Unofficial Mackie User Forums > DL1608/DL806/DL32R/ProDX Mixers

Master/My Fader V4 Wants and Speculation

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I actually use condensor mics ... they're only Behringer C2s so no worries if I damage one of them ... but so far after a dozen concerts the recordings through these channels still sound pretty good.

The main argument against using the Y was, as you say, active sources.   I would definitely not do this on battery powered mics ... but for phantom powered condensor mics where the amplifier is powered by the input phantom power, I don't think there is the possibility of the mic putting out enough voltage fluctuation to affect the other mic (other than distorting the audio signal).  If you know otherwise please share your knowledge as this is of interest to me (and probably good for a different thread  as well ;) )

I've done a lot of chorus work from 140 down to 20 and if you use more than 4 quality (condenser) overheads you doing something wrong. From about 40 on down I'm at 2 overheads and for quartets it's a XY setup. The old adage of "less is more in audio" applies well here. Gain before feedback drops 18dB for 64 mics, might as well leave the stuff off stage. The larger the chorus the less sound reinforcement is needed. Usually a couple of handhelds for solos and a emcee podium mic rounds out the list. I've seen and heard some College choirs with upward of 50 SM58's and they sounded as messy as they looked.  The idea is to get a uniform rich sound out of a group, not to cater to a bunch of Prima Donas. Good luck with that. ;)

There's no potential across pins 2 and 3 with phantom power (just the same as with battery power) the issue is the output of one preamp trying to drive the output of the other. I don't know enough to be sure if it is safe or otherwise but it is not recommended. You should certainly never use two outputs to drive a single input at line or speaker/headphone level without proper isolation (the obvious one is iPod/Phone or suchlike stereo headphone outputs into a single mixer input, if you just short T & R on the stereo plug and use a TS on the other enn you could fry the headphone amps (which aren't known to be especially robust). 


The output voltage of a mic (which would include the mic's internal preamp output if it has one)  appears to be  on the order of 50mV or less.   This is an AC fluctuation going across a line that is charged with a constant DC phantom voltage of anywhere from 10-50VDC.   I would be curious to know how a 50mV AC voltage fluctuation could damage another mic that is working with a 48V input.

So my argument that Y'ing passive mics together is harmless is based on the fact that the mics themselves do not have a power supply, and in fact the line they are connected to already carries a far greater power supply on them.   

In the case of headphone or line output jacks, those are generally powered sources and are being connected to a line that has no power.  So you are pitting two sole power sources against each other -- which I agree does not sound like a good idea.

The 48VDC phantom power is not going across the hot and cold signal connections of the mics preamp it is between pins 1 (-ve) and pins 2 & 3 (+ve) equally so 2&3 are at equal potential (meaning there is NO voltage across them, this is why it doesn't immediately destroy any dynamic mics connected to a live P48 input, don't connect your vintage ribbon mic there though). If you used a TSF jack to XLRM adapter to connect an old, jack terminated, dynamic mic to a P48 input you would almost certainly destroy the mic.  With capacitors any signal sent by mic one between pins 2 & 3 would be driving the output devices of mic 2's preamp, as the output is very low impedance compared to the desk's mic inputs that would generate a much greater current into it. Most will handle this fine but it is definitely not what they were designed for and I wouldn't take the chance of (at best) subtle distortion and at worst damage. With dynamic mics the issue of damage is minimal, they are pretty robust devices but, if you were to connect one to another then excite the diaphragm of the first, in theory, you would hear the second behaving like a loudspeaker (remember the "two tin cans and a piece of string walky-talky beloved of pre-teens wannabe 007s"? this is just the electrical version). This is exactly what you are doing when you use a y-cable to send two mics to one mic preamp. This can't be expected to sound good.


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