1. Limiter/Compressor Unit
Note: This document refers to the /J variant of the limiter compressor unit. For information
concerning the other variants refer to the Uses and Variant Information sections.
Introduction
The limiter/compressor unit contains two identical, isolated channels which are suitable for mono,
stereo or independent operation; units can be linked for multi-channel use. When limiting and
compression is not required, both channels may be by-passed. Both mono, stereo and by-pass
functions are switched by logic-controlled relays. Depending on the position of an EXTernal –
INTernal switch, these functions are either operated by front panel switches or externally
controlled by a rear panel connector. The above switches are internally illuminated from the Logic
to show the operational mode of the unit, and external tallies are available via the rear panel
connector. The reset position of the relays is BY-PASS and MONO: when power is disconnected
or switched off, the unit is in BY-PASS mode.
The limiter/compressor unit contains the following four identifiable sections:
• Limiter/Compressor board (11475): This is a motherboard which contains the
attenuator control element, the 10640 amplifier and the limiting and
compression sidechain electronics.
• The external control potentiometers, switches and gain reduction meters (which
indicate the overall gain reduction in each channel during limiting and
compression) are mounted on the front panel.
• Input and output sockets mounted on the rear panel.
• A D.C power and relay logic board, which provides a regulated +24V supply
for the 11475 board, and +5V for logic circuits controlling the relay switching.
Limiter/Compressor Board (11475)
This board contains the main audio signal processing circuits of the limiter/compressor unit. The
operation of the board is described elsewhere.
Front Panel Switches
The rotary control switches located on the front panel of the unit are multi-contact multi-layer
rotary switches. Electrical loading of the switches is provided via resistors connected across the
contacts.
Limiter/Compressor Technical Handbook Page 2
Mains Power Supply
The 24V DC input, derived from the main supply, required by the limiter/compressor board
(11475) is supplied by a power supply unit located in the limiter/compressor unit. The power
supply unit comprises a toroidal mains input transformer (TF14201), a regulator circuit (part of
11720), and associated components. The arrangement is shown as part of the limiter/compressor
block diagram EB20515.
Mains power is applied via an IEC inlet connector to the front panel mounted switch POWER. The
switch illuminates when the unit is switched on. Input voltages of 100V, 120V, 220V or 240V AC
can be selected using the voltage selector mounted on the rear panel. Primary circuit protection is
provided by a 500mA anti-surge fuse for 100V, 120V, 220V or 240V operation, the fuse is located
in the body of the voltage selector.
The 24V AC output of the toroidal transformer is connected to the secondary protection fuse, a
500mA anti-surge type, mounted on the rear panel of the unit.
Specification
Frequency Response 20Hz to 20kHz +/-0.5dB, measured at 0dBu relative to 1kHz.
Noise -75dBu with input terminated in 600 ohms, by-passed IN, compress and
limit OFF;
-55dBu with compress IN and gain make-up at maximum.
Limit Level +4dBu to +15dBu, measured with limit recovery set at 50ms, level at
+8dBu using a 1kHz sine-wave at +20dBu and +8 control adjusted to give
+8dBu output.
Limit Ratio with input level at 10dBu, increased to +20dBu the change in output level
should be 0.1dB, +/-0.1dB.
Attack Time slow: 4ms, +/-1ms
fast: 2ms, +/-1ms
being the time taken for a signal to return to within 1dB of its original
value, using an increase in input level of 10dB on an input adjusted to
+10dBu at 1kHz.
Limit Recovery this is the time taken by the control voltage to reach a new value when
an input at +20dBu is reduced in value by 10dB. The recovery times can
be set to 50ms, 100ms, 200ms, 800ms, AUTO 1 1500ms, AUTO 2
3000ms (all +/-50%).
Gain Make-up settings 0 to 20dB correspond with output level +/-0.5dB.
Limiter/Compressor Technical Handbook Page 3
Compress Ratio recovery set to 100ms
threshold to -20dBu
input 1kHz at 0dBu
for each setting of the Compress Ratio Switch the input level is increased
by 10dB and the change in output level should be:
Compress Ratio
Switch
O/P Level
Change (dBu)
Tolerance
(dB)
1.5 : 1 6.5 ± 1
2 : 1 5.0 ± 1
3 : 1 3.5 ± 1
4 : 1 2.5 ± 0.5
5 : 1 1.5 ± 0.5
Compress Recovery 100ms, 400ms, 800ms, 1500ms
AUTO 1 800ms AUTO 2 1500ms
Distortion 0.075% maximum (bypass IN, Compress and Limit OUT, input level at
+9dBu, 1KHz).
0.2% maximum (Compress IN ratio 6:1, Gain make-up to MAX, Recovery
800ms, Threshold –18dBu).
0.45% maximum (Limit IN, Compress OUT, Recovery 800ms. Input level
+22dBu, Threshold –18dBu).
Power Consumption 5.3VA
Dimensions Length: 482mm (19in)
Height: 88mm (3.26in)
Depth: 255mm (10.03in
