Studio Technologies 40 - Manual (Page 19)

Issue 1, July 1994 Model 40 User Guide
Page 20 Studio Technologies, Inc.
Talent Amplifier Output Groups
Two identical circuits are used to create
talent amplifier output groups A and B.
Each circuit provides power and stereo
audio for connecting up to four Model 35
or Model 38 Talent Amplifiers. One circuit
will be described. A double-pole/double-
throw switch selects which audio bus is
connected to the output group. From the
switch, signal connects to sections of
operational amplifier which are configured
as inverting, unity-gain amplifiers. The
selected op-amps are capable of driving
low impedance loads.
The output of the op-amp that serves the
left channel is capacitive coupled to a
power modulator circuit. The power modu-
lator circuit provides the talent amplifiers
with nominal +23Vdc, amplitude modu-
lated with left channel audio at a nominal
level of –10dBu. The modulator consists of
a 3-terminal regulator integrated circuit,
two transistors, and several discrete com-
ponents. The voltage regulator creates
nominal +26Vdc from +V UNREG. The
regulator has inherent thermal protection
so that an over-current condition will not
damage the circuitry. One of the transis-
tors is used as the actual modulator. It is
connected in a series-pass arrangement,
with the left channel audio connected to its
base. The other transistor, along with a
resistor, serves to limit the maximum
current.
The second section of op-amp provides
right channel audio to the output group.
A capacitor, along with a series resistor
protects the op amp from capacitive loads,
as well as from an accidental short circuit
to the power/left channel signal.
Over Current Detection
Two identical circuits monitor the operating
status of the talent amplifier output groups;
we will describe one. The +23Vdc with left
channel audio output signal is connected
to a network consisting of two precision
resistors and a filter capacitor which
serves as a voltage divider/filter. The
scaled voltage is connected to one section
of integrated circuit comparator. The
switching point of the comparator is set by
a reference voltage that is created from
the +15Vdc power supply rail. The output
of the comparator is used to control the
base lead of a transistor, which in turns
controls the Over Current LED. The LED
lights whenever the output voltage falls
below the threshold.
Model 35 Talent Amplifier
General Description
The Model 35 Talent Amplifier is a self-
contained module which allows head-
phones to be driven with stereo audio
provided by a talent amplifier output group
from the Model 40 Central Controller. The
major components of the Model 35 are the
power supply and headphone amplifier.
The Model 35’s circuitry is contained on
two printed circuit boards which are inter-
connected via a 5-conductor flexible
jumper cable. All active circuitry lies on the
electronics board; the connectors lie on
the connector board. No surprises here!
Power Supply
A 3-terminal adjustable integrated circuit
voltage regulator is configured to provide
+19V from the incoming +23V that is
modulated with left channel audio. The
important characteristic of this circuit is its
constant input impedance. The input
impedance is fixed at a moderately high