Stereo Input Module

stereo-input-front-panelSometimes you cannot find a PC board with just the right circuit.  This was one of those times.  I work with a lot of stereo sources, such as the Ciat-Lonbarde instruments, that output line level audio.  I needed a way to bring those into the modular at full levels, so I designed a circuit and built it myself.  It features

  • Stereo input mini-jack
  • Quarter inch left and right inputs
  • Left and right outputs
  • Mono mixed output
  • Clip indicator LEDS
  • Master gain knob

The Circuit

stereo-input-schematicThe mini-jack inputs go through the normal switching on the quarter-inch jacks so that either can be used, but not both simultaneously.  Each channel has a unity gain buffer with AC coupling and a 470K terminating resistor.  This is followed by an inverting buffer with 20X gain max.  A dual-ganged audio taper pot allows the gain to be adjusted from zero up to 20X.  The panel dial is marked to correspond roughly to the gain.  At the lower settings, a full synthesizer level input can be processed.  It’s not a mistake that the last two markings both indicate 20X.  This is due to the fact that the maximum resistance of the pot is reached a bit before the maximum rotation.

Each channel is monitored by a comparator that lights an LED whenever the signal passes 4 volts peak to peak.  An inverting unity gain mixer provides a handy mono output.






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