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I'm getting too much noise when I try to use a battery eliminator with a UM400 that is used as a wireless link from a mixer to a camera. It works fine if I use battery in the transmitter. What is the problem?


The original reply made 13 Jun 05 is below. It is long but does discuss why the problem exists. In short, the problem is due to a ground loop between the common ground between the audio cable shield, the power supply cable to the transmitter and the power supply ground to the mixer. On 15 Nov 06 we released a new product which solves the problem by isolating the ground to the transmitter. ISO9VOLT battery eliminator

This was an email from a customer: 
We are unable to use the Lectrosonic 400 series wireless mics as camera links between our mixers and the Panasonic Varicams. While the units work fine between talent and mixer, there is a significant signal to noise problem when used from the mixer to the cameras. We have traced the problem to the use of the external power modules for the transmitters and/or the battery distribution box (Hawk Woods) we are using. Comparison with a 400 transmitter powered by a disposable 9 volt battery demonstrates the desired performance. 

(Lectrosonic makes a unit called the battery eliminator which allows their transmitters to be externally powered and therefore eliminates the accidental loss of audio between mixer and camera due to an undetected battery run down. Our sound packages, mixer, receivers and transmitters are powered from a rechargeable NP 13 battery via a power distribution tap made by Hawk Woods with a variety of cables [4 pin to single and dual coaxial ] and the aforementioned battery eliminators.) 

This s/n hiss is introduced at the transmitter and is related to a ground potential between the 400 transmitters and any other ground in the package. Dc voltage from 411 transmitters to ground of mixer measures 4.5mV. Resistance, which should be 0, measures 4 or 5 ohms. This results in audible hiss between our mixers and the cameras. Changing the power cables, audio cables or battery eliminators does not solve the problem. The more equipment added to the Hawk Woods power distribution, the louder the hiss becomes. 

This is the same power scheme we have used with the rented Lectrosonic 200 series transmitters without any problem. We are shocked that this problem is unknown by Lectro and the wider sound community.

Our First reply:
The hiss problem is probably caused by a ground loop between the common battery feed to the mixer and transmitter and the audio ground to the transmitter. The switching power supply in the UM400 is noisier than the linear regulator in the UM200. This is invariably true of switching power supplies and is a trade off for their greatly improved efficiency. The reason the ground loop problem is showing up on the transmitter is that this is probably the lowest level audio in the system. When the UM400 is run from a 9 Volt internal battery, the ground loop is broken and every thing is normal.

One solution would be to star ground every thing with separate lines at the transmitter input since it is the most sensitive point in the system. The other would be to use an isolation transformer in the audio feed to the transmitter. Neither of these is very easy. The easiest solution is to use the balanced output of the 442 mixer to accomplish the same thing.

The balanced input would be wired so that pin 1 of the 442 output XLR goes to the cable shield and pin 1 of the UM400. Pin 2 of the XLR goes to pin 5 of the transmitter (line level input). Pin 4 of the UM400 goes to pin 1 of UM400 to form the line level pad. Then pin 3 of the 442 XLR goes to pin 1 of the UM400 (along with the shield of the cable). This way the balanced output of the 442 is referenced to pin 1 (local ground) of the transmitter. This will require a 2 conductor plus shield cable of course.

We will duplicate the problem here if possible with our 442 mixer and then apply the "cure". We should get this done in the next few days and will let you know how well it works.

One other thing that could be adding to the noise problem is if the UM400 is not receiving a line level signal. If the output from the mixer is fed directly to pin three of the UM400, the sensitivity to ground loop noise will be 20 dB worse than if the signal is fed at line level to pin 5 with pin 4 tied to pin 1.

Posted 1 year agoby LectroAdmin