Need guidance on use of phantom power on my PC interface

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Patrick108

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I recently worked with another musician in my home studio. When we ready to use my condenser mic which required phantom power, I switched the phantom button on my Focusrite PC Interface which had eight inputs. The switch was designed to activate phantom power for inputs 1 through 4. I connected the mic cable to input 1. Before I ever worked with her, I was of the opinion that once input 1 through 4 were activated for phantom power, I should not connect the cable for guitar or keyboard or drum machine (or any other instrument not requiring phantom power) to the other inputs 2 through 4. She disagreed with me and said that it did not matter, and that if I used the other available inputs no damage would result, either to the interface, my laptop, the DAW or the musical instrument itself. Is that true? Normally I would use inputs 5 through 8 for other instruments not requiring phantom once inputs 1 through 4 had been switched on for phantom. So I need guidance on that. Can I get away with using inputs 2 through 4 for instruments not requiring phantom power? Thanks

Patrick
 
She was correct. The only caveat is that you don't want phantom on most older "non-powered" ribbon mics. Furthermore, Wes Dooley of AEA Microphones, recommends that dynamic mics be plugged in and/or out with the phantom power off to avoid magnetization of their internal transformers.
 
Phantom power will not appear on the 1/4" inputs at all. Ribbon mics are what you should worry about although I believe Royers can take it but I won't try.
 
Track Rat said:
Phantom power will not appear on the 1/4" inputs at all.
Click to expand...

While phantom power shouldn't appear on 1/4" inputs, there are one or two pieces of gear where it does - notably some of the Golden Age preamps.
 
I have read many times over many years of BBC engineers who worked in OB vans and studios where the XLR fields had phantom power permanently live. They were gaily plugging all sorts of microphones in and out of those XLRs and the engineers have stated that there was never a problem..even with the Coles (then STC) 4038 ribbon

I can see no way that a transformer in a properly wired dynamic mic could get magnetized by spook juice? If that were possible there would be millions of 57s,58s and SM7bs so effed up... and that is just Shure!

Yes, do have 48V off when connecting a mic and unplugging it but that is to prevent large transient thumps going through the system and causing loud noises in speakers and painful bangs in cans. The BBC chaps were also drilled in the practice of "faders down. Connect. Faders up" and the inverse to that.

Things that CAN be damaged by phantom power are those things that were never intended to go anywhere near it. If peeps experiment with guitar pedals say on mic inputs.

However do take Jame's point onboard about 48V possibly appearing on line jacks. IMHO everyone involved in audio should avail themselves of a $20 digital test meter.

Dave.
 
If you want to see a ribbon plugged into phantom power, have a look at this video - nothing to worry about.
 
The whole phantom power and ribbon mic issue came about with certain old RCA ribbon mics which used a transformer with a grounded centre tap on the output. These were more common in the USA than in Europe. Any professional mic made within the last 50 years should be able to handle phantom power - as Dave says, the BBC's mic sockets are permanently phantom powered.
 
Wow, I learned something today and that's really good info.
 
I'm also sure that every manufacturer knows that products that teeter on the edge of failure will feature heavily in media, and social media so almost any product that is delicate or very sensitive to usage conditions has been weeded out. On here we have people connecting amp outputs to mixer inputs by mistake, and almost every piece of kit has been connected badly at some point. It's now very rare for users to be able to kill equipment, which can only be a good thing. Us oldies remember when loudspeakers connected with ¼" jacks and people were forever plugging mics into loudspeaker outputs. Amps would say 8 Ohms and people would parallel up speakers to load the amps with less than 2 Ohms - few died. Now the only high power via jacks comes from guitar and bass amps - but we don't get very many going pop anymore - but we do have doom and gloom merchants convincing us that many things are really, really bad!
 
Rob has it a'right, to some extent people seem reluctant nowadays to experiment* with audio gear?
I blame that on the 'meeja' to a degree...every connector or control panel explodes in a shower of sparks if tampered with!

Of course, we must be careful and not poke inside kit as a rule and certainly never anything mains powered. But as Rob has said, things like mixers, AIs, pedals and other gear handling line or mic level signals can be plugged up any old which way "to see what happens" Might hum. Might go into screaming instability or just make no noise at all but there will be no smoke!

Again, always keep volumes at zero and advance slowly.

Once last phantom power myth? Peeps worry about shorting the P power in a mixer or AI's XLR. Will do no harm. The absolute worse case dissipation of one of the 6k8 internal resistors is less than 0.4W and that is taking the juice to be a top tolerance of 52V. A 0.5W resistor will therefor be perfectly safe. There might be very cheap ass makers that fit very wee SMT chip resistors that could burn out but I have never heard of such a failure.

*50+ years ago we did some crazy stupid things that I would not go into here. Wonder we survived really!

Dave.
 
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