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Lectro SMV Input Wiring from 2 Mic Level Sources

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Hey guys,


I'm going to start by saying that this is a very specific question and has no practical use on a film set.  It's for a project I'm working on at home.


The basic idea is one SMV transmitter with a lav going into it.  Call the lav Source A.  I need a second source of mic level audio, Source B, going into the same input jack on the transmitter.  I would make an external circuit so that when there is signal present on Source B, Source A is muted and Source B is allowed to pass into the transmitter.  I was planning on achieving the mic level of Source B by padding down it's regular line level by 50db with resistors.


I was wondering what pins I would use for Source B's audio on the transmitter input jack.  From looking at the Lectro wiring diagrams, I think it's 1 & 3, but I'm not sure.  Would bias wiring matter from a padded down line level signal? 


I also ideally would want this to work with multiple lav wirings for Source A (different lavs).  Would switching between the two different mic level wirings of Source A and B like that be bad for the transmitter in any way?  I've definitely plugged and unplugged different kinds of lavs like a COS11 and B6 before and it worked fine.  I'm just wondering if there's anything I'm not seeing.



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Lav A (Source A) needs bias to operate. When you switch to Source B with an external switch/relay you are going to get full DC bias change in voltage at the mic input. This won't hurt the transmitter or receiver but you will get a helluva pop. A secondary problem in using Source B with unneeded bias applied is that if you don't short pin 4 of the 5 pin connector to ground (pin 1) you will have several mA's of current flowing into your resistor network. Any wiggling of the 5 pin connector will cause some loud static. You can put a blocking capacitor (4.7 uF with + toward pin 3 of the transmitter) in series with the pin 3 wire of your Source B to prevent 99% of this. You may still have a little leakage current causing some wiggle noise. If you can switch pin 4 to the proper pins to add and remove the mic bias, that would be best. A double throw, double pole switch or relay has enough contacts to do all this.


To answer your direct question, the SM input is very well protected and what you are doing will not hurt the transmitter. Pin 3 is correct for your padded down Source B.

Have fun experimenting.

Best Regards,

Larry Fisher


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