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How should I wire a Countryman B6 or E6 for a UM400 transmitter?


Here's a long reply posted to the RAMPS news group. It has a variety of wirings depending on the output level you need.

To the Group:
To make the measurements on the B6 we used the same setup as in the previous post, "Lectrosonics MM400a and COS11 red dot wiring" (See COS-11 test setup in FAQ How should I wire a Sanken COS-11 or other three wire microphone to a MM400 transmitter?). We had available two B6 microphones and I assume (that word again) they are the standard 6 mV/Pa units since the overload point that we found was close to the specified values. One unit was one that Carl at Countryman kindly loaned us several years ago and the other was from a customer who was having difficulties matching to 400 series transmitters. We found several factors that could cause possible problems with the B6 and a UM400a. The bias resistor for the UM400 series is 4k. This is higher than what we have used on the UM200 by a factor of 4. We chose the higher value because of the improved noise performance of the 400 series. In an effort to increase input signal levels to get past self noise, we increased the bias resistor value between our pins 3 and 4 to 4k. We had also run into problems getting enough output out of some big name low current microphones that wanted to look into a 20k (!) load.

The B6 microphones that we measured here were pulling 750uA and 950uA at 3 Volts which is 2 to 3 times higher than the B6 spec sheet. At first I thought this might be the problem since this much current would pull the operating point well below 3 Volts.

I called Carl at Countryman and learned more about B6's than the average person should know. 

What Carl said specific to the B6 is that the ideal voltage at the B6 mic terminals is 1.5 Volts at which point the mic will draw 500uA. This was a lower voltage than I expected and changes what we would recommend for biasing. The 500 uA does differ some from the Countryman web site values but products always change the most right after the moment you publish "firm" specs. 

Countryman's original recommended UM400 wiring inside the TA5F, from their website was:

  • pin 1 shield
  • pin 4 white (center conductor)
  • pins 2 and 3 install a 2.8k resistor between them.
  • This will give slightly more than a 3 dB reduction in signal, compared to our Lectro wiring recommendation.

Carl now prefers another configuration, which is to ignore our internal resistors entirely and wire a 1.5 k resistor from pin 2 to pin 3 and and a 3.3 k resistor from pin 3 to the the hot lead of the B6. The reason for the new recommendation is to reduce the high B6's sensitivity and get it closer to other commonly used mics.With our bias impedance and the resistors this will drop the signal 6 dB total below our original Lectro wring and bias the mic at 2.1 Volts. So the wiring is:

  • pin 1 shield
  • pins 2 and 3 install a 1.5k resistor between them.
  • pin 3 3.3k resistor in series with the mic's white lead (center conductor)
  • i.e., a 3.3k resistor between pin 3 and the mic's hot lead (white).

Another wiring, for more attenuation, will change our 5 Volt bias to 1.7 Volts on the B6 and drop the level 14 dB below our original wiring is as follows:

  • pin 1 shield
  • pin 3 white (center conductor)
  • pins 2 and 3 install a 1.5k resistor between them.
  • pins 1 and 2 install a 1.8k resistor between them.

So here are three wirings which will drop input levels 3, 6 and 14 dB below the Lectro recommended wiring. I agree with Carl that the 6 dB wiring is the best all around. However, there is a bit more to the dynamics of the situation than just limiting and clipping levels. The 3 dB wiring will let the B6 drive the UM400 into 10 db of limiting (compression not distortion), even with the gain at a minimum. The 6 dB wiring will be 7 dB into limiting. The 14 dB wiring will not drive the transmitter into limiting before the mic itself clips. The downside is that input noise levels will come up by the same amounts which might be a problem in very quiet environments. 

The standard B6 is spec'd at a maximum input sound level of 118 dB. We measured gentle clipping at 114 dB on the high current mic and 117 dB on the lower current mic. These measurements are probably not as precise as Countryman's since they were made at higher voltage and current levels (more gain) but still are certainly comparable to Countryman's spec sheet. Considering that these are higher current mics than other electrets and at clipping, the mic is swinging the entire 500 uA bias supply, the std B6's are hot mics indeed. The gain reduction wiring above does nothing as far as increasing the sound level limit of the microphone itself to more than 118 dB spl. Therefore, the lower gain B6 (-10dB) version may be a good spare mic choice, certainly for loud situations since it would handle 128 dB Spl.

The long and short of it is that Carl wishes wireless mic manufacturers would standardize the input circuits and if not that, then at least not change the inputs willy-nilly. I agree with Carl and certainly we are guilty of changing the input values when we went from the 200 series to the 400 series. I would add that it would be great for the wireless manufacturers, if the all various mics had similar output levels and similar bias currents. What makes it tough, is that the bias currents between manufacturers vary by 15 to 1, the output levels by 25 dB or more and recommended loads from 1k to 20k.

Carl made a very interesting proposal which was to just provide a bias voltage (say 5 Volts or 3 Volts), a DC blocked audio input and a ground and let the mic manufacturers recommend the resistor values for the drain and/or source loads and build them into the mic connector. As Carl pointed out there is lots of room inside a Switchcraft TA5F connector. 

Posted 1 year agoby LectroAdmin