After a few years of using the custom preamp/DI on his Dobros, Stacy Phillips switched to a Fishman Pocket Blender system. The advantage of the Fishman Blender is that a single cable links the instrument's two transducers (piezo pickup and electret mic) to the preamp. However, his previous preamp system has at least three features that the Fishman Blender lacked: (1) a solo- versus rhythm-level stomp switch (with status LED), (2) an always-on tuner output, and (3) a low-pass filter to attenuate the Dobro's "tink" attack transient. Stacy asked me to make the necessary modifications to his new Fishman Blender even though I assured him that such tampering would irreversibly void its warranty.
To make room for the modifications, I bypassed the Blender's side-chain jacks for its two input channels. I removed three of these four jacks entirely, and converted the function of the fourth to a pre-mute-switch master channel output (for use with a chromatic tuner). In the holes for the three absent jacks, I placed the knob for the rhythm level control (bypassed by the stomp switch for solo level), and bypass switches for two different "tink"-targeting low-pass filters (more on this below). I drilled new holes in the top of the unit for the heavy-duty DPDT "stomp" switch and red LED stomp status indicator.
You can click here to see a schematic of my modifications which I scanned from my notes (you will need your browser's "back" button to return). To preclude possible problems with the Fishman people, I have pixelated the potentially proprietary portions of this picture.
Since it can be powered by either a 9-V battery or a "wall-wart" external transformer, the Fishman Pocket Blender employs low-power active devices. For compatibility, I chose low-current TLE2062 dual op amps and an efficient red LED in my modifications. Originally I used one dual op amp in a 4th-order Butterworth low-pass filter configured like the "tink" filter in the custom Dobro preamp. However, as noted in my schematic, I subsequently removed this filter assembly because of excessive noise (the custom preamp's SSM2139 performed much more quietly in this role than the TLE2062 in this modification). Still residing in the modified Blender, however, the alternative "tink" filter is less aggressive but operates quietly. It uses a passive 2nd-order L-C network at the input of a TLE2062-based buffer. The second op-amp of this TLE2062 package buffers the signal at the wiper of the rhythm volume control which I added. This control is bypassed by the stomp switch for "solo" level (the maximum level as set by the Blender's original master volume control). In its "solo" (bypass) condition, the DPDT stomp switch also turns on the red (status) LED. When activated this LED draws either 1 mA ("normal") or 5 mA ("bright") current depending on the setting of a miniature switch that I added to the side of the unit. This switch can be used to increase LED visibility outdoors (with battery operation this would require more frequent battery changes).
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