Issue 5, April 1994
Mic-PreEminence User Guide
Page 12
Studio Technologies, Inc.
where a failure may be located. Please
refer to the Mic-PreEminence schematic
diagrams while reading this material.
Power Supplies
The Mic-PreEminence contains circuitry to
provide the
±
18Vdc, +18Vdc, and +48Vdc
required for operation. The audio circuitry
uses the
±
18Vdc, the LED level meters use
the se18Vdc, and the phantom
power feed uses the +48Vdc. A step down
transformer converts the incoming AC
mains voltage to nominal 40Vac. The pri-
mary is tapped for either 115Vac or 230Vac
mains input. A switch on the back panel
selects which tap is connected. The second-
ary of the transformer has a center tap,
which is connected as the circuit common
and is strapped to the metal chassis and the
ground wire of the line cord. The trans-
former’s secondary connects to two rectifier/
filter sections: a voltage doubler and a full
wave bridge. The voltage doubler raises
the nominal 40Vac to nominal 85Vdc. This
is then fed to an adjustable high-voltage 3-
terminal regulator which pr48Vdc.
A trim potentiometer on the circuit board is
factory adjusted to +48Vdc (
±
0.1Vdc). The
full wave bridge produces nominal
±
30Vdc.
One positive and one negative fixed voltage
3-terminal regulator produces
±
18Vdc.
Another, lower power regulator produces
+18Vdc which is used by the levels meters.
Preamplifier Circuit
The input stage is unique because it
achieves very high current gain with
extremely low noise. A transistor array
package feeds two high slew rate opera-
tional amplifiers in a non-differential configu-
ration. The input is capacitor coupled to
block the +48Vdc phantom power that may
be present. The output of the two opera-
tional amplifiers are capacitor coupled to
the output stages.
The balanced output stage uses high-
performance operational amplifiers that
allow the driving of low source impedances
(600 ohms) with a large voltage swing.
Cross feedback provides good stability
in driving balanced or unbalanced loads.
Trim potentiometers in the output stage
permit adjustment of the input common
mode rejection while maintaining a “straight
through” balanced circuit without transform-
ers. The output signal is direct coupled to
provide the best sonic performance. Ca-
pacitors installed in the circuit board can
be enabled by removing two 0 ohm resistor
jumper wires if capacitor output coupling is
required.
The unbalanced output is achieved by
disconnecting the XLR-type connector,
“padding” the signal, and connecting it to
a ¼-inch, 2-conductor jack. In keeping with
“–10” equipment, the unbalanced outputs
are not intended to drive low impedances
with a large voltage swing. The source
impedance of the unbalanced output is
approximately 200 ohms and is meant to
connect to loads of 10k ohms or greater.
Meter Circuit
Each channel contains a 5-segment LED
level meter circuit to provide the user with
an indication of the internal operating level
and the corresponding output level. The
ballistics of the meter were selected to be
a cross between those of a mechanical VU
meter, and the very fast display of a PPM
meter. The thresholds are set so that the
