Circuit Description, +/- 17V Rails | SoundCraft CPS275, CPS250

Circuit Description

The CPS250 and CPS275 power supplies provide +/-17V at 4 Amps and 5.25 Amps respectively. Both give +5V at 1 Amp, and +48V at 0.3 Amp. The PCB is SC3803.

THE +/- 17V RAILS.

This consists of two identical +17V supplies connected together to give +/-17V. Each 17V supply is a linear regulator with conventional fullwave rectification and 20,000uF reservoir capacitors. This is combined with a second power supply that sits on top to power the driver transistors and the control IC; this reduces the minimum voltage drop across the series-pass device and so improves efficiency.

The main series-pass element is a 250W discrete bipolar transistor mounted on a fan-cooled heatsink. The voltage reference and servo control amplifier are provided by a 723 regulator IC.

The circuitry of the +17V regulator is described in detail below:

The +17V Supply.

The transformer secondary is fused by F1. Secondary fuses will normally only blow in the event of serious failure, such as a major short in the AC wiring or a failed bridge rectifier.

The fullwave bridge rectifier is mounted on the heatsink, at the hottest end as it is the most heat-tolerant of the semiconductors. For safety reasons, the reservoir capacitor is discharged by bleeder resistors R39,40 at switchoff; this takes about 10 seconds. The unregulated DC voltage is approx +26 to +29V.

The 723 IC contains a 7.15V nominal Zener diode, defining the voltage that appears at Pin 6. R5 is in series with the positive input of the servo opamp (Pin 5) and is made roughly equal in resistance to the feedback divider R7,11 etc to minimise bias-current offsets.

The feedback divider R7,8,11, & PR1 derives a fraction of the output voltage and delivers it to the negative input of the servo opamp. (Pin 4) The negative feedback keeps this point also at 17.15V, so the actual output voltage is determined by the feedback divider ratio, which is trimmed over a narrow range by PR1.

The output of the servo opamp controls the output through TR1, an emitter follower driving the series-pass device TR3. The power supply to TR1 is taken from the +12V supply developed across C23. This has its 0V side connected to the positive unregulated DC supply of 26 - 29V, (at nominal mains) giving a total voltage of approx 38 - 41V for the driver; this also powers the 723 IC via R1. This subrail markedly reduces the minimum drop across the pass elements and therefore increases the efficiency.

10 Amp Schottky diode D25 on the output allows supplies to be paralleled for redundancy. This is only fitted for the CPS275 option.

Overcurrent Protection.

The power supply is protected against short-circuits and overload by a dual-mode protection system; brief overloads are dealt with by constant-current protection, while prolonged overload causes both 17V rails to shutdown. Reset is by turning the mains power off for not less than 6 seconds.

14	Technical Description

CPS250, CPS275 specifications

SoundCraft is renowned for producing high-quality audio mixing consoles that cater to a wide range of audio professionals and enthusiasts. Among their notable offerings are the SoundCraft CPS275 and CPS250 power amplifiers, which combine innovative technologies with reliable performance.

The SoundCraft CPS275 is a powerhouse designed to deliver a robust output with exceptional clarity. It features a dual-channel configuration, providing 275 watts per channel into 4 ohms. The CPS275 is built with a solid-state design, ensuring high efficiency and minimal distortion, making it suitable for both live performances and studio applications. One of its distinguishing features is the advanced thermal protection system, which safeguards the amplifier from overheating, ensuring longevity and reliability even in demanding environments.

On the other hand, the SoundCraft CPS250 is a slightly smaller model, ideal for users seeking a balance between power and portability. It offers 250 watts per channel into 4 ohms, making it perfect for smaller venues or monitors. This model emphasizes versatility and sound fidelity, featuring a class AB circuitry that enhances audio accuracy while maintaining low noise levels.

Both the CPS275 and CPS250 come equipped with comprehensive protection circuitry, which includes short-circuit, overload, and DC protection. This guarantees that both amplifiers will perform consistently without risking damage to connected equipment.

In terms of connectivity, these amplifiers feature XLR and 1/4-inch TRS inputs, allowing seamless integration with a wide array of sound systems. Additionally, the CPS series employs efficient cooling systems, ensuring optimal performance without the risk of overheating even during prolonged usage.

Both models are designed with rugged construction, making them suitable for the rigors of touring and live events. They are equipped with rack-mountable chassis, allowing for easy installation in professional setups.

Overall, the SoundCraft CPS275 and CPS250 amplifiers exemplify the brand's commitment to delivering high-quality audio solutions. With their advanced technologies, protective features, and exceptional sound quality, they cater to a variety of audio applications, making them an excellent choice for professionals in the music and live sound industries. Whether powering large PA systems or smaller setups, these amplifiers are engineered to meet the demands of any audio environment with impressive accuracy and reliability.