From the Source:
The Lehle Sunday Driver SW is a compact-format high-end preamp for all
types of electrical and acoustic instruments – a preamp that enables
electric guitars and basses, acoustic guitars, and also other stringed
instruments (such as violins, cellos and double basses) to realize all
of their musical potential. The switchable version expands the classical
Lehle Sunday Driver’s features by an On/Off footswitch.
The Lehle Sunday Driver SW, with its high-quality JFET technology,
provides two operating modes. In D (= Driver) mode, the input signal is
amplified with zero modification, to eliminate the signal losses that
long cable routings and/or effects chains can cause. The signal remains
powerful and clear, and retains its dynamics. Mode S (= Sunday)
multiplies input impedance by four, bringing out previously inaudible
details and, as gain increases, imparting to the guitar a characteristic
and unmistakable warmth. There is no distortion at any stage, and the
signal always stays clean, even at a maximum gain setting of 15 dB.
Thanks to its studio-standard signal-to-noise ratio of better than -100
dB, the Lehle Sunday Driver SW is totally free of background noise. To
exploit the dynamics of tube amplifiers to the full, the input voltage
is also rectified from the power supply socket, then filtered,
stabilized and doubled to 18V.
In OFF status, the Lehle Sunday Driver SW provides two different modes
of operation – in TB (= True Bypass) mode, the input and output are
linked with zero losses to one another via a switch with gold-plated
contacts. True Bypass mode is the best option for a small set-up with
short cable routings and for when a treble-booster or a classical fuzz
pedal is looped in after the Lehle Sunday Driver SW. The line-driver
function, located at the start of the train or in an effect chain, is
the better solution for long cables and for complex effect board
configurations. Switch the Lehle Sunday Driver SW to TS (= True Sound)
mode, and it is active as a buffer, without amplifying or modifying the
signal in any way. In this mode, the Lehle Sunday Driver SW operates as a
totally neutral line-driver equivalent to Mode D with the gain pot
turned all the way to the left. The Bypass mode option enables the Lehle
Sunday Driver SW to integrate ideally into any set-up, whether as a
battery-operated stand-alone unit, or in an existing pedal board.
Note: I removed the odd power network, which consisted of a diode rectifier and regulator, which should not be needed since it’s supposed to run on +9VDC not +9VAC. According to the site, they claim there’s a TB/Buffered switch which is not included on the schematic, nor did I see it on the PCB over at FSB when it was reversed engineered.