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Matthew Steel

Lectro LMb v2.13 changes affect mics with nonstandard wiring?

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I wanted to share our experiences with newer LMb transmitters in case somebody else runs into the same issue.


Newer LMb transmitters (those with firmware 2.13) appear to be more picky about microphone wiring than earlier versions of the LMB (2.10 and 2.11) or other Lectrosonics transmitters we have tried.


As we have been transitioning out of 600MHz we have chosen to standardize on the LMb, mostly for cost reasons, and because its features meet our needs.  We have had lots of trouble with the transmitters we bought new, which had v2.13 firmware on them.  The problems appeared to be related to low audio level which required high transmitter gain settings and resulted in high noise as well as poor immunity to RF sources like cell phones.  However we had bought a couple of older used transmitters and these did not exhibit the same problems.  It seemed unlikely that eight transmitters bought in two batches would all be broken, so I made a call to New Mexico.  I found that there was a circuit board revision that came out with v2.13, but changes were not expected to affect the audio input portion of the transmitters.


Now, note that we have been using Lectrosonics transmitters for decades and when we first began using M185 VHF transmitters we followed the microphone wiring recommendations at that time for our MKE-2 microphones:


pin 1: Shield.

pin 2: Bias (red).

pin 3: Audio (blue).

pin 4: Jump to pin 1.

pin 5: Not connected.

shell: Not connected to shield.


Through the years we bought different Lectro transmitters but we never fully retired the old ones - so we needed all our microphones to be compatible with all transmitters.  Each time we bought a new transmitter we checked out the microphones with it and the wiring we were using seemed to work fine.  This was the case with the transition to servo bias wiring even though new "universal" wiring recommendations were published along with the input change.  Our existing wiring appeared to perform equally well with M185, UM200c, UM400a, SMa, and (older) LMb transmitters so we stuck with it.


The solution turns out to be simple (but relatively painful) because the currently recommended "universal" wiring (Figure 5) does indeed perform satisfactorily with all the transmitter models we own, and so all that is required is to rewire all our MKE-2 mics to the newer wiring.  The painful part is that we have about 50 microphones that need to be rewired.


I would find it interesting if any of the Lectro insiders here would have any comments.  Specifically, knowing the way our mics were wired, would the new PCB or firmware be expected to make this kind of difference or is this totally unexpected?  Also, would a Lectro M150 be expected to behave the same way as the MKE-2's?  Should I rewire ours into M-152's if I want to use them with servo bias transmitters?

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Hi Matthew,


First off, thanks for posting these details so that we may comment and hopefully provide some enlightenment on the subject. 


The firmware changes have not affected mic wiring for this transmitter. If you were to load v2.10 or v2.11 firmware into a recently purchased LMb, the results (with the microphones) would be the same.


Back in 2005, when we introduced the SM Series, we updated our mic wiring recommendations for two types of inputs: "servo bias only" (to work with the then new SM Series, the UM400a, later the LMa, and every subsequent bodypack transmitter introduced since then), and "compatible" wiring that would work both with the servo bias input units, and the older units (UM200a,b,c, UM400, LM, etc.)


It was possible for lav mics wired for the older "legacy" transmitters (such as the M150 you mention) to work, if perhaps not ideally, with servo input transmitters due to a voltage offset spec for the op amp used at that time. 


In 2015, that part was changed in all Lectrosonics bodypack transmitters, and the result is that old lav mics, or any lav mics not wired to either the "compatible" or "servo bias only" standards, will no longer work very well, which is what you've experienced with your newer LMbs.


So, although perhaps painful, the correct solution is to have your microphones re-wired to the 2005 standards, either:


"Compatible" (will work with servo or non-servo inputs):

Shield to pin 1

Bias (likely red) to pin 3

1k resistor between audio (likely white) and pin 1

Wire jumper from pin 4 to pin 2




"Servo bias only" wiring (generally the best audio performance with most lav mics):

Shield to pin 1

Bias (likely red) to pin 3

Audio (likely white) to pin 5

Wire jumper from pin 4 to pin 2


For details on wiring for specific lav mic models, please visit https://www.lectrosonics.com/Support/Microphone-Wiring/uhf-transmitter-5-pin-input-jack-wiring.html




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Yes, when I discovered that rewiring the mics would fix it I decided to bite the bullet and rewire the mics to present day recommendations.  We still need compatibility so I'll do the "compatible" wiring and I already got some resistors for that purpose.  I do the wiring myself so I can do a few at a time as I am able.

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Are you telling me that ceramic clad 5W wirewound resistors won't work?


Just kidding - Yes, I got 1/8 watt metal film 1% as recommended in this other thread.  I would not have thought about metal vs carbon otherwise.  It still amazes me that when I was learning the resistor color code the only options for the tolerance band were gold and silver - and now 1% resistors are only two cents each.

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