Preamp Mixer Stomp-Box for Dobro Guitar

Posted on Feb 6, 2014

I designed and built this stomp-box forGrammy-winning resophonic guitar player Stacy Phillips. Stacy typically uses two transducers on his Dobro-style instruments: a piezo element mounted internally and a gooseneck-mounted electret microphone positioned an inch or so above the resonator cover. When I built this preamp in 1996, his microphone cons

Preamp Mixer Stomp-Box for Dobro Guitar
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isted ofan AudioTechnica AT831b capsule connected to anAT8530 power module. The power module could use batteries or 5-52 V phantom power, and had an XLR male output jack. A boost/cut tone control centered on 1 kHz (this is the approximate fundamental resonant frequency of the Dobro`s resonator and proves to be important for setting up the instrument`s amplified sound). A defeatable low-pass filter to eliminate a high-frequency "tink" sound, which Stacy found irritating. We identified this sound as pick (attack transient) noise with an audible bandwidth of 12-20 kHz. Link here to a FFT output (Fast Fourier Transform) from Sound Designer II which helped us determine this (please use your browser`s "back" button to return). Compact and rugged construction suitable for portability and road use. Stacy reluctantly agreed to let the unit`s power supply use an external wall-wart transformer rather than batteries. From my notes I have scanned the Dobro preamp`s schematic diagrams and modification records. You can see these images by clicking the links below. (You will need to use your browser`s "back" button to return. ) Circuit The unit has two independent input channels feeding an Analog Devices OP275-based mixer, with a rotary fader for each. The 1/4-inch input for the piezo transducer leads to an SSM2139-based low-noise non-inverting gain stage with a 1 M-ohm input impedance. The XLR female microphone input, with selectable +15V...

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