countryman B6 mic and UM400a wiring

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". 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.

Best Regards,
Larry Fisher
Lectrosonics

Thank you very much to both of you for all this enlightening, and to
Lectrosonics and Countryman for being so close to real life.

Larry, the UM400"a" is a new hardware or software version? It is yet
available?


Fernando


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