The Allison Research Kepex 500 was released in 1970 as a keyable program expander designed for professional multi-track studio use. The Allison Research Kepex 500 was developed to address the noise problems associated with multi-track recording, including tape hiss, amplifier noise, air conditioner rumble, room echo, intertrack leakage and other unwanted sounds that could build up across multiple recorded tracks. Rather than operating as a simple on/off gate, the Allison Research Kepex 500 used a wide-range, wide-band gain expander circuit that could be adjusted to absorb low-level noises while preserving the wanted programme material.
The Allison Research Kepex 500 was intended for studios working with individual tape tracks, where each channel could contribute its own layer of background noise. By placing the Allison Research Kepex 500 across separate tracks, engineers could keep individual parts cleaner, reduce background studio noise, control tape hiss and minimise the cumulative effect of noise during mixdown. The unit’s design made it especially relevant to multi-track recording, where the combined noise from many channels could become a practical production issue.
The Allison Research Kepex 500 also offered creative control beyond basic noise reduction. The brochure describes its ability to control presence without frequency equalisation, reduce effective microphone-to-instrument distance without overdubbing problems, remove echo from a percussive track by mistake, clean up old noisy masters and create automatic fades. This positioned the Allison Research Kepex 500 not only as a corrective tool, but also as a studio processor capable of shaping programme dynamics in a controlled and repeatable way.
A key feature of the Allison Research Kepex 500 was its external keying input. This allowed the gain of the unit to be controlled by a separate audio signal, turning the Allison Research Kepex 500 into a gain amplifier with an extreme dynamic range. The key input gave engineers and producers additional flexibility for programme-controlled effects, externally triggered expansion and other forms of dynamic signal manipulation.
The Allison Research Kepex 500 included solid-state sequential light metering for visual indication of operating gain. This metering circuit could also be used independently as a peak-reading VU meter without using the expander circuit. The brochure notes that this gave the recording engineer accurate monitoring of transient peaks beyond the capability of conventional moving-coil meters, including older and newer light VU meters.
The Allison Research Kepex 500 used a modular design built around a single 152mm x 152mm printed circuit card, with everything except the power supply mounted on the module. Any number of channels up to 16 could be installed in the Model 508 or 516 rack-mounted chassis, which included the power supply and necessary customer connectors. The Allison Research Kepex 500 could also be fitted into the Model 501 single-channel module for console installation. In either configuration, the module remained removable and interchangeable.
The later Allison Research Kepex 501 adapted the Kepex 500 circuitry into a single-channel console module for custom installation directly into a mixing desk. Designed for Kepex processing “on the board”, the Kepex 501 placed one channel of Kepex 500 expansion in a shielded 38mm-wide case with a rear-mounted connector for connection to an external power supply. The module was offered in a choice of two standard colours, Sky-blue or Avocado, with special console-matching colours available on request.
Features
- Keyable program expander designed for professional multi-track studio recording
- Wide-range, wide-band gain expander circuit
- Designed to reduce tape hiss, amplifier noise, air conditioner rumble, room echo and intertrack leakage
- Can be used on individual tracks to reduce cumulative noise during mixdown
- External keying input allows gain control from a separate audio signal
- Can control presence without frequency equalisation
- Can reduce effective microphone-to-instrument distance without overdubbing problems
- Can remove echo from percussive tracks and assist with cleaning noisy masters
- Can be used for automatic fades
- Solid-state sequential light metering for instantaneous gain indication
- Meter circuit can be used independently as a peak-reading VU meter
- Modular removable and interchangeable card design
- Compatible with Model 508 and 516 rack-mounted chassis for systems up to 16 channels
- Compatible with Model 501 single-channel console installation module
