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1. INTRODUCTION
Audiophile Circuits League . -Discrete Core Ladder VCF is a classical transistor
ladder design.A fully discrete, low noise, low distortion audio path is used in the
whole circuitry.
The Discrete Core Ladder VCF 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 �ilter output individually.
(The outputs at different stages then could be mixed together using a polarizing
mixer to achieve different �ilter slopes and types.)
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 attenuated by the knob named
"RESONANCE CV". When resonance is turned fully, up self oscillation will occur,
enabling to use the VCF as an quadrature VCO since each stage output signal is
phase shifted by 90° to the previous stage.
Resonance 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 2 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 resonance and therefore no self oscillation at low cutoff
frequencies.
By switching "Q LIN" to "--", the resonance will not depend on the cutoff setting,
so the cutoff setting will have no in�luence on the Resonance setting of the �ilter.
Self oscillation is possible down to frequencies lower than 1 Hz.
If "Q LIN" is switched to "/--", the resonance will be decreased, if the cutoff
frequency is decreased, so at very low cutoff-frequencies, no self-oscillation is
achievable.
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 VCF 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 "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.
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