I made this two-channel preamplifier/mixer in 1996. I wanted a high-quality dual preamp to independently set EQ and volume levels for my mandolin and fiddle (each equipped with a piezo transducer). During performances, this enabled rapid transitions between the two instruments by simply muting one channel and turning on the other.
Click here for the schematic (please use your browser's "back" button to return). Each input channel uses a low-noise Analog Devices SSM2139 op amp, has 1 M-ohm input impedance, and is followed by an SSM2139-based active filter stage providing bass, midrange and treble boost/cut controls. The front-end gain and tone controls are accessible through the aluminum enclosure as screwdriver-operated presets. Each channel's post-tone control signal level indicator (green LED) and near-clip indicator (red LED) circuits allow optimal input gain trimming. Two Analog Devices OP-275-based summing mixers drive two line outputs at 1/4-inch female jacks: For the output to a monitor amplifier (and also the balanced output; see below), the two channels can be switched on and off independently and without "pops;" the state of these switches is indicated by yellow LEDs. The channel levels for this mixer is controlled by easily accessible rotary faders. The other mixer is independent of the mute switches and faders; I used its output for continuous hook-up to a chromatic tuner. Thus, the monitor and balanced outputs can be muted while tuning if desired. An SSM2139-based servo-type low-impedance balanced line driver circuit is linked to the male XLR output, and is used for direct interface to a PA system. Series capacitors and zener diode clamps protect this circuit against any phantom power from the PA system. Internal except for the required 9-VAC "Wall-wart" transformer, the unit's +/-15 VDC highly regulated and filtered power supply allows all active stages to operate at near their design maximum signal/noise ratios.
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