The Collins 26C is versatile for any application requiring a high-quality, moderate-output program amplifier. Its output limiting feature makes it particularly suited for feeding into a radio transmitter, recording device, or other equipment where precise output level regulation is essential.
The Collins 26C Limiting Amplifier is a powerful compressor designed to enhance audio quality in both amateur and professional communication services. While traditionally adopted in broadcasting, where compression must be carefully limited to preserve original sound dynamics, the Collins 26C is engineered to excel in voice-only transmission scenarios, where clarity and intelligibility are paramount.
With the Collins 26C, users gain a range of significant advantages:
- Consistent modulation: Achieve complete and stable modulation for improved audio transmission.
- Reduced interference: Prevents adjacent channel interference from accidental or chronic over-modulation.
- Automatic gain control: Minimises the need for manual adjustments, maintaining optimal levels even with fluctuating input.
- Enhanced circuit protection: Protects transmitter circuits from damage due to severe over-modulation peaks.
The Collins 26C is built with a unique, sophisticated circuit designed to handle rapid and extensive variations in input levels, automatically adjusting gain for seamless performance. For those interested in the technical details of its design and application, a comprehensive bulletin is available upon request. Ideal for both broadcast and high-frequency use, the Collins 26C Limiting Amplifier is an essential tool for any audio setup prioritising voice clarity and consistent output.
The gain reduction in the Collins 26C Limiting Amplifier activates rapidly, within about 2 milliseconds, controlled by the charging time of condenser C9. The gain returns to normal approximately 3 seconds after input levels decrease, governed by C9’s discharge through resistor R24.
These response times suit general uses for transmitting both voice and music. Users can adjust these characteristics by modifying C9 and R24. For voice-only applications, reducing the return time by decreasing R24’s size may help maintain maximum gain. However, the combined time constant of C9 and R24 should not fall below 0.1 second.
For music-focused applications, increasing the time constant by enlarging R24, C9, or both may soften gain changes, preserving musical expression often lost in high-amplitude limiting.
Metering Circuits
The Collins 26C is equipped with two panel instruments, allowing a thorough check on the amplifier and limiting circuits. A three-range volt-milliammeter can be activated via push buttons to measure the plate current for each of the five amplifier tubes individually, along with the plate supply voltage from the rectifier-filter system. The wiring design ensures that even if the meter, push buttons, or associated wiring fails, it won’t disrupt amplifier operation. Push buttons come with multiple contacts for improved reliability, and an external terminal (marked “M”) is available for using the meter outside the amplifier.
A rectifier-type volume indicator meter, controlled by the V.I. SELECTOR switch on the amplifier panel, serves three different functions. In the FILAMENT position, the meter connects via a multiplier resistor across the heater supply voltage to the amplifier. The resistor is calibrated so that a “0” reading on the scale reflects the correct voltage, with allowable tolerances specified in the “OPERATION” section of this manual.
In the OUTPUT position, the meter connects across the 500-ohm output terminals, measuring power level in decibels above 6 mW. The amplifier’s output level is determined by adding the setting of the V.I. RANGE switch to the meter’s scale reading. For instance, if the V.I. RANGE switch is set to 10 and the meter reads -3, the output level is +7 dB. When the V.I. RANGE is set at zero, a zero reading on the meter corresponds to an absolute level of 0 dB, referenced to 6 mW.
