C84 Microphone Preamp

The SCA C84 is a transformerless microphone preamp 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.

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