Studio Technologies 40 - Manual (Page 18)

Model 40 User Guide Issue 1, July 1994
Studio Technologies, Inc. Page 19
bridge is connected as the circuit com-
mon, as well as being strapped to the
metal chassis and the ground pin of the
power entry connector. +V UNREG is
used by the talent amplifier output group’s
power modulator circuitry. The center tap
of the transformer creates +V LED—can
you figure out how this works? For filter-
ing, an electrolytic capacitor is connected
from +V LED to circuit common.
For service assistance, several test points
are included on the printed circuit board:
±15Vdc, +V UNREG, and circuit common.
Analog Circuitry
The Model 40 takes advantage of an
excellent series of audio-specific inte-
grated circuits from Analog Devices. Using
these parts saved us from using literally
hundreds of additional components. More
importantly, performance levels were
achieved that would have been difficult,
if not impossible, to obtain with more
conventional circuitry. These Analog
Devices parts have the common prefix of
SSM, indicating their roots in a company
called Solid State Microelectronics for
Music, purchased, by way of Precision
Monolithics, a few years ago. Hats off
to the guys and gals at Analog Devices!
Line Inputs
The Model 40 contains two identical
stereo line input circuits. The line inputs
are compatible with balanced or unbal-
anced signals with nominal levels of –
10dBV to +10dBu. The exact purpose of
the line input circuits is to receive the
audio signal, separate out common-mode
hum and/or noise, unbalance the signal,
and attenuate it to the –10dBu internal
operating level. Each line input circuit
contains a differential input integrated
circuit, followed by a variable gain reduc-
tion stage. The line input signals are direct
coupled to SSM-2141 differential (bal-
anced) line receiver integrated circuits.
The ’2141 has excellent common mode
rejection, low noise, and high slew rate. It
contains two internal 25k ohm series input
resistors, individually laser trimmed for
accuracy. By design, the ’2141 acts as
a unity-gain device.
The output of the SSM-2141 is capacitive
coupled to one section of dual potent-
iometer. The coupling capacitor is a non-
polar type, allowing for small DC voltages
of unknown polarity to be received on a
line input. The wiper of the pot is con-
nected to one section of op amp config-
ured as an inverting, unity-gain buffer. The
output of the op amp serves as the internal
signal “bus.”
Signal Present LEDs
Four identical signal present circuits moni-
tor the audio level on the internal audio
buses; left and right for input 1, and left
and right for input 2. Audio signals enter
each meter circuit via an operational ampli-
fier configured as a half-wave synchronous
rectifier. The resulting DC output is
smoothed via a resistor/capacitor low pass
filter. The output of the low pass filter is
connected to the input of one section of
integrated circuit comparator. Two preci-
sion resistors are connected in series with
the +15Vdc power supply rail to create a
DC reference voltage. This reference
voltage is used as the switching point for
the comparator. The output of the com-
parator is used to control the base lead of
a transistor, a transistor which directly
controls the Signal Present LED.