Dan Appleby Posted April 6, 2009 Share Posted April 6, 2009 Hello all you lovely blue-roomers, Here's my scenario... I have a small 8ch desk, with 'globally' switched phantom power (I.e phantom power to all channels or none at all), I have two lectern mics which I'd like to y-split into the same desk - so essentially, the mic will get two lots of phantom power. Will this have any adverse affect on the mic, desk or sound quality? Just want to sanity check this - it's late, my brain is frazzled, I'm knackered and can't seem to find the answer anywhere... I'm guessing it will be fine. Any help much appreciated!! Link to comment Share on other sites More sharing options...
J Pearce Posted April 6, 2009 Share Posted April 6, 2009 All points should be at the same potential so can't see any issue there. Link to comment Share on other sites More sharing options...
mi-ul Posted April 6, 2009 Share Posted April 6, 2009 As an alternative - do you have direct outs on the channels? Mi-ul Link to comment Share on other sites More sharing options...
Dan Appleby Posted April 6, 2009 Author Share Posted April 6, 2009 Thanks for the advice Jon. As an alternative - do you have direct outs on the channels? Nope! Link to comment Share on other sites More sharing options...
david.elsbury Posted April 6, 2009 Share Posted April 6, 2009 Why do you need to split the same mic into two channels (for a lecturn) anyway? Another solution would be to put a pin 1 lift onto one of the channels. Since it's already getting power via the other channel, this will allow the audio to pass but won't pass phantom. Link to comment Share on other sites More sharing options...
David Duffy Posted April 6, 2009 Share Posted April 6, 2009 Even with pin 1 snipped on one input, it would still get phantom power from both inputs. The return of both would be via the still connected pin 1. The only way to only have one input providing phantom power is capacitors in line with pins 2 & 3 of one input. Link to comment Share on other sites More sharing options...
J Pearce Posted April 7, 2009 Share Posted April 7, 2009 But there really shouldn't be any issue with both inputs providing phantom. Both phantom outputs (on the mic inputs) should be at the same voltage on the basis that they will be derived from the same power rails. Assuming a well soldered cable, both lines should have comparable dc resistance, so the only current flow is from desk to mic. No desk to desk current can flow as both male XLRs are at the same potential. Therefore the desk is fine. As both are at the same potential they will not add, so the the total voltage presented to the mic should be exactly what leaves one input of the desk. Therefore the mic is fine. If anyone is still concerned they could implement dc blocking capacitors in one XLR shell as suggested above, or use an isolating 1:1 transformer on one leg of the split. Link to comment Share on other sites More sharing options...
Rob_Beech Posted April 7, 2009 Share Posted April 7, 2009 No problems at all. The voltage will stay the same, however the amount of current it is able to draw will go up, but as this isn't needed it won't matter. Imagine a widow maker with 2 13A plugs to 1 32A socket. It's the same sort of thing, only without the danger of death and we're talking DC not AC, same voltage, just a different maximum load. Link to comment Share on other sites More sharing options...
j_b Posted April 7, 2009 Share Posted April 7, 2009 The voltage at the mic will go up, but it won't be a problem. Generally, the 48V power rail from the desk feeds to pins 2 & 3 equally via 6k8 resistors (some desks are different). If you Y-split the mic into two channels, then there will be 6k8s in parallel to each pin, i.e. 3k4. The output of the mic to the phantom power circuit in the desk forms a first order low pass filter, so halfing the resistance that it's reeding will double the cut-off frequency, but that's probably in the 1-10Hz region anyway, so you'll not notice it. Link to comment Share on other sites More sharing options...
Mixermend Posted April 7, 2009 Share Posted April 7, 2009 With a parallel feed of Phantom from 2 channels the actual voltage presented to the microphone will be doubled. With respectable designs this may not matter - but as a matter of principle I would prefer to block one feed. Capacitors in an XLR plug will work well. Link to comment Share on other sites More sharing options...
j_b Posted April 7, 2009 Share Posted April 7, 2009 It won't be doubled but it will be raised. It depends on the mic as to how much it goes up. Go measure one. Many mics quote a wide operating range for phantom power anyway, e.g. 9-52V. Link to comment Share on other sites More sharing options...
paulears Posted April 7, 2009 Share Posted April 7, 2009 er, sorry? Doubled or raised? I'm sure lorry drivers trying to get 24v from a 12V battery will be very happy with this new phenomenon. 48V phantom power is 48V. A single power supply connected to a load via two paths doesn't change voltage in DC circuits. If the Y split is used, then current capacity as has been said, goes up - but the voltage can't - unless I'm missing the points somewhere. Care to share the physics Mixermend and JB? As for the need for it - I've always found it a good way of taking a single mic on an analogue desk and being able to apply two different sets of processing for two different people. It's also been handy in the past for having FOH on one fader and F/B only on the next to get two long faders. First came across it in 1994 when Mr Davidson insisted I did it that way. Oddly, it worked for what he wanted, so I've used it quite a few times since. Link to comment Share on other sites More sharing options...
jamesperrett Posted April 8, 2009 Share Posted April 8, 2009 er, sorry? Doubled or raised? I'm sure lorry drivers trying to get 24v from a 12V battery will be very happy with this new phenomenon. 48V phantom power is 48V. A single power supply connected to a load via two paths doesn't change voltage in DC circuits. If the Y split is used, then current capacity as has been said, goes up - but the voltage can't - unless I'm missing the points somewhere. Care to share the physics Mixermend and JB? A lorry driver trying to start a lorry with a duff battery is seeing a similar effect - the lights probably work fine but as soon as the extra load from the starter motor is applied everything dims thanks to the internal resistance of the battery. A phantom power supply has a deliberately high source resistance. If the DC load presented to the phantom power supply is a constant resistance then the decreased resistance between the signal pins and +48V caused by the two sets of limiting resistors in parallel will cause the DC voltage on the signal pins to increase. While many mics don't present a purely resistive DC load, there is usally a resistive element which will cause at least some increase in voltage. Cheers James. Link to comment Share on other sites More sharing options...
Mixermend Posted April 8, 2009 Share Posted April 8, 2009 As the last poster has stated - it is all about source internal resistance (to continue the battery analogy). So assuming a resistive load - say 2000 ohms - the 48 volt Phantom feeds through the 2 x 6800 ohm resistors in parallel (3400 ohms), so the current flowing is 48 divided by 2000 + 3400 = 0.00888 amps. So the voltage across the load (2000 ohms) is I x R or 2000 x 0.00888 = 17.76 volts. If we now parallel 2 channels, the source resistance is now 1700 ohms, so the current flowing now is 48 divided by 2000 + 1700 = 0.013 amps. So the voltage across the load (2000 ohms) is now 0.013 x 2000 = 26 volts. So not doubled (my mistake in a previous post) - and also not likely to cause a problem with almost any microphone. Phew! Link to comment Share on other sites More sharing options...
KevinE Posted April 8, 2009 Share Posted April 8, 2009 I'm struggling to understand what it is you're trying to do. You have two lectern mics and you want to plug them into how many channels? sorry for sounding dense. Link to comment Share on other sites More sharing options...
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