FEA Labs Dual-Band SMX Optical Compressor


FEA Labs

This design is a direct result of many requests to model the very popular Trace Elliot SMX dual compressor pedal. This optical compressor is the original FEA Dual Band Optical Compressor with the variable Attack, Release and Crossover Frequency controls removed. This compressor does not have the limiter function that the full featured FEA Dual Band compressor has. The crossover frequency, attack and release times are set to the same values as the Trace Elliot SMX dual compressor pedal. The FEA and Trace Elliot compressors have vast differences of internal circuitry and functionality.


  • Small footprint - 4 1/2 inches by 3 1/2 inches
  • Dual optical compressors for separate Low frequency (LO) and High frequency (HI) dynamics control.
  • The frequencies above 5KHz are routed around the compressor circuits and are mixed back into the output amplifiers at unity gain to preserve clarity.
  • Output GAIN controls for the HI and LO compressors.
  • Both bands exhibit soft-knee compression at lower RATIO settings and hard-knee at maximum RATIO settings. The COMP LED indicates when the THRESHOLD has been reached. This LED does not indicate the RELEASE response.
  • The compressor side-chains use precision full-wave rectification of the audio signals in the THRESHOLD circuits to improve tracking. This also reduces the possibility of “pumping” with very low frequencies.
  • HI BYPASS foot switch. This allows the compressor to only control the Low frequencies (if desired) and to pass the High frequencies to the outputs unaffected. There is a red LED to indicate the HF BYPASS is on.
    NOTE: With HI BYPASS active the HI GAIN control is inactive.
  • ACTIVATE foot switch places the compressor unit in the signal chain or in Direct Bypass mode. In Direct Bypass mode the signal at the input is directly connected to the outputs and does not pass through any electronics.
  • The compressors have minimal components in the signal paths to maintain the utmost signal quality. The signal compression is accomplished with only resistive components (Light Dependent Resistors or Photocell). All resistors in the circuit are low noise 1% metal film type. The signal coupling capacitors are tight tolerance, quiet, multi-layer and metallized polyester film type. The amplifiers used in the signal path are JFET input, low-noise, low-distortion “hi-fi quality” devices.
  • The power supplies onboard voltage charge pump allows the circuitry to operate at 18volts (+9 and -9 volt rails) from a single 9-volt battery or AC power adapter of 9 volts or greater. This allows the signal plenty of headroom from active electronic guitars and aggressive playing techniques (i.e. pop and slap).
  • The onboard power supply can accommodate power adapter voltages up to 25 volts DC with protection from reversed polarity. The “switch on” power supply current is less than 1µA (micro amp) on the signal ground at the INPUT jack. This is approximately 45,000 times (-93dB) less than the commonly used method of connecting the battery's negative terminal to ground via the sleeve of the plug inserted into the INPUT jack. The “switch on” sensing method used in the FEA Dual-Band SMX Optical Compressor keeps nearly all of the circuit's generated white noise and transient currents out of the INPUT stage signal ground. Extreme measures have been taken to keep the power and signal paths as clean as possible.
    NOTE: Unplug the cord from the INPUT jack when not in use to prolong battery life.
  • The power and grounds for the signal path circuitry are separated from the side-chain power and grounds to protect the audio signal from spurious noise. The power for the signal amplifiers is exceptionally clean, filtered twice for each rail and all filter stages are oversized.
  • HI and LO/FULL output jacks are configured so the outputs of the individual compressors may be routed to different amplifiers or effects. The HI output is useful if you wish to use effects made strictly for guitar and route a clean LO signal to another amplifier or mixer. If only the LO/FULL output jack is used, both the HI and LO signals are mixed together for a composite full range output signal.

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