Studio Technologies 40 - Manual (Page 21)

Issue 1, July 1994 Model 40 User Guide
Page 22 Studio Technologies, Inc.
impedance is fixed at a moderately high
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
vigor when encountering audio transients.
An LED indicator shows that +19V power
is present. Two resistors and a capacitor
are used to create a low-impedance
reference voltage of approximately +9.5V.
This reference voltage is used to set the
operating point for all the analog circuitry.
Talent Preamplifier
Two identical sections of preamplifier
serve the talent input. For clarity we will
describe only the section that serves the
left/mono input. Signal enters the pream-
plifier via blocking capacitors. These are
specifically provided to block +48V
phantom voltage that may be present on
microphone signals. From the blocking
capacitors the signal enters one section of
operational amplifier that is configured to
act as a differential amplifier. This allows
balanced or unbalanced signals to be
connected.
A switch controls the gain of the op amp,
providing a voltage gain of 20dB when set
to the microphone position, or an attenua-
tion of 30dB when set to the line position.
Signal diodes protect the inputs of the
op amp from destruction due to an
over-voltage condition. Compensation
capacitors are provided to ensure stability
at the two gain settings. (Note that the
gain set switch is a four-pole/double-throw
type, allowing one switch to control both
preamplifier sections.)
The output of the op amp is connected to
one section of a dual audio-taper potenti-
ometer. This pot is the user control that
sets the level of the talent signal fed to the
headphone outputs. Signal from the wiper
of the pot is capacitive coupled to the input
of another section of operation amplifier.
This op amp has a fixed voltage gain of
25dB. The output of this op amp is
connected to the left headphone amplifier
and the talent input stereo/mono switch. In
the stereo position the output of the left
preamp is connected only to the input of
the left headphone amp; the output of the
right preamp is connected to the input of
the right headphone amp. In the mono
position the output of the left preamp is
connected to the inputs of both the left and
right headphone amps; the right preampli-
fier is not connected to anything.
Headphone 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.
Cue mix audio (audio from the Model 40)
enters the amplifier via an electrolytic
capacitor and is connected to a log taper
potentiometer. The “pot” is used to set the
output level of the cue signal. From the pot
the signal is connected to one section of