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Phantom power being fed into mixer outs during live gigs.
Old 16th November 2009
  #1
Gear Maniac
 
Oscar G. Hinde's Avatar
 

Phantom power being fed into mixer outs during live gigs.

Hi folks! I'm having this recurring problem during the tour I'm currently doing. Mabe you could suggest something specific to solve it.

I'm a keyboard player and my current set up consists of an 88 key Korg M3 and a Nord Electro 2 rack mixed through a Mackie VLZ mixer. The balanced stereo out from the mixer is what the sound guy recieves. This is the first time I'm using this setup.
What I'm encountering in most gigs is a forgetfull AE who fails to switch off the phantom power of the lines he gives to me. For some reason most venues have the phantom on by default. I know this can't be good for my mixer (or mabe it's really no trouble?).
In any case, appart from remembering to remind the guy to switch it off (a remark that often gets ignored in the middle of pre-gig setup stress), is there anything I can place in between the mixer and the cables to eliminate this problem? I know I can just throw a couple of DIs in there, but one of the reasons I got that mixer in the first place was to have a balanced stereo output and not need DIs.

Any sugestions?

Thanks!
Old 16th November 2009
  #2
Lives for gear
 

Hi
You are right that your desk won't like phantom on it's output unless it is transformer output (which your Mackie won't be).
Several choices.
DI box (easy to do and you probably have some around).
Transformer 'isolation' boxes. (basically a DI but 1:1 ratio with hopefully transformers that will handle the line level signal better).
Get a 'tech' to put some capacitors in a little box to 'isolate' the phantom power. This is likely to be MUCH cheaper than getting decent transformers, the parts being a few Dollars.
4 capacitors, 4 resistors, a box, and 2 male and 2 female XLRs.
Positive of the caps to the MALE XLR, negative to female. Resistors (say 100K Ohms) from the negative cap ends to ground.
Matt S
Old 16th November 2009
  #3
Gear Maniac
 
Oscar G. Hinde's Avatar
 

Hi Matt! Thanks a lot for your informative reply. Looks like some DIY is in order! Goody, I love DIY. I'll call one of my tech friends to lend a hand. Only one question. Up to what extent will this solution mess up with sound quality? Mi intuition and basic electronics knowledge tells me "not much", however the transformer approach might be better?

In any case, you rock!
Old 16th November 2009
  #4
Lives for gear
 

Hi
If you use something like 220uF 63 or 100 Volt caps and put a 'polypropylene' bypass on each (0u1 or 0u22 100 Volt minimum) then it will not affect the audio significantly especially since it has already been through about 40 before it gets out of the desk.
A transformer is a 'better' option in some respects but not necessarily sonically unless you are up for a decent Lundahl or Jensen. Note the distortion and max level specifications are often at 40 or 50 Hz and the max permitted level drops rapidly as you get to 20 or 10 Hz. You should be looking at a 600:600 Ohm type (line driving) not a 10K:10K 'bridging'. You could use the 'bridging' at the 'receive' end of the cable with a short hop into their mixer input, but you are more likely to lose it as it won't be next to your desk.
Note the polarity of the caps, the positive end goes to the 'source' of the phantom power, hence the suggestion of XLR connectors which will help you get it plugged up the right way for use.
If you feel that the 'true' isolation of a transformer may help with potential (pun) ground induced hum then this would really be the way to go. Get the bigger chequebook out!
Matt S
Old 18th November 2009
  #5
Gear Maniac
 
Oscar G. Hinde's Avatar
 

Oki doki. You are a very helpfull guy! I'll see how this works out.

Thanks a million!
Old 18th November 2009
  #6
Lives for gear
 
deve's Avatar
 

Radial has a xformer box called twin iso. Good quality transformers.
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