The LA-4 is an optical compressor designed and built by UREI. It is inspired by some of its predecessors - namely the 1176 and LA-2A.
With a double pot design in the primary controls, the LA-4 allowed the user to select between ratios of 2:1, 4:1, 8:1, 12:1, and 20:1. It also reversed the orientation of the primary controls, and renamed them “threshold” and “output”, bringing the feel closer to an 1176. However, the LA-4 still retained the optical design and did not bring in controls for attack and release.
The LA-4 also saw the end of the class A design, and the introduction of an early integrated circuit (IC) chip. The LA-4 also added the convenience of front panel switching to allow for stereo linking and un-linking of two units - very convenient, considering that the LA-3A and LA-4 were often used as stereo devices. The most interesting change however, is the introduction of an LED based optical design, which I’m certain was a cost saving measure, and may have combated the “panel aging” factor found in the original T4 (It was originally recommended that T4s be periodically replaced, just like tubes).
Around 1980 the LA-4 received more design changes as well as a component upgrade of the original ICs. The LA-4 had a run from 1977 to 1989 and saw blackface and silverface iterations.
Input: Balanced bridging differential amplifier
Input Impedance: 40 kohms - balanced ; 20 kohms - unbalanced
Equivalent Input Noise: less than -90 dBm; I/O terminated with 600 ohm load
Maximum Input Level: Determined by rear panel switch to allow for optimum operation in different level environments; High Range: Max +20 dB ; Low Range: Max 0 dB;
Gain: Adjustable with front panel gain control. Max 20 dB with input level switch in LOW position, or 40 dB with switch in HIGH position
Frequency Respone: ±0.5 db, 20 Hz - 20 kHz
Attack Time: 1 to 10 ms for 63% correction, depending on signal waveform
Release Time: 100 ms to 1 s for 63% return, depending on duration of limiting
Compression Ratio: 2:1, 4:1, 8:1, 12:1 or 20:1; switch selectable from front panel
Limiting Threshold: Adjustable with front panel threshold control and rear panel input level switch. Minimum level (±2 dB) to achieve 1 dB limiting is:
Ratio: 2:1 ; Low Range -61 dB ; High Range: -41 dB
Ratio: 4,8,12,20:1 ; Low Range -45 dB ; High Range -25 dB
Output: Floating, transformer isolated
Output Load: 150 ohms or greater
Power Output: +24 dBm into a 600 ohm load; +20 db into a 150 ohm load
Power Requirements: 100 to 125 VAC or 200 to 250 VAC , 50/60 Hz, switch selectable, less than 10 W
Environment: Operating +0C to +50C; Storage -20C to +60C;
Connections: Input and output, through rear chassis barrier strip. Stereo interconnect through phono jack. Power through 3-wire IEC style connector.
Indicators: Standard VU meter, switch selectable to read output level (0 VU referecnce +4 dBm or + 8 dBm), or amount of gain reduction. LED indicator for input overload. LED power indicator.
Dimensions: 216 mm W x 89 mm H rack panel, depth behind panel is 203 mm
Finish: Panel is 3.18 mm brushed, clear anodized aluminium in two shades. Chasis is cadmium plated steel
Weight: 2.95 Kg
The Gear Rack