The CrossOver Filter is a smartly configured pair of Resonant State-Variable Filters with Voltage Control. The playable design presents a powerful little toolbox for creatively transforming audio signals!
VIDEO
The driving forces behind the design of the COFilter were two-fold – firstly I wanted to bring the filtering out of the Modular range so that I could make use of it more easily in the studio. But I’d also been wondering about re-imagining the Cross-Over units that often sit in PA racks to split audio into separate frequency bands – these always tended to be a bit limited/boring so how exciting it would be to make a playable and powerful design for studio usage which took a mono signal and split it into several frequency bands for further processing/panning.
The circuitry of the COFilter was completely redesigned in 2019 – adding balanced input, voltage-controlled resonance and upgrading the core components. From the outside, the new version appears almost identical to the old (the only clear difference is the Drive control – now with black rather than white line). A new 2FW Expander will follow along offering individual I/O for the Filters and further CV inputs for Frequency & Resonance, with a distinct view to allow dual use for stereo processing work.
The COFilter consists of the following parts (see Block Diagram image above):
The mono (electronically balanced) line level 1/4″ input is buffered before passing through a variable gain Preamp with clean gain up to +26dB. You will generally be applying some gain to bring signals up to roughly match the self-oscillation levels of the filters.
The two State-Variable Filters (2-pole, 12dB/Oct) are identical but have their Inputs and Outputs arranged differently. Each covers the full audio range (approx. 20Hz to 20kHz) and resonance (Q) is variable up to self-oscillation at which point a pure sine-wave is produced. Each filter has a CV Summer which mixes the large black Cutoff control with any Control Voltage modulation via the polarizing FMod control – modulation is switchable between an External CV source via the blue 4mm Banana socket or Internal Self-Modulation from the filter’s Band-Pass output (this gives some fairly chaotic audio modulation possibilities!). The filters are calibrated to accurately track 1V/Oct over 5+ Octaves.
The Output conditioning and mixing stages add further flexibility to the COFilter. Filtering can alter both the amplitude and phase relationship of signals and, when mixing the signals (and perhaps a dry feed via an external mixer), different effects can be achieved by inverting the phase of signals. Each output band has a polarity switch which also allows quick muting (centre-off) which is followed by a Level control before the individual 1/4″ Output (Impedance balanced). The three bands can either be taken out to individual destinations or can be internally re-mixed via normalisation on the Low and Mid outputs – when nothing is plugged in to either one, then the signal passes on to mix with the High output.
Some notes and ideas for usage – see also the INSTRUCTIONS : mini-jack-to-banana cable .
While the COFilter is presented as a self-contained standalone device, it is designed based on my experience building Modular systems – not only is it really a ‘module’ which could fit into a different frame for use in a larger system setup. It also has two expansion headers on the back of the main circuit-board which can hook to the optional/forthcoming Crossover Expander panel – this opens up the filters, presenting an extra Audio Input per filter, along with the individual filter responses (Low/Band/High) and extra CV inputs for each filter’s Cutoff and Resonance.
The standalone COFilter is housed in a custom aluminium enclosure (4.5 x 5.25 x 3.5 inch) with PCB material front and side panels. The case holds a DC/DC converter which converts the 12VDC external supply (minimum 300mA, 2.1mm centre positive) to the bipolar +/-15V internal power. A 90-250VAC worldwide power pack is included with interchangeable plug socket adaptors.
