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Phantom power and Dynamic mics

Richard Bridge 12 Jun 12 - 06:00 PM
John J 12 Jun 12 - 06:15 PM
GUEST,josepp 12 Jun 12 - 06:17 PM
treewind 12 Jun 12 - 06:54 PM
GUEST,Rightsound 12 Jun 12 - 08:34 PM
SteveMansfield 13 Jun 12 - 01:52 AM
treewind 13 Jun 12 - 02:44 AM
SteveMansfield 13 Jun 12 - 03:17 AM
GUEST,Ray 13 Jun 12 - 04:30 AM
Bernard 13 Jun 12 - 05:42 AM
John J 13 Jun 12 - 06:28 AM
GUEST,Guest, MTB 13 Jun 12 - 07:43 AM
GUEST,Ray 13 Jun 12 - 12:19 PM
GUEST,Albert O'Balsom 13 Jun 12 - 12:29 PM
GUEST,Albert O'Balsom 13 Jun 12 - 01:12 PM
GUEST,Ray 14 Jun 12 - 04:25 AM
Tim Leaning 14 Jun 12 - 04:58 AM
Bernard 14 Jun 12 - 08:24 PM
GUEST,Max Reiner 14 Jun 12 - 08:50 PM
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Subject: Phantom power and Dynamic mics
From: Richard Bridge
Date: 12 Jun 12 - 06:00 PM

I'm always reluctant to put phantom on (most cheap desks have global phantom not channel-by-channel) unless I have a complete rack of condenser mics (which has never happened).

There are two issues hiding here.

First, if I get forgetful and senile and put a jack-socket-to-cannon up in the stage box and forget to put the other way round adaptor on the other end at the desk and put it in the line in - I'm going to fry something.   

Second - I just don't like the idea of putting phantom up on a dynamic mic.   MIGHT it alter mic characteristics? I ask because we were performing (but not desk driving) recently and the mics - mostly SM58s but a few Beta 87s (which is how I know that global phantom (it was a Behringer desk) was on) - we were singing on the SM58s - seemed to be a bit "on or off". Normally it's easy-ish to listen to yourself in the backwash of the FoH speakers - or in the foldback if it's any good - and fade your own vox in or out to get the precise balance you want as the song varies, by backing off the mic a spot. This time it wasn't. Either one was there (and a bit too much there) or one vanished.

Foldback as such was not audible but might have been contributing slightly.

Views? COULD putting phantom onto an SM58 change its behaviour?


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Subject: RE: Phantom power and Dynamic mics
From: John J
Date: 12 Jun 12 - 06:15 PM

I would have thought so.

Applying volts to the coil on a dynamic mic is going to bias it and I would expect that to alter the mic's response. I imagine the current will be limited by the desk to a couple of milliamps. Don't take that as gospel, it's only my guess.

It might be worth a PM to Bernard, this is right up his street.

JJ


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,josepp
Date: 12 Jun 12 - 06:17 PM

Seems to me you answered your own question.


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Subject: RE: Phantom power and Dynamic mics
From: treewind
Date: 12 Jun 12 - 06:54 PM

It shouldn't make any difference at all. With properly wired phantom power, both legs are at 48V and no current flows though the mic coil.

Plugging a dynamic into a live phantom powered connection will not cause current to flow either, except a tiny amount for a few microseconds if one connector makes contact before the other, but only enough to charge up the capacitance of the cable, which really isn't much.

I'm more wary of plugging a condenser mic into a phantom powered socket - ideally you should plug it in first, then switch the power on. In practice it's usually harmless, but some manufacturers advise against it in their instructions.


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Rightsound
Date: 12 Jun 12 - 08:34 PM

Hi there.

I regularly run a mixture of condenser and dynamic mics Including SM58 and similar, on a global phantom powered Behringer and other makes of desk. I can say I have not noticed any issues with changing dynamic microphone responses.

Balanced cable systems are used because they can allow a small wanted signal to be transmitted and received along its length with minimal degradation, and yet large sources of interference can be tolerated and separated from the wanted signal. this applies at both ends of the cable.

