ACL Discrete core ladder VCF

Amazing filter, 3 outdoor inputs used for mixing! Intuitive design.

The VCF ladder design consists of 4 stages, providing an buffered output for each of the 4 stages separately in order to
get a 6dB, 12dB, 18dB and 24dB/octave -sloped filter output individually.
(The outputs at different stages then could be mixed together using a polarizing mixer to achieve different filter slopes
and even different bandpass behaviours.)
The cutoff frequency of the VCF can be controlled manually using the knob named „CUTOFF“ and by 3 independent
external control voltages, connected to the input jacks named „1V/OCT“, „CV1“ and/or „CV2“, while „CV1“ and „CV2“
inputs got an attenuator each (Knobs named „CV1“ and „CV2“).
The VCF´s resonance can be controlled manually and by an external CV connected to the „RESONANCE“ input and is
attenuverted by the knob named „RESONANCE CV“
When resonance is turned fully, up self oscillation will occur, enabling to use the VCF as an voltage controlled quadrature
VCO since each stage output signal is phase shifted by 90° to the previous stage.
Resonance is is provided by feeding back the output of the last stage to the input of the 1st stage.
A switch called „Q LIN“ switches between 3 different feedback settings, affecting the frequency linearity of the feedback
path:
In a very classic design of a lowpass adder VCF, the lower frequencies are damped more than higher frequencies,
generating a loss of self oscillation at low cutoff frequencies.
This behaviour can be achieved by switching „Q LIN“ to LOW“ or „MID“, assigning 2 different cutoff frequencies of high
pass filter in the feedback loop of the filter.
By switching „Q LIN“ to „HI“ the cutoff frequency of that high pass filter is lower than 1 Hz, enabling self oscillation even
down to the LFO range of frequencies,
The cutoff frequency is temperature stabilized and shows good tracking with 1V/Oct. to up to 5 octaves.
3 audio signals can be connected, being summed together to one signal in order to enter the VCF.
When approaching 8V PP, the ladder will start to saturate, creating a awesome creamy and compressed sound, which
will bend and evolve more and more to a very strongly saturated and distorted sound, the further the ladder gets
overdriven by the signal.
In many ladder designs, the lower frequencies gets more attenuated (bass loss), when increasing resonance,
approximating a bandpass behaviour at very high resonance settings.
We spent a knob called „BASS BOOST“ for restoring the low frequencies and boosting them at high resonance settings.
That way you can make very bombastic bass sounds, since the sound might got higher resonance settings and still kept
it´s bass fundament.
When not overdriven by input audio signals, the overall distortion of the fully discrete audio signal circuitry is quite low
due to the use of low distortion discrete buffers, enabling a very clean sine wave when self-oscillating no too strongly,
unlike some other designs.
The use of matched components made it possible to keep the CV-feedtrough quite low.