ULXS4 or ULXP4 receiver and remote mixer - audible hum when connected

FAQ #3670 Updated October 26, 2017

Question:

I have a ULX4 receiver that is not rack mounted. It is connected to a mixer that is 100 feet away. I am using the ULX4 XLR output and a balanced input on the mixer, but I still have a low level of audible hum. What could be the cause and the solution?

Answer:

SITUATION
The ULX4 receiver is sitting on a shelf, therefore not mounted in a grounded metal rack. A long audio cable (over 100 feet or 30 meters) connects the active-balanced, mic-level, XLR output to a balanced, mic-level, XLR input on a mixer. The AC grounds of the receiver and the mixer are different, so there is current flow on the interconnecting audio cable shield, inducing hum into the cable's inner conductors.


POSSIBLE SOURCE OF THE PROBLEM
The ULX4 receiver power supply has a two prong AC plug. This type of power supply often has a small value internal capacitor that connects to one side of the AC line in order to reduce Electro-Magnetic Interference from the switching power supply. This capacitor can allow a very small amount of current to leak onto the ULX4 chassis; the amount of current is not a hazard, but can be the source of audible hum. If the ULX4 is mounted in a metal rack, and the rack is AC grounded, this leakage current will be drained to AC ground. But in this case, the ULX4 is sitting on a shelf and there is no path to AC ground.

POSSIBLE SOLUTIONS
a) Mount the ULX4 in a grounded metal rack or ground the ULX4 chassis to a nearby AC outlet. This will provide a ground path for any leakage current.

b) On the female XLR connector at the ULX4 receiver, disconnect the cable shield from pin 1 and, if necessary, from the XLR shield-can grounding lug. This will eliminate the current flowing on the audio cable shield.

c) At times, a transformer-balanced output can provided a better hum solution than an active balanced output. Connect a Shure A95U transformer to the 1/4" phone jack output of the ULX4 receiver and use this transformer-balanced XLR output instead of the ULX4 active-balanced XLR output. Solution "b" may also be required for the female XLR connector mated to the A95U.

d) Use the ULX4 XLR output and set it to Line level. Adjust the mixer input accordingly to handle the Line level signal. This will improve the signal-to-noise ratio, but may not completely eliminate the hum.

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