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SCA C84 Microphone Preamp Module

SCA C84 Microphone Preamp Module


The Seventh Circle Audio C84 is a transformerless microphone preamplifier based on the "Double Balanced" circuit first published in 1984 by Graham John Cohen and since used by many notable manufacturers. The C84 is superb on acoustic instruments, percussion, and any source that needs to be captured as accurately as possible. The C84 is also inexpensive and easy to build. Available as a DIY kit or fully assembled mic preamp module.


CH02-SP-C84 subpanel must be ordered separately.


    Dual DC Servos

    The C84 uses two DC servo circuits. The first biases the input stage for minimum common mode offset voltage, the second minimizes differential offset voltage.

    High Gain

    The C84 can provide 70dB of gain, more than enough for most applications, especially with modern high-output condenser microphones.

    Stepped Gain Control

    A 12 position Grayhill switch provides accurate and repeatable gain settings in 5dB steps from 15dB to 70dB.

    Gain Trim

    Since 5dB is often too big a gain step, the C84 also has a gain trim control, which provides up to 6dB of cut at any gain setting. This allows very fine adjustment of gain between steps.

    Phase Reverse

    Inverts the signal phase.

    Phantom Switch

    Slow-rise phantom power minimizes pops and thumps.

    Neutrik Connectors

    High quality PC mount connectors eliminate cabling and ease assembly.

    Transformerless DC-Coupled Signal Path

    With the exception of the phantom blocking capacitors, the C84 uses direct coupling throughout to minimize distortion and unwanted "coloration".

    Polypropylene Coupling Capacitors

    Instead of an input transformer, the C84 uses a combination of high-quality polypropylene film capacitors and bipolar electrolytic caps to block +48V phantom power while maintaining flat frequency response to below 10Hz.

    Discrete Z-Buffer Output Circuit

    Instead of an output transformer, the C84 uses a pair of discrete, high-current cross-coupled buffers to drive the output. This circuit can effortlessly drive a 600 ohm load to +32dBu.

    Flexible Power Supply

    On-board voltage regulators can accommodate power supplies from +/-15VDC up to +/-35VDC, allowing the use of op-amps with +/-24VDC requirements. Potentiometers allow easy voltage adjustment, making experimenting fast and simple. Local regulation also keeps inter-channel crosstalk extremely low when multiple preamps share the same power supply.

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