Lectro MM400a and B6 levels

To the Group:
To make the measurements on the MM400a (minature weather proof) and 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. I'm
not going to go into as much detail here as on the previous Cos-11 and
B6 posts for use with the UM400a and MM400a, since it is just more of
the same. We found that the B6 overloads at between 114 and 117 dB spl
and that value is essentially the same as their spec sheet value of
118 dB spl. Our sound pressure arrangement was more practical than
exact. Since the microphones swing their entire bias supply of 500 uA
at the 114 to 118 dB spl input, they are fairly hot mics. This will
develop 1 Volt pk at the normal input of a MM400a. The maximum level
that the MM400a will handle before limiting is 0.42 Volts peak. I want
to emphasize that this is limiting and not clipping. The signal will
still be clean just compressed. So a B6 at maximum signal swing can
drive the MM400a into 7 dB of compression and the B6 itself, will
start to clip before the transmitter audio circuits begin to clip.

To reduce the gain of the standard B6 another 4 dB, put a 4k resistor
across the B6 leads in the connector, i.e., from the center pin of the
connector to ground. Three dB is due to a reduced load and a dB or so
due to reduced current. This will get the max output of the B6 close
to the max non limiting input of the MM400a. If this is still too hot
and you are uncomfortable with running the MM400a at minimum gain,
then I recommend the reduced gain B6 which is down 10 dB from the
standard unit.

One last subtle point; it is possible to get the limit light to come
on with a microphone that is in distortion and clipping because most
of the time the mic is not centered voltage wise by the bias supply.
This means the microphone can generate larger voltage excursions in
one direction than in the other. Another way of saying it, is that the
mic can be clipped on one polarity of the waveform and be clean on the
other. So even though an electret mic is clipping severely, the output
level can be increased by 6 dB or so with even louder signals. One
side of the waveform is clipped and can produce little additional
level but the other side can keep increasing in voltage.This
asymmetric clipping doesn't actually sound too bad because the
microphone goes into major distortion at a gradually. The one sided
clipping is mainly 2nd harmonics and has a warm sound until the unit
is driven even harder and the distortion becomes gross or both sides
begin to clip producing third harmonic distortion. The ear is fooled
with this one sided clipping because the distortion has a relatively
gentle onset. Way back in prehistoric days, we made 7 watt, battery
powered lecterns from which comes the name "Lectro" sonics. Anyway,
the original designer found that setting the amplifier off center so
that it began clipping on one side well before the other sounded much
better than having the amplifier clip symmetrically. This wasn't
clever design, but just serendipity.

Best Regards,
Larry Fisher
Lectrosonics