Microphone Preamps - Shure AMS8100 Service Manual

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Microphone Preamps

Channel EQ
Channel LED Circuit
25A1041 (RA)
Shure AMS8100 Automatic Microphone Mixer
The microphone preamps convert the current signals from the two
microphone elements of an AMS microphone into voltage signals.
The microphone preamps are only suitable for use with Shure AMS
microphones. Conventional, balanced microphones usually have about
a 150 Ω impedance and supply a small voltage for a conventional micro-
phone to amplify.
However, AMS microphones are unbalanced, have two microphone
capsules per microphone, have high impedance outputs, and supply a
current output. Each output from the AMS microphone, taken between
the red conductor and shield and between the black conductor and
shield, has an approximate equivalent output impedance of 51 kΩ.
U801 and associated components form two identical preamps; one
for the front microphone capsule, and one for the rear microphone cap-
sule. Both channels are simple current to voltage converters with R8015
and R8014 determining the gain.
Bias for the microphones is supplied from the 7.5 Vdc supply via
R8012 and R8009. L801–L804, R8001–R8002, and C801–C802 form a
broadband rf filter to keep rf from getting into the subsequent circuitry.
After the microphone preamp, the signal from the front microphone
element (from the U801B output) goes through the equalizer (EQ) and
then the channel fader gain stage. This gain stage, comprised of U812A
and associated components, has a gain of 32 dB (40 times).
Following the channel fader stage is a buffer, U803B, that has a gain
of 0 dB (1 times). This buffer is used to lower the impedance of a signal
fed into the direct-out jack if it is modified to be a send/return insert.
Equalizing (EQ) is provided to make different microphones sound
similar. The low frequency EQ is a single pole (6 dB per octave) low cut.
It is a simple resistance-capacitance (RC) filter comprised of C834,
VR802, and R8147. The high frequency EQ is a boost or cut Baxandall
type filter. Its frequency and boost, or cut amount, is defined by C836,
VR804, R8151, R8152, R8148, and R8149.
U807C, D816, and surrounding resistors, are hooked up as a full
wave peak detector. If the signal at D816, pin 3, reaches about
Vdc peak, U807, pin 14, goes low. When this pin goes low (–15 Vdc),
Q814 conducts and pulls current through the red LED inside LED801.
C826 acts as a pulse stretcher to extend the on-time of the LED
on peaks.
3
Products
7.5
Characteristics