Phantom Blocking - Mackie 1202

[Oddball]

Hi everyone,

 

I was playing a gig the other day and I use a Mackie 1202VLZ to submix my keyboards. I then use the control room out for my amp and the main xlr outs for FOH.

 

At the last gig, in between soundcheck and the gig the guy running FOH somehow turned on phantom on the channels I was using and this didn't play well with my Mackie. It seemed to mute anything happening in the desk. As soon as I realised what he had done and asked him to take the phantom off everything worked again.

 

Now I assumed something as sturdy as a 1202 would have some kind of phantom blocking system built in but it doesn't seem to. Is there something I can make up for the future?

 

Thanks,

 

Rich

[paulears]

Soundcraft desks do equally weird things, but on those it just makes meters all indicate full! I've not come across any damage from accidental phantom up a mixer output, but what happens is very unpredictable. My Yamaha seems to ignore it, a Soundcraft Lx7 does the meter thing as does a Peavy but a small Soundcraft Folio ignores it, as does a small Behringer. Odd!

[david.elsbury]

A couple of options here, you could put a 1:1 isolating transformer between your desk and the FOH desk, which will also help if there is a ground difference between FOH and yourself, (though there shouldn't be!) or alternatively, if you make/get some short leads or adaptors which lift pin 1 on the XLRs, the audio will still pass as it is a balanced signal but the phantom cannot come back into your mixer.

 

David

[Bobbsy]

Years ago for bigger than 24 input shows I had to gang together two Soundcraft mixers (a 200D and a Live 3/2) and had to be careful of exactly the problems Paul describes. The tranformers David mention are probably the best solution but, being very stingy, my solution as to wire a little Maplins plastic box with electrolytic caps (100uf bipolar from memory but it was a couple of decades ago) on pins 2 and 3 of a pass through on XLRs). Worked for years despite being a quick bodge.

[Shez]

It never hurts (if the option is available) to run through DIs in this situation. It's what I would generally do when running sound as it completely isolates the performers' equipment from mine. I'm usually more concerned about what nasties they might send my way than the other way around though!

[dbuckley]

Unless one can afford decent transformers, caps would be the way to go. 47uF will do, 50v rated. The +ve is pointed to the source of phantom.

 

I'd also put 10k to earth each side of each cap. Limit voltage buildup.

[timsabre]

alternatively, if you make/get some short leads or adaptors which lift pin 1 on the XLRs, the audio will still pass as it is a balanced signal but the phantom cannot come back into your mixer.

 

Although true in theory, this probably won't work in practice, if there is any common earth path (and there almost certainly will be) then the phantom will still come through.

 

DI or electrolytic cap as discussed above is the correct way. Nonpolarised (NP) caps are better in this situation than normal polarised ones as they will deal with offset voltages on either side.

[KevinE]

I assume you must be going out of the mackie line outs and into the FOH mic channels? Surely only the mic inputs will have phantom on them. Can't you plug into the FOH mixer line inputs?

[sam.spoons]

That would seem to be the way to go but the Mackie 1202 does have an output pad which reduces the line level outputs to mic level so they designed it with sub-mixing in mind, all the more surprising that it has problems with phantom power. I'll try to test mine sometime soon and post results.