Mic-PreEminence User Guide
Issue 5, April 1994
Studio Technologies, Inc.
Page 13
first green LED lights at an internal level of
–12dBu, the second at –5dBu, and the third
at 0dBu. The yellow LED lights at +5dBu.
The red LED lights at +12dBu. Audio enters
the meter circuit via an operational amplifier
configured as a half-wave rectifier. The
audio is picked up at the point at which the
input circuit couples to the line driver circuit.
The rectifier produces a DC voltage which
is representative of the audio level in the
preamplifier circuit. This DC is fed to a
meter driver integrated circuit. This IC
contains circuitry to directly control up to ten
LEDs, although the Mic-PreEminence uses
five. The circuit is configured to operate in
the series mode to reduce current draw;
one LED or all five, the current is the same!
The meter IC and the LEDs are powered
by a separate source of +18Vdc. This
eliminates the chance that clicks will enter
the audio signal via the meter circuit. Note
that since the unbalanced outputs are
simply “padded” versions of the balanced
outputs, the meters display correctly for
both balanced and unbalanced operation.
Troubleshooting
The following procedures must be
performed by a qualified technician.
Operating the Mic-PreEminence with
the top and/or bottom cover removed ex-
poses the technician to points in the power
input section with hazardous voltages.
Equipment Required
• Dual Trace Oscilloscope
• Sound Technology 1710A Distortion
Measurement System
• Digital Voltmeter
• Audio Cables as required
Power Supplies
The first step in troubleshooting the
Mic-PreEminence is to remove the top
cover (two Phillips head screws on each
side panel), and check the DC voltages at
the test points listed on the schematic. It is
common for failures associated with con-
temporary audio equipment to be power
supply related. The components most
likely to fail in the power supply are the four
3-terminal regulators (one each for +18,
+18 (meter power), –18, and +48) and the
large filter capacitors. Check and set the
adjustment potentiometer for the +48V
(
±
0.1Vdc) supply if the adjustable regulator
is replaced.
Analog Problems
If the power supplies are not at fault, use
standard troubleshooting procedures to
locate the problem. A simple technique is
to connect an audio signal to the input and
trace the signal (again, using the schemat-
ics) until the source of the problem is found.
Troubleshooting the Mic-PreEminence can
require a bit of patience, as it is a balanced
device; AC
and DC voltages must be
checked.
