Oakley 3U User Manual - page 12
The tone control is based around the classic TS and SD-1 design and is built around U3B. Its
a simple circuit that works well in practice and produces enough tonal variation to be very
useful. At the lowest setting of the Tone pot, called 'timbre' on the suggested front panel, C1
acts in conjunction with R10 to produce a single pole low pass filter. At the high end of the
tone pot's travel, C1 now mostly acts in conjunction with the feedback resistor R9 and R5 and
produces a shelving high pass filter. This boosts high frequencies and produces a much
sharper sound. This is particularly apparent in the 'distortion' mode since this part of the
circuit produces much more higher harmonics due to the more severe clipping affect.
The output of the tone control stage is fed to the other end of the balance pot, called 'mix out'
on the front panel. This is a simple arrangement that lets the user apportion the mix of
distorted and straight-through signal. The relatively high impedance output of the wiper of
balance pot would not be able to drive any other external modules direct, so it is buffered and
amplified by a simple inverting amplifier U2B. R14 provides protection and stability for the
output signal.
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