises when leads are moved etc. Most higher end desks have separate phantom switches, but with budgets desks this can present a problem.

Cheers
 
#4
Centauri,

I'm not sure I get what you’re saying,

Pins 2 and 3 carry the signal and have the phantom power applied to them. (1 is ground, 2 is hot, 3 is not) (+- 24V)

However, any resistances in leads and connectors can upset the balance and cause hum and other crackling noises when leads are moved etc.
Click to expand...

But that’s pretty much the case whether phantom power is turned on or off.

I think were saying the same thing here. I’ve just never had phantom exacerbate a noise issue.

I believe microphones almost always have their case and shield tied together. It’s a safety thing.
 
#5
Pin 1 = Gnd
Pin 2 = Non-inverted signal + phantom power
Pin 3 = Inverted signal + phantom power

I've seen the phantom power vary between +18V and +48V on commercial equipment (mixers / mic preamps etc) and most phantom powered equipment is designed to work over that range. If the mic has no Gnd connection except for the shield then assuming all else is equal, it wont know that the phantom power is even there.
 
#6
Oops

I'm guilty of having let some info-pollution seeping into my cranium. The 48V is not from +- 24V. Having built some supplies I should know better.

Here is a nice explanation about phantom:

http://www.new-line.nl/default.asp?i=61
(dont worry about the T-power stuff unless you've got Schoeps and Nagra stuff kicking around too)

I'm still disturbed by all this talk about ungrounded microphones.

IT'S A SAFTEY THING.

The phantom power will not hurt your 421. Leave the ground connected.
 
#7
Yes the Gnd should be connected to the shield but some mics use a CT coupling transformer on the output with the CT grounded. It is the Gnd connection to the transformer that must not be present for safe operation of Phantom Power, the Gnd connection to Shield should still be there regardless.
 
#8
To further clarify, the microphone capsule (or sometimes transformer assy) has two wires, hot and cold. These are connected to pins 2 and 3 on the XLR. There is no connection from the capsule to ground, the ground is to shield the casing only. So if we have equal voltage on both pins 2 and 3, there is no voltage appearing across the capsule.

I’ve just never had phantom exacerbate a noise issue
Click to expand...

Presence of phantom power can increase current in mic lead wiring, especially if an unbalance exists, and can cause noisy leads and connectors to sound worse - if all leads are in good condition, the phantom should not increase noise.

Cheers
 
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