Discrete Core Ladder VCF

Transistor ladder VCF

The VCF ladder design consists of 4 stages, providing a buffered output for each of the 4 stages separately in order to get a 6dB, 12dB, 18dB and 24dB/octave -sloped filter outputs individually. (The outputs at different stages then could be mixed together using a polarizing mixer to achieve different filter slopes and even different bandpass behaviors.) 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 get 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 attenuverted by the knob named „RESONANCE CV“. When resonance is turned up fully, self oscillation will occur, enabling use of the VCF as a voltage controlled 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 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 behavior 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 added 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 get higher resonance settings and still keep
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-feedthrough quite low.

http://audiophilecircuitsleague.com/products/discrete-core-ladder-vcf.php