that this circuit can
on/y
be used with balanced, floating
(ungrounded), transformer-coupled input mixers such
as the Shure M67 or SE30. The resistors should be 1%
tolerance or better to assure close matching. With new
batteries, this supply will operate an SM80 for approx-
imately 200 hours.
LOW-FREQUENCY RESPONSE SWITCH
T h e S M 8 0 h a s a t h r e e - p o s i t i o n l o w - f r e q u e n c y
response switch located on its handle. The switch is
recessed to avoid accidental movement, but may be
easily moved with the fingertips. The user may select
either flat response, low-frequency rolloff of 6 dB per oc-
tave below 100 Hz, or low-frequency cutoff of 18 dB per
octave below 80 Hz (see Figure 1).
ATTENUATOR RING
The SM80 has a switchable 10 dB capacitive atten-
uator to prevent high sound pressure levels from over-
loading the microphone’s internal electronics. The
attenuator is engaged by rotating the actuator ring
directly below the grille assembly. In the “10” position,
the output of the microphone is reduced by 10 dB, in-
creasing the maximum sound pressure level at clipping
by 10 dB.
The attenuator ring may be locked in either the “0” or
“10” position as follows. Unscrew the grille and car-
tridge assembly by unscrewing counterclockwise from
the top. Turn the actuator ring to the “0” or “10” position
as desired. Insert the actuator ring lock (small curved
piece of plastic) in the area behind the actuator ring be-
tween the pin and the edge of the slot, thereby preven-
ting the ring from turning. Replace the grille and car-
tridge assembly.
The amount of attenuation can be increased by
changing the value of the capacitor in the 10 dB at-
tenuator switch assembly. Note that this change must
be done carefully and with subsequent cleaning to
avoid possible signal-to-noise degradation. Unscrew
the grille and cartridge assembly (counterclockwise
from top). Lift the 10 dB attenuator actuator ring up and
over the screw threads. Grasp the center contact of the
10 dB attenuator switch assembly and lift upward to
remove it. Obtain a 5% NPO monolithic ceramic
c a p a c i t o r ( C e n t r a l a b M O N O - K A P C N s e r i e s o r
equivalent) of the required value for the desired attenua-
tion:
Attenuation
Capacitance
-15 dB
150 pF
-20 dB
270 pF
-25 dB
560 pF
-30 dB
.001 µF
Using long-nose pliers and a low-wattage, pencil-type
soldering iron, carefully remove the capacitor from the
switch assembly and replace it with the new value. Take
care not to touch the switch assembly with the solder-
ing iron. To remove possible contamination, after
soldering wash the entire switch assembly in a mild
detergent solution, rinse it in distilled water, soak it in
alcohol to remove the water, and allow it to air dry.
Reassemble the SM80. Note the new attenuation value
on the SM80 attenuator ring with a small label.
MIXER OVERLOAD
The SM80 output is about 15 dB higher than most
dynamic microphones, in order to provide optimum
signal-to-noise ratio. When used at moderate to high
SPLs, this additional output may overload the mixer
input. A resistive attenuator can be inserted in the
microphone line ahead of the mixer to minimize this.
The Shure Model A15AS Attenuator (15, 20 or 25 dB
switch selectable) is specially designed for use with
condenser microphones. A convenient 15 dB attenuator
design is shown in Figure 9. The resistors shown are
1/2-watt, 1% tolerance, and the circuit may be pack-
aged in a Switchcraft S3FM adapter housing. The 15 dB
attenuator can be used between the SM80 and the PS1A
(or other power supply), or between the PS1A and the
mixer. Two of these attenuators may be used in series
to provide 30 dB of attenuation. Note that commercially
available 150-ohm attenuators (such as the Shure Model
A15A) are not recommended, due to loading.
15 dB ATTENUATOR CIRCUIT
FIGURE 9
WIND NOISE
The excellent frequency response of professional
condenser microphones such as the SM80 makes them
quite sensitive to wind, breath and air currents from ven-
tilation equipment. The resulting low-frequency micro-
phone output may cause mixer overload or other prob-
lems. The Model A81G Pop Filter Grille attenuates
breath popping sounds when the microphone is close-
talked, and permits its use outdoors with minimal
pickup of rushing and rumbling sounds.
Slip the A81G over the SM80 until the inside of the
A81G touches the top of the microphone. Tighten the
A81G by rotating the knurled collar clockwise from the
bottom. When removing the A81G, loosen the knurled
collar first (otherwise the cartridge will unscrew with
the A81G).
For outdoor use under very windy conditions, the
Model A81WS Heavy-Duty Windscreen is recom-
mended.
CIRCUIT DESCRIPTION
A block diagram of the SM80 is shown in Figure 10.
The capacitor cartridge is followed by a switch-con-
trolled capacitive attenuator stage which provides for
10 dB attenuation at the cartridge output. The signal is
fed to a field-effect transistor (FET) impedance con-
version stage. The FET output drives an active low-
frequency response (high-pass) filter controlled by a
three-position switch. The filter output from a com-
pound transistor, Class A, emitter-follower amplifier is
transformer-coupled, providing a balanced output to the
RFI protection filter at the microphone connector. An
active, constant-current power supply circuit regulates
the phantom voltage, allowing the SM80 to operate over
a very wide range of voltages. A reverse voltage protec-
tion diode guards against miswired cables and equip-
ment. The circuit contains five semiconductors to pro-
vide low noise, low distortion, wide frequency response
and ultra-reliable operation over a very wide range of
operating conditions.
4
