Custom Preamp/Mixer/Stomp-Box/DI for Dobro Guitar

I designed and built this stomp-box for Grammy-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 consisted of an AudioTechnica AT831b capsule connected to an AT8530 power module. The power module could use batteries or 5-52 V phantom power, and had an XLR male output jack.

Stacy's specifications for this preamplifier included:

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 Description: 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 phantom power, is coupled to an Analog Devices SSM2017 microphone preamplifier chip. Each preamp stage has an input gain trim control accessible by screwdriver through the unit's cast aluminum enclosure for pre-setting the optimal signal/noise ratio. Following the mixer is a bypassable fourth-order Butterworth low-pass filter with 24 dB of attenuation at 14 kHz. This is followed by a defeatable, variable boost/cut second-order active filter centered on 1 kHz. A heavy-duty DPDT stomp switch allows bypass of a master volume control which follows these filter stages, and also sets the status of a jumbo green LED--"on" for solo (bypass). Signal to a tuner output buffer (with a 1/4-inch female jack) and an LED signal level indicator circuit (relative green intensity = signal level, red = near clip threshold) is independent of the volume control and stomp switch status. The audio path leading to the main balanced and line outputs passes through an Analog Devices SSM2402 analog switch, as a "pop"-less muting circuit, which is controlled by a panel switch, for muted tuning when desired; a blue LED indicates muted status. An OP275 configured as a non-inverting unity-gain buffer drives the line-level 1/4-inch capacitor-coupled output. The low-impedance, balanced male XLR output is driven by a SSM2139-based transformerless balanced line driver circuit which is protected from house phantom power by capacitor coupling and zener diode clamps to ground. Except for the 9 VAC wall-wart transformer, the power supply is internal and provides the preamp's active components with highly regulated and filtered +/-15 VDC. The entire unit is housed in a compact 2.06 x 3.68 x 4.68-inch (H x W x L) cast-aluminum enclosure.

Stacy Phillips used this preamp system from late-1996 until mid-2003, when he switched to a modified Fishman Pocket Blender. At that time we dispensed with the AT831b mic's power module and let the blender handle that function. Stacy can now use a single cable for both transducers (rather than separate mic and piezo cables) and no longer fusses with the cumbersome AT8530 power module.

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