The Gates 28-CO Limiting Amplifier is designed for use with broadcast equipment to effectively double transmitter power. Without a limiting amplifier, audio levels must remain low to prevent overmodulation at peaks. By limiting peaks by 3 decibels, this amplifier enhances modulation, providing a stronger audio signal and improved reception range.
This amplifier combines two widely used limiting circuits into a single unit, enabling high compression levels with minimal distortion compared to no compression. The Gates 28-CO employs both a differential bridge and an electronic suppressor grid circuit. The differential bridge, built around 60mA lamps, offers pure resistance.
A tube rectifier processes part of the output audio voltage, sending it to the suppressor grids of the first audio stage. This ensures rapid response to peaks while maintaining low distortion at higher compression levels.
A release timing switch on the rear chassis provides three adjustable release cycles (50 to 300 milliseconds), allowing customisation by the user. The limiter’s effect time is preset to approximately 15 milliseconds.
The amplifier of the Gates 28-CO consists of four audio stages divided into two amplifiers. The first two stages handle limiting, while the last two provide voltage gain. Its overall gain is comparable to a two-stage amplifier.
Two front-panel controls manage gain and modulation. The limiter control adjusts gain for optimal signal peaking, while the master gain control fine-tunes modulation percentage. A high-speed 3½” decibel meter displays compression levels, calibrated to indicate when 3½ decibels of compression occurs. Additional features include a headphone jack, pilot lamp, and starting switch.
The Gates 28-CO features a steel-grey chassis with a deep gloss finish and light-grey lettering. Its vertical rear chassis design integrates a self-contained power supply. Components are encased and impregnated for longevity and temperature stability.
