Gain Control Amplifier-Line Level

Ideal Use Cases

The RDL ST-GCA3 is an automatic gain control amplifier for systems where input levels can vary substantially but output levels must remain tightly managed. Its SLOW mode is intended for leveling music sources, using an 8-second response for a 20 dB average-level adjustment. FAST mode responds in 3 seconds for the same adjustment, making it suitable for systems exposed to loud audio bursts or continually inconsistent levels. The provided application guidance specifically identifies paging, intercom, and microphone-mixing systems where intelligibility is important.

With a +4 dBu balanced nominal input and both balanced line- and microphone-level outputs, the ST-GCA3 can serve installations that need controlled gain delivered at either output level.

Why Choose This Model

At the core of the ST-GCA3 is a wide-dynamic-range AGC system offering more than 40 dB of control while maintaining the output level within 3 dB. The input accepts signals from -45 dBu to +20 dBu for rated output, with a maximum input level of +26 dBu. This operating range supports sources with considerable level variation.

The two response settings allow the gain behavior to match the program material. SLOW mode provides gradual adjustment for music, while FAST mode supplies quicker correction for paging, intercom, microphone mixing, and systems prone to abrupt bursts. The design also provides slow, inaudible gain reduction, positive fast gain reduction, and tight dynamic control.

Installation and Integration

The unit measures 0.7 inches high by 3 inches wide by 1.6 inches deep, equivalent to 1.7 by 7.6 by 3.9 centimeters. It operates from 24 Vdc at 40 mA and supports either a ground-referenced or floating SupplyFlex power input configuration. Recommended power supplies are the PS-24AS for the USA and the PS-24AX for international use; the power supply is not included.

The balanced input is nominally +4 dBu. Its two outputs provide a +4 dBu, 150-ohm balanced line signal with a nominal +0/-3 dB rating and a -45 dBu balanced microphone signal into 150 ohms. The specified ambient operating range is 0° C to 55° C. A 50 ms on-delay ramps the output from the muted state to the fully active condition.

Performance Highlights

Frequency response extends from 10 Hz to 20 kHz within ±0.75 dB. THD+N is specified at less than 0.3% from 50 Hz to 20 kHz, with a typical value of 0.05% at 1 kHz. Noise is specified at less than -70 dB with no input signal and the output unmuted, less than -80 dB with no input signal and the output muted, and less than -50 dB when an input signal is present while the output is muted.

The mute threshold is 46 dB below maximum input and 28 dB below normal input. Muting response is 15 seconds for a 50 dB change. Together with the characterized SLOW and FAST response times, these specifications provide clear timing and level information for system design.

What Makes It Different

The ST-GCA3 combines wide-range automatic gain control, two defined response modes, and balanced line- and microphone-level outputs in one compact unit. Its front-panel feedback uses two variable-intensity LEDs: a green LED shows audio expansion, while a yellow LED shows audio gain reduction. At the normal input level, both LEDs display equal intensity, indicating equal available expansion and gain reduction.

This combination allows the installer to see how the AGC is responding while choosing between gradual music leveling and faster correction of inconsistent or burst-prone sources. The published response times, mute behavior, input range, and output levels also make its operating characteristics readily identifiable before integration.

Final Assessment

The RDL ST-GCA3 is a focused solution for automatic audio-level management where balanced connectivity, more than 40 dB of AGC range, and a choice of slow or fast correction are required. Its compact dimensions, line and microphone outputs, specified low-noise performance, and variable-intensity status indicators make it particularly relevant to music leveling, paging, intercom, and microphone-mixing applications. A compatible 24 Vdc power supply must be selected separately.