Issue 5, April 1994
Mic-PreEminence User Guide
Page 14
Studio Technologies, Inc.
Alignment
Alignment is required after any repair not
involving the power supply or level meters.
The following paragraphs will aid in aligning
the Mic-PreEminence circuits, thus restoring
proper operation. It is important to perform
the alignment procedures whenever you
have replaced any of the audio-related
components. Alignment can be tricky and,
if at all possible, factory repair and realign-
ment is recommended.
Caution: DO NOT attempt to adjust any
of the common mode reject/output balance
trim pots inside the unit before you under-
stand the entire alignment procedure and
are equipped to go through the entire
process.
All signal measurements are to made in
reference to circuit common test point. This
point is common for the +18, +18 (meter),
+48, and –18V power supplies; pin 1 of the
four XLR-type connectors; sleeve of the
¼-inch, 2-conductor jacks; chassis ground;
and earth ground via the mains power cord.
The common test point is located on the
right side of the circuit board, near the
power supplies, and is labeled COM.
Common Mode Reject/Output
Balance
Any repair of the Mic-PreEminence audio
circuitry requires adjustment of the two trim
potentiometers associated with the repaired
channel. These trim pots serve a dual
function: setting the input common mode
rejection and the output circuit level bal-
ance. From now on we will refer to this
procedure as setting the CMR (common
mode rejection). Input common mode
rejection relates to the ability of the
Mic-PreEminence to ignore signals to
input XLR pins 2 and 3 that are common in
phase and amplitude. The output balance
relates to the output circuitry producing
equal amplitude excursions in the positive
and negative direction (in the case of the
balanced output, when measuring with
respect to pin 1, if pin 2 is 2V, then pin 3
should be –2V).
The widely used 1710A Distortion
Measurement System from Sound
Technologies is an excellent, although
somewhat older, audio test set. This instru-
ment, along with an oscilloscope, will allow
easy CMR adjustment. The following test
procedure refers directly to settings on the
1710A and the oscilloscope.
1)
On Mic-PreEminence: front panel gain
control at MIN, phantom power OFF,
phase reverse NORMAL, output to BAL,
and power to ON.
2)
On 1710A: generator at –10dBm,
oscillator set to low distortion, frequency
1kHz, source impedance 150 ohms,
noise filter on at 30kHz, hum filter on,
analyzer mode to volts power, analyzer
input range switch to +10.
3)
Connect 1710A generator output to
Mic-PreEminence mic input: one termi-
nal of balanced output to pin 2
and pin
3, the CT (center tap) terminal to pin 1.
The other balanced output terminal is
not used. This puts a common mode
signal on the Mic-PreEminence mic
input.
