Studio Technologies 40 - Manual (Page 17)

Issue 1, July 1994 Model 40 User Guide
Page 18 Studio Technologies, Inc.
Power Supplies
The Model 40 contains two independent
linear power supply circuits. We felt that
reliability would be enhanced by splitting
the circuitry into two main groups, and
then optimizing a power supply for each.
Using this scheme, the Model 40 will
operate reliably, even with wide swings
in ambient temperature, varied operating
duty cycles, and mains voltage fluctua-
tions. The result is a unit that should prove
quite hard to kill! The power supplies use
separate step down transformers, the
primary side of each containing two 115V
windings. This allows them to be config-
ured for nominal mains voltages of 100V,
120V, or 220/240V. For 100V and 120V
operation the primaries are connected in
parallel; for 220/240V operation they are
connected in series. The configuration is
performed using jumper straps on the
printed circuit board. For safety, a fuse is
in series with the incoming mains power.
Because the Model 40 is intended for
continuous operation, a power switch is
not included. This serves several pur-
poses: eliminating the chance of a power
switch being accidently turned off, increas-
ing the physical isolation between the
nasty 50/60Hz fields and the sensitive
analog circuitry, and eliminating the
physical space required by a switch.
The first power supply generates filtered
and regulated ±15Vdc. The two 18V sec-
ondaries are connected in series, with the
series connection point providing circuit
common, as well as being strapped to the
metal chassis and the ground pin of the
power entry connector. The transformer’s
secondary is fed to a full wave diode
bridge. The output of the bridge is filtered
with electrolytic capacitors, producing
nominal ±22Vdc. Two integrated circuit
regulators produce the ±15Vdc from the
unregulated voltages. Capacitors on the
outputs of the regulators provide stability.
The ±15Vdc is utilized by most of the
analog circuitry.
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. The +15V regulator
will, worst case only, go to –0.7Vdc; the
–15V regulator to +0.7V. This is important
when supplying bipolar loads, such as
operational 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
supplies can “come up” (get to its operat-
ing voltage) sooner than the other. This
voltage 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, etc.!
The protection diodes keep this condition
from happening.
The second power supply generates two
unregulated DC voltages: +V UNREG,
which ranges from approximately +30 to
+55Vdc, and +V LED which ranges from
+15 to +28Vdc. Its transformer has dual
15V secondaries for 120V and 220/240V
operation, and dual 17V secondaries for
100V operation. The secondary windings
are connected in series, and then go to a
full wave diode bridge and an electrolytic
filter capacitor. The negative pin of the