Studio Technologies 742A User Manual

Page 25

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Model 742A User Guide

Issue 1, July 2012

Studio Technologies, Inc.

Page 25

Audio Mixer

Compressor Conﬁgurations
The compressor circuits that control the

dynamic range of the main output buses

can be conﬁgured to operate from one

of three modes. Two conﬁguration jump-

ers, one for each compressor circuit, are

used to select which mode is active. From

the factory the jumpers are placed across

the pins labeled +10 dBu. This sets the

thresholds to be 6 dB above the main

outputs’ +4 dBu nominal output level.

Moving the jumpers to the +6 dBu posi-

tions sets the thresholds to be 2 dB above

the +4 dBu nominal output level. To dis-

able compressor operation place the

jumpers on the pins labeled OFF. Finding

the compressor conﬁguration locations is

not difﬁcult. When viewed with the cover

off, they are located near the center of the

printed circuit board. The jumper sections

are labeled COMPRESSOR 1 and COM-

PRESSOR 2. The factory default has the

jumpers set to their +10 dBu positions.

Monitor Output Conﬁgurations
The monitor section’s line-level outputs

are conﬁgured to operate from one of

three modes. Two conﬁguration jumpers,

one for each monitor output, are used to

select the desired mode. From the factory

the jumpers are placed across the pins

labeled POST. This conﬁgures the moni-

tor outputs so that they are after (“post”)

the level controls. Moving the jumpers to

the PRE –10 locations sets the monitor

outputs to be electrically before (“pre”) the

level controls and to have a nominal level

of –10 dBu. This is appropriate when con-

necting the monitor outputs to the inputs

of most ampliﬁed loudspeakers. Moving

the jumpers to the PRE +4 locations sets

the monitor outputs to be “pre” the level

controls but with a +4 dBu nominal level.

This mode allows the monitor outputs

to serve as an additional set of line-level

main outputs. Finding the conﬁguration

locations for the monitor outputs is not

difﬁcult. When viewed with the cover off,

they are located at the back of the printed

circuit board, near the ¼-inch 3-conductor

jacks that are associated with the monitor

outputs. The jumper sections are labeled

MONITOR 1 and MONITOR 2. The factory

default has the jumpers set to the POST

positions.

Reference Tone Frequency

Conﬁguration
Two conﬁguration jumpers are used to

select the nominal frequency range of the

reference tone oscillator. A trim potentiom-

eter is then used to set the frequency to

the exact desired value. From the factory

the frequency of the reference tone is set

to be 400 Hz. This was accomplished dur-

ing the initial factory testing process in two

steps. The ﬁrst step was to set the jumpers

to the two locations that are labeled 400

Hz. Then the trim pot labeled FREQ was

adjusted so that a 400 Hz signal was gen-

erated, as measured at the ¼-inch

3-conductor tone output jack.
To change the frequency to 1 kHz start

by moving the jumpers to the positions

labeled 1 kHz. Then apply power to the

Model 742A using a DC source in the

range of 10 to 18 volts. (Do not connect

an AC mains source as this exposes the

technician to a safety hazard!) Then, using

a frequency counter or other measuring

device, adjust the trim pot labeled FREQ

so that a 1 kHz signal is present at the

¼-inch 3-conductor tone output jack. Find-

ing the reference tone frequency conﬁgu-

ration location on the printed circuit board

is not difﬁcult. When viewed from the front