C-Audio ST Series - User Manual

The C-Audio ST Series comprises high-performance stereo power amplifiers, designed to meet the demands of professional audio applications. The ST600 and ST1000 models are robust and reliable units, engineered for long-term, operational reliability in "real-world" environments.

Function Description

The ST Series amplifiers are designed to provide optimum long-term performance, operational reliability, and sonic quality. They feature a compact amplifier package, measuring just 2U high. These amplifiers are suitable for driving a wide range of loudspeaker loads and are equipped with protection circuits to safeguard against faults and misuse. The design emphasizes sonic quality and stability, enhanced by "audiophile-grade" circuit topologies.

Important Technical Specifications

The ST600 and ST1000 models offer different power outputs, as detailed in the Power Rating Table. For the ST600, continuous average power output into 4 ohms is 260W per channel (both channels driven) and 300W (bridged). Into 8 ohms, it delivers 130W per channel (both channels driven) and 800W (bridged). For the ST1000, continuous average power output into 4 ohms is 680W per channel (both channels driven) and 1000W (bridged). Into 8 ohms, it delivers 340W per channel (both channels driven) and 2000W (bridged). These power figures are measured at 1kHz into 0.1% distortion.

Key technical specifications for the ST600 include:

• Inputs: Electronically balanced, XLR (female) connectors.
• Input Impedance (Balanced): 20k ohms.
• Common Mode Rejection: < -50dB at 20kHz, < -80dB at 1kHz.
• Input Sensitivity: +4dBu.
• Gain: 32dB.
• Outputs: Binding posts and 3-pin IEC connector for power.
• Output Impedance @ 1kHz: <32m ohms.
• Slew Rate: 50V/µs (ST600), 40V/µs (ST400).
• Rise Time: 3µs.
• DC Offset: <50mV.
• Performance:
  • Frequency Response: 10Hz - 50kHz (+0/-1dB).
  • Total Harmonic Distortion (20Hz - 20kHz): <0.06%.
  • Intermodulation Distortion (1kHz): <0.06%.
  • Signal to Noise Ratio: -100dB.
  • Damping Factor: >250.
• Voltage Requirements: 220/240V AC ±20%, 50Hz.
• Weight: 15 kg (net), 19 kg (shipping).
• Dimensions: 482 mm (width), 410 mm (depth), 88 mm (height).

Key technical specifications for the ST1000 include:

• Inputs: Electronically balanced, XLR (female) connectors.
• Input Impedance (Balanced): 20k ohms.
• Common Mode Rejection: < -50dB at 20kHz, < -80dB at 1kHz.
• Input Sensitivity: +4dBu.
• Gain: 32dB.
• Outputs: Binding posts and 3-pin IEC connector for power.
• Output Impedance @ 1kHz: <32m ohms.
• Slew Rate: 50V/µs.
• Rise Time: 3µs.
• DC Offset: <50mV.
• Performance:
  • Frequency Response: 10Hz - 50kHz (+0/-1dB).
  • Total Harmonic Distortion (20Hz - 20kHz): <0.06%.
  • Intermodulation Distortion (1kHz): <0.06%.
  • Signal to Noise Ratio: -100dB.
  • Damping Factor: >250.
• Voltage Requirements: 220/240V AC ±20%, 50Hz.
• Weight: 20 kg (net), 24 kg (shipping).
• Dimensions: 482 mm (width), 427 mm (depth), 108 mm (height).

Usage Features

The ST Series amplifiers are designed for ease of use and integration into professional audio systems.

• Compact Size: The 2U high chassis allows for efficient rack mounting, saving space in equipment racks.
• Full Headroom: High CMRR balanced inputs on XLR connectors and outputs on 5-way binding posts ensure robust connectivity and signal integrity.
• Product Reliability and Service Life: Optimized through optimum power supply design and efficient output stage efficiency, these amplifiers are built for demanding use.
• In-built Protection: Protection circuitry safeguards against faults and misuse, enhancing the longevity of the unit and connected equipment.
• Ground Lift Switch: Fitted to help eliminate system hum loops, ensuring clean audio reproduction.
• Large-scale Use: Common assemblies and comprehensive visual checks simplify servicing and minimize spare parts inventory, making maintenance straightforward.
• Bridged Mode: The amplifiers can be operated in bridged mono mode, providing higher power output for specific applications. A switch on the rear panel allows selection between stereo and bridged modes.
• LED Indicators: Front panel LEDs indicate the status of the amplifier, including power, signal presence, and clip/protect warnings.
• Cooling System: A forced cooling fan is provided to maintain optimal operating temperatures.

Maintenance Features

The ST Series amplifiers are designed with maintainability in mind, incorporating features that simplify servicing and ensure long-term reliability.

• Modular Design: The use of common assemblies across the series simplifies troubleshooting and parts replacement.
• Visual Checks: Comprehensive visual checks are part of the maintenance procedure, allowing for quick identification of potential issues.
• Modification Procedure: A detailed modification procedure is provided for all ST amplifiers, irrespective of their issue. This procedure outlines specific component changes (e.g., capacitors, diodes, resistors) to improve performance and reliability.
  • Capacitor Replacements: Several capacitors (C20, C28, C10, C32, C17, C40, C16, C39, C12, C13, C35, C36) are to be replaced with specified values and voltage ratings.
  • Diode Replacements: Diodes (1N4148) are to be soldered in reverse parallel from IC2 pin 3 to 0V and IC4 pin 3 to 0V.
  • Output Transistor Capacitors: Capacitors fitted across the two outside pins of the K1058 transistors should be removed.
  • Ceramic Capacitors: Fit 220pF, 1KV ceramic capacitors between Drain and Source of TR6 and TR17.
• Issue Specific Changes: Specific procedures are outlined for different PCB issues (Issue 6, Issue 4 & 5 with Modification Daughter Boards, Issue 4 & 5 with Resistor-Resistor-Diode-Capacitor Networks, and Previous Issues). These procedures involve removing and replacing specific resistors, zeners, and fitting new components as per detailed figures.
• Output Transistor Maintenance: The fixing screws for the K1058 output transistors and the insulating pad between the transistor and heatsink should be removed. The heatsink should be rubbed down with medium abrasive paper to remove any burrs or remnants of insulating pad. The mounting faces of the power transistors should be cleaned, and a thin, even layer of heatsink compound applied. The devices should be re-assembled, ensuring the fixing screws are secure but not overtightened.
• Final Checks: After any modification or repair, a series of checks is recommended to re-test the amplifier.
  • Output Stage Check: Measure between Drain and Source of each transistor with a multi-meter set on a low Ohms range. A short-circuit reading indicates one or more transistors are faulty. Measure between Gate and Source of each transistor with a multi-meter set on the 2k Ohms range. A reading of less than 560 Ohms indicates a faulty transistor.
  • Zobel Resistors Check: Verify R34, R35, R106, R69, R71 & R109 (10R, 2.5W parts) for signs of having over-heated.
  • Short-Circuit Test: Drive the amplifier into a 4 Ohm load to an output level just prior to the clip point. Connect a short-circuit across the amplifier output for approximately ten seconds. Remove the short circuit and check the output returns to its previous level. If output transistors fail during this test, they should be replaced, and the current limiting protection circuits should be checked for faulty components.

The ST Series amplifiers are designed to deliver reliable, high-quality audio performance in demanding professional environments, backed by comprehensive maintenance guidelines to ensure their longevity and operational integrity.