Studio Technologies 40 - Manual (Page 20)

Model 40 User Guide Issue 1, July 1994
Studio Technologies, Inc. Page 21
value, approximately 2K ohms, and does
not vary appreciably with load. This is
important so that the left channel audio
signal is not significantly attenuated, nor
distorted by normal fluctuations in the
power draw. A moderately large capacitor
is connected across the output of the
regulator. This capacitor provides a re-
serve of energy to allow the left and right
channel audio amplifiers to respond with
gusto when encountering audio transients.
An LED indicator shows that +19V power
is present. Two resistors and a capacitor
create a +9.5V reference. This reference
is used to set the operating point of the
circuitry.
Audio Amplifier
Two identical amplifier circuits provide the
left and right headphone outputs. The
circuits are designed to produce a maxi-
mum voltage swing, rather than to source
a large amount of power. This is the cor-
rect means of driving contemporary head-
phones, most of which have a load
impedance of 250 ohms or higher. The
reality is that with most phones a high
output level is obtained via a large voltage
swing, not through power.
For simplicity, only the left channel circuit
will be described. Audio enters the ampli-
fier via an electrolytic capacitor and is
connected to a log taper potentiometer.
The “pot” is used to set the output level.
From the pot the signal is connected to
one section of integrated circuit opera-
tional amplifier via resistors and a switch
which performs the mono function. In the
mono position the switch connects the left
channel signal to the right channel ampli-
fier, while dropping the level by 6dB. The
op amp, along with two transistors and
supporting components form the left
channel output stage. A low pass filter in
the feedback loop helps to provide stabil-
ity. The amplifier’s output is capacitor
coupled, via a series resistor, to the output
connectors. The capacitor changes the
audio output signal from being biased at
approximately +9.5V, to being biased at
signal common. The series resistor limits
the output current in the event of a shorted
output load. The output capacitor was
selected for sonic performance, rather
than what the math told us to select. We
used our ears, not the numbers!
Model 38 Talent Amplifier
General Description
The Model 38 Talent Amplifier is a self-
contained module which allows head-
phones to be driven with a mix of audio
from the Model 40 Central Controller,
along with another signal that we refer to
as the talent signal. The talent signal can
be stereo or mono, microphone or line
level, allowing an individual headphone
mix to be created.
The major components of the Model 38
are the power supply, talent preamplifier,
and headphone amplifier. The Model 38’s
circuitry is contained on two printed circuit
boards, which are interconnected via two
5-conductor flexible jumper cable. All
active circuitry lies on the electronics
board; the connectors lie on the connector
board.
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