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Re: KM100 series wireless Phantom Power problem
Author: Craig Stauffer
Date: 01/06/2012
Thanks Gentlemen. David Hable in Vancouver (our tech guru for fixing all things to do with the film industry) wanted me to share his views with you, so here it is...

Part 1: Lectrosonics should be using a balanced input on their plug-on transmitters, so the audio input signal would be the difference between the signal on pin 2 and pin 3. Mixers do this but wireless mics do not. Lectrosonics simply adapted their good ol’ single ended mic input, which works fine with lavalier mics on the body pack transmitters, to the plug-ons. They made assumptions about how the balanced output is done on most of the mics and fudged the input to work with them. This works most of the time. One assumption Lectro made was that pin 2 always has a signal on it. By connecting pin 3 to AC ground, both transformer and non-transformer output mics are covered. In their defense, I am not aware of any wireless mic transmitter that has a balanced input, even though it would be simple to implement and would save a lot of headaches..



Part 2. The Neumann KM100 is a very special case. It is a non-transformer output mic, and the output is actually NOT balanced!!! If you were to connect the mic to a phantom power supply or mixer input, and put an oscilloscope on pins 2 and 3 of the mic, you would see that all (or very nearly all) the signal is on pin 3, and pin 2 has zero, or almost zero, audio. Why Neumann made this design choice is unclear. Technically it is incorrect but as long as the impedance at both pins is the same, noise cancellation will still occur, this being the reason behind a balanced signal in the first place. So, as long as you plug into a balanced input like a mixer, you will never be aware of this oddity. Had Neumann decided to use pin 2 instead of pin 3 as the pin with the audio on it, no problem either. But some guy in a lab somewhere in Deutschland got a bee in his bonnet and made a very odd decision, we’ll probably never know why.



So, in order for your plug-on to get audio from the KM100, we have to wire the adapter cable backwards, reversing pins 2 and 3 to get the audio to the transmitter. Ergo we have reverse phase, and so on and so on. One could probably bastardize the mic to put audio on Pin 2, but the signal would still be the negative phase, as there is NO positive phase present on that mic… The only other solution is to get a 48V mic supply that has a transformer in it, and I’ve never seen one. So, for now you’re stuck with hitting the phase reversal switch.  
 
Reply
Craig Stauffer
01/05/2012
Uwe Sattler/NeumannUSA
01/05/2012
Craig Stauffer
01/05/2012
Uwe Sattler/NeumannUSA
01/05/2012
Craig Stauffer
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Martin Schneider/Neumann
01/06/2012
Uwe Sattler/NeumannUSA
01/06/2012
Craig Stauffer
01/06/2012
David Satz
01/07/2012
Martin Schneider/Neumann
01/07/2012
Uwe Sattler/NeumannUSA
01/09/2012