KNOBS
Gain A - Volume control for channel A vintage VCA. Gain A also affects the
saturation/distortion of the signal through the Tube circuit.
Gain B - Volume control for channel B vintage VCA. Gain B also affects the
saturation/distortion of the signal through the Tube circuit.
Tube - Tube bias voltage control. When L8 is switched Off, it is the bias for just PIN 3; when
L8 is switched On, it is the bias for PIN 3 & PIN 8
Drive - Adds more gain to the mixed signal A+B (+C+D with Curiouser) by an operational
amplifier (op-amp) before the tube stage. It has a maximum gain of x 32. Medium settings
can cause the op-amp to clip, and higher settings cause asymmetrical wave folding. The
hard-clipping of the op-amp is smoothed out by the Tube stage, which follows. Drive should
be used in tandem with the Gain A & Gain B to achieve the required amount of
saturation/distortion. In practice, Drive sets the upper limit of saturation/distortion of the
Tube circuit, and Gain A & Gain B are the finer CV-able control of the
Drive/saturation/distortion.
FB - This is a VCA placed in the feedback path. This control works in tandem with FB switch,
which needs to be switched on (down). It has two functions, feedback duties and tuning
when the module is used as a VCO.
• FB knob has an interesting range of effects over the timbre and is highly interactive
with other settings.
• Inverting the Output attenuverter’s polarity (ACW from the mid-point) has a
significant effect on the Feedback timbre.
Tone – This is an overall tone shaping circuit for the ‘wet’ Tube signal. After the Tube circuit,
the signal is split and passes in parallel through an active 2-pole low pass and high pass
filters. These are then recombined through a crossfader. Tone is the crossfader control for
blending between the high passed and low passed signals. (For further info, see TRIMMERS)
Mix – After the two vintage VCA channels are mixed and before the Tube stage, the signal is
split into Dry and Wet. Wet then goes through the Tube and Tone stages; Dry goes direct to
Mix. At this point, the signals are recombined through the Mix crossfader, blending
between Dry and Wet signals. It should be noted that Dry is exactly the signal that arrived at
the inputs. The vintage VCAs impart some colour on the sound, and the Gain of the VCAs
will obviously affect the amplitude of the Dry signal at Mix. (For further info, see
TRIMMERS)
Out – Attenuverter for more flexibility. This is perfect when using Rabbit Hole as a drum
voice mixer and plan to mix with a bassline. The phase of Rabbit Hole’s output can be
flipped to fix kick/bass phase issues. Additionally, this has an effect on the timbre when
using the FB feedback circuit. Clockwise from 12 o’clock, the output signal is in the same
phase as the input signal. Anticlockwise from 12 o’clock, the output signal phase is rotated
180o with respect to the input signal.
