Universal Audio 175 B Operating Instructions Manual

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER

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OPERATING INSTRUCTIONS

UNIVERSAL AUDIO INC.

Model 175 B

LIMITING AMPLIFIER

The Universal Model 175 B is an extremely versatile professional quality peak
limiting amplifier designed for use in the recording, broadcasting and motion
picture industries as well as other high quality audio systems. The 175 B is a
true limiting amplifier capable of high gain, low distortion, linear amplification
at levels below the threshold of limiting and with excellent limiting
characteristics and negligible increase in distortion at levels above.

The model 175 B is designed to be mounted in a standard 19” rack. The unit

requires only 3½” of rack space, and the front panel is hinged to provide
accessibility to the inner components. The highest quality military type printed
circuitry is combined with 1% precision resistors and over rated components
throughout to insure maximum performance and reliability. Plug-in type “FP”
electrolytic capacitors are used for ease of replacement.

The 175 B has a self contained power supply, a VU meter that is switchable to
indicate input or output level as well as gain reduction in dB, and input and
output attenuators graduated in 2 dB steps, with vernier adjustments in excess
of 2dB for each step.

The limiter is provided with input and output jacks on the front panel as well as
a terminal strip on the rear of the unit for interconnecting the amplifier with a
system. The terminal strip also provides for connecting a remote VU meter if
desired.

Separate adjustments of the attack and release times are provided on the front
panel as well as a limit defeat switch mounted on the rear of the attack control to
allow for the wide variety of program material that the 175 B will be expected to
handle. Checking the unit for balance, or rebalancing may be accomplished
without the need for test equipment, because an internal signal for balancing is
provided on the position of the test switch. Screw-driver adjustment holes are
provided on the front panel so that the VU meter may be easily observed with
the panel closed during balancing.

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER

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OPERATION

GAIN

The gain of the amplifier can be changed by altering resistance in the grid
circuits of V1. Jumpers have been provided for this purpose (see schematic
diagram). Unless otherwise specified, the unit is wired with the two jumpers
connected for low gain (24 dB). If desired, the jumpers can be relocated to the
high gain (37 dB) position.

VU METER

Unless otherwise specified, the 175 B is wired for +8 dBm zero reading on the
VU meter. If desired the VU meter can be wired for a +4 dBm zero reading by
relocating a jumper wire in the meter switching circuit (see schematic diagram).

Terminals provided on the rear of the unit for connecting a remote VU meter. A
strapping arrangement on the VU meter terminal board (see schematic diagram)
allows the remote VU meter to perform gain reduction while the internal meter
is still usable to indicate input and output level.

ATTENUATORS

The input and output attenuators are both adjustable from 0 to 40 dB. For this
reason the 175 B may be utilised at any point in the system where input signal
level is between -24 dBm and +13 dBm. The attenuators are adjusted for proper
operating levels as follows:

a. Rotate the input and output attenuators fully counter clockwise

(maximum attenuation).

b. Interconnect the unit with the system through either the input and output

jacks on the front panel or the terminal strip on the rear.

c. Feed a sample of the program material into the system.

d. Set the meter switch to GR position.

e. Decrease attenuation with the input attenuator until the desired amount

of limiting is observed on the meter (limiting should not exceed 3 to 4 dB
except where the program material contains extremely loud peaks).

f. Set the meter switch to OUTPUT position.

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER

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g. Decrease attenuation with the output attenuator until the desired output

level is observed on the meter.

h. Return the meter switch to GR position to verify amount of limiting

during operation.

Note: The output level may be increased 6 dB by replacing the output attenuator
with Langevin model AT 601 attenuator.

ATTACK AND RELEASE TIME

The ATTACK and RELEASE potentiometers should be adjusted for optimum
results in accordance with the type of program material being used. The
following general suggestions are given for use of ATTACK and RELEASE
adjustments:

a. For vocal or narration program material with minimum low frequency

information, best results are usually obtained by adjusting for very fast
attack and release times.

b. The release time should be lengthened as the low frequency content of the

program material increase.

c. The attack time should be lengthened when the program material

contains tremolo or vibrato which the limiter might possibly follow.

The limit defeat switch is located on the attack potentiometer. By turning the
attack control full counter clockwise, the limiting action may be defeated and the
unit perform as a normal line amplifier.