The Phantom power is injected onto the cable so it presents as interference (or as a common mode signal), and thus is cancelled out before it reaches either pre-amps, or the sensitive microphone transducer coils. There should be no standing current running through the microphones voice coil, and so should not affect its response.

With regards plugging unbalance plugs in to balanced sockets with phantom power connected, the phantom powering is done through closely matched 6810R (typically)resistors. These provide a current limiting characteristic, and so even if the socket was completely shorted, the phantom power supply will not be damaged.

However care must be taken when using ribbon microphones. Most ribbon microphones will be damaged if they're connected to phantom power sources.

It is always sensible to switch the phantom power system off and allow any stored charge on the supplies capacitors to dissipate before connecting or disconnecting any XLR device.

Possible explanations for any perceived change in microphone performance - there could be many, but I do wonder if the mixer board may have been damaged, such that the DC blocking of the phantom power to the pre-amps has been compromised. This can possibly happen if you hot plug into a live desk.


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Subject: RE: Phantom power and Dynamic mics
From: SteveMansfield
Date: 13 Jun 12 - 01:52 AM

So just to clarify for the technical ignoramuses like me - the general consensus is that there is no harm in running Microvox and the like on a desk that's got (global) phantom power turned on, because some of the mics need it?

That was / is certainly my understanding, but is it right?


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Subject: RE: Phantom power and Dynamic mics
From: treewind
Date: 13 Jun 12 - 02:44 AM

Microvox battery boxes always have a jack socket on them. If your mixer has a line input socket, you can use that with a jack-jack cable and phantom power will not be an issue (and as Microvox are quite high output, the lower sensitivity of the line input isn't a problem)

If you want to use an XLR mic cable, you have to solve the problem of how to connect that to the Microvox box. A DI box is the standard foolproof solution. A simple jack - XLRM cable will short pin 3 to ground and pin 2 with phantom power will be connected to the signal output on the Microvox box. I don't recall whether Microvox has a blocking capacitor to keep the DC out, but I seem to remember having some trouble with this arrangement - some phantom power supplies don't work well if you short the pins to ground. The 6k8 resistor limits the current to 8mA, but on some cheap mixers that's enough to make the power supply drop out of regulation and you get hum.

I have made several battery boxes for Microvox mics with DC-isolated and impedance-balanced XLR connectors on them. They work VERY well.


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Subject: RE: Phantom power and Dynamic mics
From: SteveMansfield
Date: 13 Jun 12 - 03:17 AM

Thanks treewind. As it happens I always route my Microvox through a (passive) DI box, because I've been advised to do so, it works well, and when I work with sound engineers they always seem to appreciate being given a feed from a DI box.

I was just idly wondering about the effect having the phantom power switched on has, and I think I understand a bit better now ... .


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Ray
Date: 13 Jun 12 - 04:30 AM

My understanding is that "Phantom" power is so called because dynamic mics can't see it and it has no effect on them. As long as you are running the dynamic mic in balanced mode there should be no problem.

Where you are most likely to come unstuck is connecting an unbalanced mic to a balanced input via an XLR plug/socket. (i.e. to connect an unbalanced mic or other source [with only two wires - screen and signal] to an XLR socket, you simply need to connect the spare XLR pin to earth. But don't do it if there's any chance that phantom will be turned on!


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Subject: RE: Phantom power and Dynamic mics
From: Bernard
Date: 13 Jun 12 - 05:42 AM

The only issue with 'Phantom' that you need to actively avoid is plugging or unplugging a device with the mic channel 'open' - the resulting 'pop' through the speakers could write off a delicate tweeter.

Otherwise, there should be no issues because of the original design brief as mentioned by Anahata.

However, if a desk (or add-on phantom power PSU such as an old AKG N66E we have here at work for mixers not having phantom) has been poorly designed, plugging in an unbalanced device (XLR pins 1 & 3 strapped) could cause phantom to be shorted out.

