Universal Audio 175 B User 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.

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER
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BALANCING ADJUSTMENTS
Inherent stability eliminates the need for frequent balancing of the 175 B.
However, if thumping is noticed or components are replaced, the unit should be
balanced as follows:
a. If using TEST 1 position, remove any input signal from the amplifier.
b. Set the attack and release controls to approximate mid settings.
c. Rotate the input and output attenuators fully counter clockwise
(maximum attenuation).
d. Set the meter switch to OUTPUT position.
e. Set test switch to TEST 1 position.
NOTE: If desired, balanced can be performed by applying a 3-volt external
signal to the input jacks and setting the test switch to TEST 2 position.
f. Rotate the output attenuator clockwise until an indication is observed on
the meter.
g. Adjust first the PLATE BALANCE and then the CATHODE BALANCE
controls for minimum meter deflection.
h. Rotate the output attenuator fully clockwise (zero attenuation).
i. Adjust first the PLATE BALANCE and then the CATHODE BALANCE
controls for minimum meter deflection. Note meter indication.
j. Adjust the PLATE BALANCE control slightly clockwise until a change in
meter indication is observed. Then attempt to adjust CATHODE
BALANCE control for meter indication less than noted in step i.
1) If meter indication is less than that noted in step i, perform step j
repeatedly until absolute minimum meter indication is obtained.
2) If meter indication is not less than that noted in step i, adjust the
PLATE BALANCE control slightly counter clockwise until a change
in the meter indication is observed. Then adjust the CATHODE
BALANCE control for minimum deflection. Perform sub-setup (2)
repeatedly until absolute minimum meter deflection is obtained.

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER
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NOTE: The absolute minimum meter deflection should be in the
order of -20 dB. If the two sections of the 6BC8 are not reasonably
symmetrical, optimum balancing cannot be obtained.
NOTE: The fastest attack or release time is attained by turning the control full
clockwise.

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER
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PERFORMANCE SPECIFICATIONS
FREQUENCY RESPONCE:
Without limiting: ± 1 dB, from 20 cps t0 20 kc
With 5 dB limiting: ± 1 dB, from 20 cps t0 20 kc
MAXIMUM POWER OUTPUT
After output attenuator with limiting switch off to prevent limiting at +24 dBm
less than 1% THD from 50 cps to 15 kc.
GAIN
Jumper in low gain position: 24 dB
Jumper in high gain position 37 dB
MINIMUM INPUT LEVEL TO ACCOMPLISH LIMITING
Jumper in low gain position: -24 dBm
Jumper in high gain position: -24 dBm
MAXIMUM OUTPUT AT THERSHOLD OF LIMITING: +13 dBm
INPUT AND OUTPUT IMPEDANCE: 600 ohms, unbalanced
SIGNAL TO NOISE RATIO: 80 dB
COMPRESSION RATIO (above threshold of limiting): 12 to 1
ATTACK TIME: Adjustable from 100 to 1000 microseconds
REALEASE TIME: Adjustable from 27 to 527 milliseconds
TUBE COMPLEMENT: One each, 6BC8, 12AX7, 12BH7, 6AL5, 0B2, GZ34
DIMENSIONS: Width: 19 inches
Height: 3-1/2 inches
Depth: 9-1/4 inches
WEIGHT: 17 pounds

UNIVERSAL AUDIO Model 175 B LIMITING AMPLIFIER
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