Sometimes 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 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.