We used to have some Sennheiser 'Black Fire' VHF radio mic receivers (BF1051 and BF1052 kits - really they were Chaiyo with a badge job!) that didn't like phantom squirted up their XLR output... they started making all sorts of frying noises after a few minutes, though no damaged ever occurred. Fortunately they also had a line out on jack, so we just had to remember to use that!

Richard's opening post suggested he had issues - they were more likely to be problems within the desk or cables than the microphones, I would suggest.

I would add that it's unusual these days to find desks with 48v phantom - 24v or even 15v is more the norm, as 'studio condensers' requiring 48v are rarely used by live musicians. Some microphones, typically AudioTechnica, require as little as 1.5v, many only need 9v, such as AKG C451, C1000 and the like.

The purpose of phantom is to apply a charge to the capacitor plate - electrets need a lot less than true condensers which do require 48v.


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Subject: RE: Phantom power and Dynamic mics
From: John J
Date: 13 Jun 12 - 06:28 AM

My comments were made assuming that electrets were being used - hence the concerns about bias.

Note to self: RTFQ!

JJ


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Guest, MTB
Date: 13 Jun 12 - 07:43 AM

"The purpose of phantom is to apply a charge to the capacitor plate - electrets need a lot less than true condensers which do require 48v"

Electret microphones contain a capsule which is permanently polarized.
They also contain a very simple pre-amplifier, often a single field-effect transistor. This requires a very small amount of power which is often provided by a phantom power adapter which may be internal to the microphone (eg AKG C1000) or in an extended XLR plug (eg AKG C419PP).


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Ray
Date: 13 Jun 12 - 12:19 PM

"I would add that it's unusual these days to find desks with 48v phantom - 24v or even 15v is more the norm, as 'studio condensers' requiring 48v are rarely used by live musicians. Some microphones, typically AudioTechnica, require as little as 1.5v, many only need 9v, such as AKG C451, C1000 and the like."

And thereby hangs the other problem! There are a lot more used for phantom power than for microphones - e.g. powering DI boxes and pre-amps - better than having your battery go flat half way through a gig [and cheaper]! By way of example, I have an AER compact 60 whose phantom won't power a Baggs ParaDI.


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Albert O'Balsom
Date: 13 Jun 12 - 12:29 PM

I have an AER compact 60. Spec says channel 2 provides 30 V phantom power. Is that insufficient to drive a Baggs Para DI?


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Albert O'Balsom
Date: 13 Jun 12 - 01:12 PM

Just looked at Baggs Para DI spec online. Says 30-48 V.


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Ray
Date: 14 Jun 12 - 04:25 AM

That may be the case but, on mine, it doesn't even light up the "phantom powered" LED. The PADI works with everything else and its quite an old Compact 60, so perhaps they've changed the spec?


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Subject: RE: Phantom power and Dynamic mics
From: Tim Leaning
Date: 14 Jun 12 - 04:58 AM

"Richard's opening post suggested he had issues - they were more likely to be problems within the desk or cables than the microphones, I would suggest."

I have had problems previously that turned out to be the desk jockey constantly twiddling the knobs... seems he thought that when I sang a quieter bit it needed turning up and obviously this was leading to some overly loud bits ..:)


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Subject: RE: Phantom power and Dynamic mics
From: Bernard
Date: 14 Jun 12 - 08:24 PM

"I have had problems previously that turned out to be the desk jockey constantly twiddling the knobs"

I prefer to call it 'digital interference'... some people's digits interfere more than others!


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Subject: RE: Phantom power and Dynamic mics
From: GUEST,Max Reiner
Date: 14 Jun 12 - 08:50 PM

Hey, this is all very interesting. Been a long time since I got this involved with mics. :) Nowaday, we do a lot of point and shoot using the shotgun condenser on the digital camera. We do mainly talent MCUs using this kind of micing.. Any longer two-shots, whatever, are with music over. And/or wild dialogue dropped in.

Getting smarter or too lazy in my "golden years." :)


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