Phantom Power and Dinamic Microphone

[LICORNE]

Most recent small analogue mixer have one all switch on phantom power on all channels.
I FOUND AN ARTICLE PERTAINING TO DYNAMIC MICS CONNECTING TO PHANTOM POWER. WHAT IS THE BOTTOM LINE ELECTRONIC EFFECT?

Can phantom power damage other audio equipments ?
The 48V phantom power is very high voltage compared to the voltages normal audio equipments are designed to handle. You should be very careful to check that you do not supply phantom power to the inputs where you have anything else than balanced microphones or other equipments designed to handle 48V phantom power properly. Applying phantom power to equipment not designed to hand it migh result a damage caused to that equipment. Especially consumer equipments which are connected to the XLR input through some kind of converter adapter or special cable are in danger. The safest bet when interfacing to XLR input which cna have 48V phantom power the safest choice is to use an audio balancing/isolation transformer between your signal source and XLR input.

Does turning on the phantom power could do any damage to dynamic mics ?
Providing dynamic microphones are balanced types and wired with twin screened all the way through the phantom power will do no physical harm. So there should be no problem with most popular dynamics with correct balanced wiring. Modern dynamic microphones with balanced connectors are constructed so that the microphone element is completely floating so adding phantom power voltages does not affect it when you are using proper balanced cabling.
Many older dynamic mics have a center tap of the transformer in the mic grounded to the body of the mic, and to the shield of the cable. This could short the phantom voltage to ground, and could fry the transformer. It is easy to tell if this is the case with your mics. An ohmmeter or continuity checker will tell you if there is a DC circuit between either pin #2 or #3 and the cable shield (pin #1) or the mic housing. If so, don't use that mic with the phantom turned on. Good luck.
Do not try to connect microphones with unbalanced conenctor to a microphone input with phantom power on, because the current from the pahntom power supply will flow though your microphone and can damage it.

What about dynamic mics built in the 70`S?

[Richard C.]

Hello,

I looked at a few mixer schematics and web sites about this and found out that phantom power current seems to be limited to 10ma. It is passed thru two balanced resistors to pins 2&3, OR thru a resistor to the center tap of the mic input transformer. I have found phantom power "on" dynamic mics several times and there wasn't any ill effects. If there is only one switch, and you think it might hurt your non condenser mics, why not just cut out the resistors on that XLR mic input used for dynamic mics?

Richard C.

[Todd W. White]

As long as the positive and negative sides of the power supply stay constant, ie, the current flow is identical to both side of the microphone element, they negate each other and it doens't hurt the microphone.

[donaldpatten]

Good info above. The only possible damage would be to your speakers and ears. Do not switch the phantom power with the volume up. You will not hurt a dynamic mic with 10 ma. That current with a 200 Ohm mic would produce 2 Volts. This is in the range of signal levels I have measured from an SM58 with a rock singer screaming.

Don

[LICORNE]

For permanent installations requiring rack mount,there`s a model the mv 800 with seperate phantom on each channell.

http://www.yamaha.com/yamahavgn/Docu...udio/mv800.pdf

[Richard C.]

Don,

Do you have an idea what the SPL would be at the mic to produce 2V of signal? This seems to be maybe 100db (or so) above a nominal 2mV signal. I sure didn't know you could get 2V out of an SM58! Wouldn't a mic severly clip (or at least distort)at that level?

The TOA mic mixer at work has phantom power jumpers in each mic input module that would be cut when using a dynamic mic. No one had ever cut these jumpers, and they were backfeeding Shure wireless receivers. The worst part was feeding these receivers (output switched to line level) to a mic input with the mic gain turned down. The TOA gain pot is naturally "after" the preamp, sooo much distortion - no headroom.

Richard C.

[raymond.shpeley]

Depending on the input circuit, phantom power could reduce hum and noise rejection by unbalancing the relative load impedance on each leg. With 1% resistors this could reduce common mode rejection to -40 dB or so.

Check out this article by Bill Whitlock.

Also check out the other articles on the Jensen site. They're excellent for solving audio problems.

Ray

[donaldpatten]

Spec on the SM58 is -54.5 dBV at 1Pa. Since 1Pa=94 dB SPL, and One Volt output = 0 dBV. At 1 Volt output the dB SPL = 54.5+ 94=148.5 ! ? In theory anyway. I never thought about it. We just connected a mic to a voltmeter to see how hot a signal we might get out of the mic by screaming at zero distance. Try it. This led to a preamp design with gain change not just post volume control.
Don