February 25, 2018

The 24dB Filter/VCA

This is a 4-pole (24dB) multi mode VCF with an unusual topology in that it doesn't have separate outputs for its different responses (LP, HP and BP), but instead it has three inputs and one mixed output. The three inputs are placed upfront of the filters and each input has a manual level control that determines the amount of signal that goes into each filter. 
The 24dB Filter has a 24dB lowpass slope, a 24dB highpass and the bandpass slope is 6dB highpass and 18dB lowpass.

As the input jacks are normalized, the filter can also be used to create a variable slope for one input signal. This way one can dial in specific responses by setting the levels of the respective inputs. As when a signal is connected to only the LP input the normalization passes the signal on to the next HP and BP inputs. But, one could also input a signal to the LP input, another signal to the HP input and a third signal to the BP input. So the filter can be used as a three input mixer with separate level controls. The output would then be a blend of the first signal treated by the LP filter, the second signal treated by the HP filter and the third signal treated by the BP filter. For example, setting the LP knob to 10 and the HP knob to 7 gives you a resonant notch filter with more emphasis on the lowpass section.

One can crossfade one signal to an other with LP input & HP input. When the LP and HP inputs are used for the same signal they can pass this signal unaltered at specific mixer settings, as the LP and HP slope are fully complementary, in this case it will even suppress the resonant peak, although that peak will be present on the BP input signal.

The LP, HP & BP inputs are DC-coupled so that also CV signals can be processed and it means it can also be used as a slew limiter when you feed it CV. With high resonance, it then imparts very interesting behavior on the resulting control voltage!

Both Cutoff frequency and resonance are under manual and CV control. There's a 1V/Oct CV input to the Cutoff frequency and two supplementary FM inputs with attenuators. The filter maintains a steady perceived volume level at all resonance settings. This filter will not go into self oscillation. 

Mod II is the level input knob of the second cutoff modulation input. If there is no jack in the second modulation input opening this knob will introduce a smooth, subtle and very natural distortion effect on the resonant peak that is similar to tube distortion (all harmonic distortion). When a jack is connected it sets the amount of cutoff modulation. Often the first cutoff modulation input and level knob are used to control the tracking of the filter to a keyboard, and then the second modulation input can set the level of e.g. a filter sweep.

The filter reacts exceptionally well on audio rate modulations signals, which can add a further range of new and interesting timbres to the already quite versatile module.

Next to the main filter output there's also a VCA output for amplitude modulation of the filter (unattenuated CV input to the VCA and no control over initial gain of the VCA). The VCA has an exponential curve.

You can use this filter to get some really interesting slowly evolving waveforms. Example: patch a fat PW modulated square wave into the LP input, and a triangle wave through a slowly sweeping wavefolder into the HP input. Now sweep the filter cutoff with a mixture of very slow sine LFO plus an audio rate sine that is just slightly detuned from the two audio input waveforms... Bizarre spectral 'crossfading' from one timbre to the next, shifting from the PWM osc to the wavefolded triangle, and producing complex combinations at the points in between...

Here is the flowchart of the 24dB Filter/VCA :

Here are some videos of the module. The second one at Novars workshop greatly explains how the filter is designed.

Thanks again to Todd for providing the picture of the module.

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