I've built a few of the "Tillman" style preamps, into DIY pedals, cables, and into guitars. About a year ago I installed one in my Pbass. I've been playing it over a year and finally getting around to posting it here.
About the switch: I used a sub-mini toggle on the pickguard, mounted between the tone control and output jack, for active/passive switch, and it's powered by +9V via TRS jack, and TRS cable from my pedal board. Or I can use a standard (TS) cable, for passive only, in which case the switch acts as a mute or kill switch. The switch is silent, both electrically and mechanically. In the schematic below, you'll see the switch is only SPDT, merely selects the passive signal or the buffered signal. In other words the JFET gate is always connected, but keep in mind, that load impedance is on the order of 10M and very low capacitance too. Key point: not switching power to the JFET eliminates pops when changing the switch.
When active, the volume boost is moderate, I'd guess about 2 to 3 dB. One reason for this mod was to get a hotter signal for an envelope filter pedal that wasn't fully triggering. Since then I've ditched that pedal but I use this preamp, on and off, all the time. Note the volume control is before the JFET. At about 7 and above on the volume control, the boost transitions into mild compression. Being a FET, this is a nice sound, IMO, similar to a tube.
Note the drain and source resistor values need to be adjusted for the particular J201B. But once you get it dialed in, it's stable. I've been using a few of these style preamps in my rigs for several years and they're all still working great.
Here's the schematic. I had previously modified the tone control but that's obviously independent of the boost mod.
About the switch: I used a sub-mini toggle on the pickguard, mounted between the tone control and output jack, for active/passive switch, and it's powered by +9V via TRS jack, and TRS cable from my pedal board. Or I can use a standard (TS) cable, for passive only, in which case the switch acts as a mute or kill switch. The switch is silent, both electrically and mechanically. In the schematic below, you'll see the switch is only SPDT, merely selects the passive signal or the buffered signal. In other words the JFET gate is always connected, but keep in mind, that load impedance is on the order of 10M and very low capacitance too. Key point: not switching power to the JFET eliminates pops when changing the switch.
When active, the volume boost is moderate, I'd guess about 2 to 3 dB. One reason for this mod was to get a hotter signal for an envelope filter pedal that wasn't fully triggering. Since then I've ditched that pedal but I use this preamp, on and off, all the time. Note the volume control is before the JFET. At about 7 and above on the volume control, the boost transitions into mild compression. Being a FET, this is a nice sound, IMO, similar to a tube.
Note the drain and source resistor values need to be adjusted for the particular J201B. But once you get it dialed in, it's stable. I've been using a few of these style preamps in my rigs for several years and they're all still working great.
Here's the schematic. I had previously modified the tone control but that's obviously independent of the boost mod.
