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I'm having level and/or distortion problems when I try to connect a universal 48 Volt power supply and condensor mic to a UM400. What is the correct wiring?

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Some phantom powered mics have a balanced and floating output and some have both outputs balanced but referenced to ground. Most outputs are electronic but you can think of the two cases as being a floating transformer winding or a center tapped transformer winding. Either way works fine into a balanced (mixer) input. The fully floating output does have some common mode noise advantages when operating into a less than perfect balanced system.

In the case of the Lectro transmitter, the input is unbalanced and you have to unbalance the mic output. (As far I know, the universal box is an innocent bystander here.) The problem is that the two different balanced systems require different wiring and what is right for one is very wrong for the other.

Fortunately, you can try one way and then the other and pick the one that gives the best results. By high and low, I am referring to plus and minus polarity from the mic. By best results, I mean loudest and clearest. Generally the differences will be dramatic.

For the fully floating balanced output, ground the shield at the TA5F pin 1, the low side wire (from XLR pin 3) at the TA5F pin 1 and the high wire (from XLR pin 2) to TA5F pin 3. This grounds the shield at the transmitter where the RF is the highest and ground references the mic low side at the transmitter.

For the balanced but ground referenced mic output (center tapped) everything is the same but the low side (XLR pin 3) is not connected to anything. If pin 3 were to be grounded, in this case, half the transformer winding is shorted to ground since the winding is grounded both at one end and the center. If it is an electronic output referenced to ground (Schoeps and some others) then that output is shorted and distortion will rise on the other output.

There are several ways of making a universal setup that will work with both types of mics. The first is to use a balanced to unbalanced transformer. The drawback here is that you need a very good transformer. The second way is to put 200 to 500 Ohm resistors in series with the low side signal (pin 3 XLR signal) and tie it to ground. This will not short a ground referenced output to hard ground but to a 200 to 500 Ohm load. A floating output will have one end of the signal referenced to ground through the same 200 to 500 Ohm resistor. The down side is that you have a resistor in the mic line and will lose a little signal but that usually isn't a problem.

After all of this was written, a customer sent in his wiring solution. Though it doesn't have the shield grounded at the transmitter, which bothers me a little, he has had good luck with the following wiring. 

" I believe all's well now. I power the mics with a stand alone 48VDC power supply. I have two of them. A Neumann and Sennheiser. Both lost in effect about 10dB or so when the cable adapter to the input of the UM400 transmitter unbalanced the signal by grounding pin 3 to 1 as suggested in your wiring scheme for self powered mic level sources. Lifting pin 3 from ground brought the signal level back to normal. The wiring I am using now is this: The XLR pin 1 is tied to the shield but the shield is open at the transmitter TA5 end. Pin 2 (high) of the XLR goes to the transmitter input (TA5F pin 3) and XLR pin 3 (low) goes to pin 1 of the TA5F. The levels now are fine."

As long as the 48 Volt box ground doesn't get tied to the transmitter ground through something like a common power supply, this should work fine with either variety of mic, floating or ground referenced.

A similar problem in the UH400a transmitter was fixed in the following way.

(See FAQ#049-WIRELESS for UH400a fix)

Posted 1 year agoby LectroAdmin
#95