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Mic Level/Line Level

Here is a reply given to this question on the RAMPS group:   There was a query about whether to send mic level or line level to a camera that always had a mic preamp in the signal chain and used an attenuator to convert line level down to mic level anyway. The answers ranged from it didn't make much difference to the line level would pick up less interference. Both are reasonable answers. In general, it is better to keep signal levels high from the source and if necessary attenuate them at the input to the "load" (in the case described, the camera audio input). If the signal is attenuated to mic level at the source, then any noise picked up in the cable enters the camera at full noise level. If the signal is at line level at the source and then attenuated at the camera, the noise picked up in the cable is attenuated also. This could improve your noise rejection by 30 dB or more.

A case in point: We made a law enforcement wireless system a few years ago that went in the trunk of a police car. Our system provided audio to their VHS video recorder that was tied to a small video camera mounted behind the windshield. The officer wore a transmitter. The distributor buying the wireless systems from us specified mic level to feed the recorders since the ALC (automatic level control) was for that input. In some installations there was severe alternator whine because of ground loops and other clicks as equipment turned on and off. The grounds to the system included the antenna ground to the car chassis at the car whip antenna, the audio ground to the recorder, the power supply ground to the receiver, and a digital ground from our squelch circuit back to their readout box. Those were just the grounds to our receiver and didn't count the grounds to the recorder, camera, etc. The installers in different states and cities all had their own way of hooking things up and varied from good to horrible. An isolation transformer in the audio line usually fixed the problem but was too much money. We finally convinced the distributor to use an attenuator at the input to the recorder and let us send a line level signal. This reduced the ground loop noise by 30 or 40 dB and "solved" the problem. The distances involved were only a few feet and it wasn't really a cable pickup problem. Later on we convinced the distributor that the ALC wasn't a good idea anyway because it confused juries as to what was going on and made it hard to hear. In addition, the ALC was really upset by gunshots, as were the officers of course. Sending line level out of our receiver to the line level in on the recorder made for a bullet proof installation, so to speak.  

So the rule of thumb is high levels at the output and attenuate as necessary at the input, even though it won't make a difference most of the time.  

Here is an adapter for the UCR100 to attenuate its output down to balanced mic level.

The mic level input on the UM400a has the new servo input with much more gain for low impedance signals than the UM400 with the older input. The increased gain improves performance with dynamic microphones. However, switching back and forth between the UM400 and UM400a transmitters will require constant readjustment of the gain setting if you are using the MC40 cable. To make life simpler we have a MC41 cable that will work with either transmitter type and matches the gain to within a few decibels. If you want to build your own, the wiring of the TA5F is as follows:

  • pin 1 is ground
  • pin 2 N/C
  • pin 3 to a 3k resistor in series with the mic level wire.
  • pin 4 jumped to pin 1 (sets the servo bias to zero)
  • pin 5 N/C

This places resistance in the audio line and reduces the input to the UM400 by only a few decibels but reduces the UM400a by 20 dB and matches them very closely. The MC41 can be used with all the servo bias mics such as the SMa, SMQa, SMDa, UM400a, UM450, and LMa. It will also work well with any of the older style transmitters.

Posted 5 months agoby LectroAdmin