Phantom Power question

[Peter B]

The phantom power voltage across pins 1 and 2 of the XLR cables connected to my interface drops off the more condenser mics I connect. With no mics connected the voltage across pins 1 and 2 sits at 47.2 volts, but as I plug more mics in it can get down as low as 41 volts. I noticed something was wrong when a microphone with a phantom power light didn't have the phantom power light on, but was still working. Is this a problem? Does the output from a condenser mic drop off with falling phantom power? I've not got the equipment to measure mic o/p vs. phantom power voltage, but hopefully someone will know the answer.

 

[John Willett]

This is normal with a poor power supply.

You are drawing more and more current and the PSU can't cope.

I had this in a mixer once, where up to 4 mics were fine and the rest no.

In the end I cut the phantom bus-bar in the mixer and ran the others from external phantom power units.

The spec. normally says 48V +4V - so down to 44V is fine, but lower is out of spec.

Normally a mic. will still "work" way below this voltage; but it will not like long cables and it will be less able to cope with transients.

Personally I would never use a mic. with too low a voltage.

I hope this helps.

 

[Peter B]

Hello John. Yes it helps a lot, thanks. I have a couple of stand-alone phantom power supplies so I think I'll follow your advice and run mics from them which should leave a bit more drive in the interface power supply for the others. It's a pretty old Lexicon interface, so it looks as though its phantom PSU is tiring a bit. Maybe I should start thinking about a replacement.

 

[witzendoz]

I have a valve 2 channel preamp, I notice that if using 2 mics with a curtain combination of mics, especially my AKG C4000b there is a slight hum which I suspect is the phantom volts dropping, using 2 Rode NT2's no problem. Some mics pull more amps (load) than others.

Alan

 

[Peter B]

Just out of curiosity I've done some (very subjective) tests to try to find out if the output from a condenser mic. reduces with reducing phantom power voltage. It looks as though it does, which seems reasonable since for the same movement of the diaphragm there would presumably be less voltage variation if less voltage is applied. I couldn't tell if it's a linear relationship though, or whether it affects the frequency response. Perhaps there's an electronics wiz. somewhere with the necessary kit to have a look at this, I'd be interested in the answer. I bet there are thousands of out of spec phantom power supplies out there on the low side, so what price your £1000 microphone?