A beautiful sounding and hugely versatile Dual Multi-mode VCF:

Featuring two completely independent Vintage SEM style filters, with a central mixing bus for both audio and frequency control voltages which allow the two VCF's to be combined in both serial and parallel modes, and VCF1 can be inverted against VCF2. So the two VCF's can be combined to behave as a 24dB SEM filter, vocal formant filter and much more!

Each of the two filters are based on the vintage SEM filter, with FET transistor buffers and feature Low Pass, Band Pass and High Pass outputs and a useful Vary output which can be faded from LP to Notch to HP. All outputs can be used simultaneously.

We have added some exciting extra features unavailable on the original SEM filter, including CV contol of Resonance, overdriving of the VCF core and full self oscillation!

VCF1 and VCF2 each have a four way switch to select LP, BP, HP or VARY to the mixing bus, this is also available as an extra individual output.

The two independant filters can be combined in three different ways:

Serial (Cascade) Mode

The mode toggle switch should be set to CASCADE position to select this mode.This connects the OUT 1 output from filter one to the IN 2 input of filter 2, and both filter one and two outputs are routed to the inbuilt cross-fader, the output of which is routed to the D-OUT output.So the signal is first treated by filter one, then this filtered signal is fed for further filtering by filter two.The four way rotary switches select the filter type for each filter signal sent to the Cross Fader, and the AUDIO MIX contol allows crossfading between the filter one and filter two outputs, so in this case we are cross-fading between one and two filters. So if, for example, both filter one and filter two are set to Low Pass (LP) mode, then the AUDI MIX contol will fade between 12dB and 24dB Low Pass filter slopes.

As another example, selecting LP filter for filter one and HP filtering for filter two will combine to give a Band Pass responce, but the frequency and resonance for each component can be individually controlled. This is quite different from using the BP setting on one of the individual filters, as it doubles the BP filter slope from 6dB to 12dB, but allows much more control over the BP characteristics too, andin this case the AUDIO MIX control would fade between LP (filter one) and BP (filter one and two in series). And of course any filter type from filter one and filter two can be combined.

Parallel (Dual) Mode

The mode toggle switch should be set to CASCADE position, and the INVERT switch set to VCF2 position to select this mode.This internally connects the output of filter one to the VCF 1 crossfader input, and the output of filter two to the VCF 2 crossfader input, so the crossfader output D-OUT is a mix of filter one and two. With the AUDIO MIX contol at the 12 'oclock position a 50/50 mix of filter one and filter two will be heard, but if the AuDIO Mix is turned to zero (VCF 1) position then only the output of filter one will be heard, and if turned to the VCF2 position then only VCF2 will be heard, and as the AUDIO MIX control can be voltage controlled we can automate cross-fading between the two filters. As an example, using the four way rotary switchs we could set VCF1 to LP mode and VCF2 to HP mode, then manually (with the AUDI MIX control) cross fade from a LP to HP filter type, or we could use a 0 to +5V or higher contol voltage to do this automatically under voltage control. We could even use the vary outputs for both VCF1 and VCF2 to feed the crossfader, so it is possible to continually vary the invididual filter type at the same time as cross fading between filters one and two!

INVERTED Parallel (Dual) Mode

The mode toggle switch should be set to CASCADE position, and the INVERT switch set to INVERT position to select this mode.This is the same as Parallel mode (above) however the output of VCF2 is inverted before being sent to the crossfader VCF2 input, so VCF 2 is inverted against VCF1, which causes phase cancellations between the two filters when they are mixed together..