Studio Technologies 80 1997 - Manual (Page 13)

Issue 4, March 1997 Model 80 User Guide
Page 16 Studio Technologies, Inc.
The output of the bridge is filtered with
electrolytic capacitors, producing unregu-
lated ±20Vdc. Two integrated circuit
regulators produce the ±15Vdc from the
unregulated voltages. Capacitors on the
outputs of the regulators provide stability.
The ±15Vdc is used by the analog cir-
cuitry. The +15Vdc is also used by the
meter circuits. The –15Vdc is also used by
the power present LED.
Interesting technical note department:
notice that a diode is connected from
the output pin of each regulator to circuit
common. These serve to keep the
±15Vdc rails at, worst case, one diode
drop (0.7V) away from ground in the
opposite polarity. This is important when
supplying bipolar loads, such as opera-
tional amplifiers, etc. Without these diodes
the regulators can “latch-up” when mains
power is applied or removed. When mains
power is initially applied one of the sup-
plies can “come up” (get to its operating
voltage) sooner than the other. This volt-
age is fed back through the loads (the
op-amps) to the output pin of the other
regulator that is still coming up to full
voltage. Upon seeing this unexpected
opposite polarity voltage on its output pin,
the regulator may get very unhappy,
possibly latching into a nonoperating
state, drawing lots of current, burning up,
etcetera! The protection diodes keep this
condition from happening.
For service assistance, several test points
are included on the printed circuit board:
±20V unregulated, ±15Vdc and circuit
common.
Analog Circuitry
The Model 80 takes advantage of an
excellent series of audio-specific inte-
grated circuits from Analog Devices. Using
these parts saved us from using literally
dozens 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!
Audio Input
The Model 80 contains one stereo line-
level input circuit. The input is compatible
with balanced or unbalanced signals
having a nominal level range of –10 to
+10dBu, with relative differences in signal
level of up to 3dB. The function of the
input circuitry is to receive the left and
right audio signals, separate out common-
mode hum and noise, unbalance the
signals, and process them such that both
the left and right signals are presented to
the Model 80’s internal audio buses at
precisely –2dBu.
Left and right line-level audio enters
the Model 80 by way of two ¼-inch
3-conductor phone jacks. The loop-
through feature is implemented by simply
connecting the left input/left loop-through
and right input/right loop-through jacks
electrically in parallel. From the input jacks
the audio is also directly coupled into two
SSM-2143 differential (balanced) line
receiver integrated circuits. The ’2143s
have excellent common mode rejection,
low noise, and high slew rate. Intended
for professional audio applications, they
contain two 12k ohm input resistors,
individually laser trimmed for accuracy. By