phantom power

[benweblight]

as you probably know I have been looking at buying a studio/pa mixer,

all of them (in my budget) have globally switched phantom power.

 

if I have some mics which are powered and some which are not and the phantom is on, will this damage/have any effect on the non powered mics?

 

if it does is there anything I could make which would get rid of the phantom going down th line but keep the audio?

 

ps: I'm not actually doing this, before you start accusing me of electricuting performers or something

[Brian]

You will be OK as long as the non-phantom mics are balanced.

 

The phantom power is applied to both the hot and cold pins (2 and 3) of the XLR. The current returns via the ground connection. If you plug a non-phantom mic in, all that happens is that both sides of the capsule are raised to around 42 volts, since they are both at this potential nothing happens.

 

If you connect an un-balanced mic to a phantom input then the capsule will see 42 volts across it fed from a 6k8 resistor. The current that flows is therefore limited to a max of 6mA. No harm will come to the desk, the mic might suffer.

 

To isolate the phantom power you could make up an XLR bodge plug which had two capacitors in series with pins 2 and 3 with pin 1 carried straight through.

[IanG]

Also, older ribbon mics can be damaged by phantom, although I doubt most people will ever have the need to use one.

[mikienorth]

Something I found out the hard way is bad with phantom is this...

Dont send a line feed down a multi into a balanced input (using turnarounds) whilst phantom is on.

My personal experience of this is as follows...

I was a holiday company technician working with a resident band (keyboards/midi & singer, aka a duo when not in the brochure) and the keyboardist didn't like the idea of running into my desk via DI boxes.

I had set up to give him a DI box per side to plug into, including the lovely use of phantom as they were active DI's

Like I said, he didn't like them, so unplugged them, shoved a couple of turnarounds in and plugged into the output of his setup without my brain being fast enough to say stop.

 

Result: alot of very fried midi kit as 48V went in the wrong places

 

The next duo asked to use DI...

 

Put turnarounds at both ends when trying a stunt like that, jacks dont go in balanced inputs in that sort of way.

 

If you are reading this Brian, I am sorry, but these things happened!

[Brian]

Result: alot of very fried midi kit as 48V went in the wrong places....If you are reading this Brian, I am sorry, but these things happened!

No problem. it's always interesting.

 

Do you remember what brand of mixer it was? As I said in my post, the spec for phantom power calls for 6k8 resistors in each leg of the phantom power which should limit the current to something fairly safe/survivable. I guess maybe the mixer designer didn't follow that spec.

[mikienorth]

Sorry, I meant the guy whose kit I killed who's name is brian, I hope it isn't you, he comes from up north like me.

I will clarify a little here.

I had a Sprit FX16, sending phantom to my DI's into which he should have plugged straight out of his Seck mixer.

He took the DI's out, shoved on a pair of turnarounds and plugged them into the back of his veritable mountain of MIDI equipment and FX boxes and what have you. I don't know where they went, he plugged them in, I just gave him two points, initially via DI, from my stage box.

As a result, and this is where I am at a loss as to where the wiring went in his racks, a few of his MIDI drum machines, vocoders, and god only knows what else he had in there, sort of raised the white flag with a smell of unhappiness mid gig. Yes, they didn't give up instantly, it took a sustained assault, and I wasn't looking.

I don't know how he had his stuff set up, but I never plugged into the top of his Seck, as I would have expected to do, but there you go, the joy of blending musicians and technology.

[benweblight]

answered my question befor I asked it mikie north

 

but does phantom affect mono line jack ins on mic channels,

 

I mean is it ok if I run a stereo source on two mono jacks?

[paulears]

this is confusing - if you were supplying 48v TO your di - then there isn't any socket on it spare that has voltage of any kind on it - certainly the ins/outs won't. What he might have done is remove your DI box and and then plug, via the genderbender into an XLR output, or more likely, an XLR to unbalanced jack lead - the tip of which would have voltage present, but as brian said there isn't much current there that ought to cause any real damage. When I've had this happen, the usual symptom of the feeder device being unhappy is fizzing, hissing or distortion - usually so bad that everyone knows something is wrong. Most modern designs are quite well designed - maybe because of passive mic splitters - and don't object very much. The only exception I found was Sountracs 8 buss studio desks which had diodes in the pre-amp that were very close to their limit - plugging in a bal/un-bal with 48V on it killed the channel stone dead - of course it took quite a few channels to die before anyone realised what was going on. In the mk 2 they upped the diode spec and the problem never reoccured again.

[mikienorth]

This isn't as easy one to explain as it happened 18 months ago, and it gets deeper still.

I was using a pair of Behringer Ultra Di boxes, with phantom on to save putting batteries in them all the time. He literally took them out of the cables, put them to one side and plugged straight through, thus sending 48v into his own equipment.

I dont know where it went or what it did exactly as he would not let me look properly. It made odd noises then stopped working mid gig.

What stopped working I am not sure about but as I said above, he had a large rack of MIDI gear, mostly quite old, and all playing their parts live as he pressed his keyboard keys, rather that just recording and playing along as I have seen others do. Some of his equipment such was self built and he claimed to hold a degree in electronics or something similar.

 

I dont know how MIDI kit works, I have never used it.

 

He wouldn't let me near his gear, or listen to anything I said during the entire period I was working with him. He was the older, more experienced one of us, and I was only a 22 yo time served electrician who had been working in theatre, film and music since the age of 14 holding no qualifications outside my apprenticeship. Oh, and I was the venue's only technician, on my own with any other expertise a day's flight away.

 

My first post was just an addition to others in this thread saying an experience I'd had with phantom and just putting it here to point it out to others to be careful. I know most people here know more about this sort of thing than me, but I was just adding information to what benweblight was asking about the potential of damaging equipment.

[paulears]

I wouldn't worry - sounds to me like you didn't do anything wrong - especially as you gave him the correct kit - i.e. 2 x di boxes. if he chooses to unplug them and do his own thing, wash your hands and say "what a shame". In my experience of seck mixers (the first one I ever built from a kit of parts) they cut a few corners but sound ok. one of the corners was almost certainly any form of dc isolation on the inputs. again, not your problem. I've never had any issues with putting phantom where it shouldn't go, apart from a denon md player - all thay happened here was the noise floor increased - a low frequency hissy/rumbly noise. Very obvious and cured as quickly with a button prod.