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CrossOver Filter [Archive]


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Introducing the CrossOver Filter - a smartly configured pair of Resonant State-Variable Filters with Voltage Control.

The driving forces behind the design of the COFilter were two-fold - firstly I wanted to bring the filtering of the PRC3A SV-Filter out of the old 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. Once I began thinking how I would (as always) update the PRC3A design, the idea of two SV-Filters one-after-the-other (ie. in series) swung into view and, after only a few prototypes, I arrived at the design presented here.
The COFilter consists of the following parts (see Block Diagram image above):
- Input Preamp
- Voltage Controlled Resonant Filters
- Output Mixing

The mono (unbalanced) 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 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 (though, note, they are not temperature compensated).
It is the chaining arrangement of the two filters that sets the CrossOver apart from other filter designs. In Series mode (which could be seen as the 'normal' CrossOver mode) the 2nd filter takes its input from the HighPass output of the 1st filter - this means that the resulting Mid band has its low-side controlled by Cutoff 1 and its high-side controlled by Cutoff 2 (alongside the Low band output coming from Filter 1 and the High band coming from Filter 2). Switching to Parallel mode means that both filters take the same signal direct from the Preamp - in this mode, the Mid band is actually a 2nd Low-pass, the same as the main Low band output, but independently controlled.

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:
- in Series mode, as both filters cover the full audio spectrum, you generally want to make sure that Cutoff 2 is set beyond the position of Cutoff 1. A typical starting point may have Cutoff 1 fully counter-clockwise and Cutoff 2 fully clockwise.
- if you set the Low/High points for the Mid band in Series mode, then switching to Parallel mode will 'knock out' the low cut of Filter 1.
- mixing signals 'in phase' will tend to reinforce resonant peaks, while inverting a band can generate somewhat hollow sounds.
- if you pan the different outputs, the sounds will move in the stereo field as you alter the cutoffs. (I have a hunch that stereo bass-frequency phasing may not be great for vinyl cutting..).
- note that the nature of the State-Variable filters means that you will still get a little audio pass-through when fully filtering (eg. Low band output with Cutoff1 fully down, or High band output with Cutoff 2 fully up).
- for accurate tracking, ensure adequate warm-up time - scaling, if required, is adjusted by trimmers inside on the main circuit board.
- for details on interfacing with the 4mm banana sockets, check the instructions or consider the minijack-to-banana interface cables I offer.

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, but it also has three expansion headers on the back of the main circuit-board which can hook to the optional 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 an extra 1V/Oct CV input. This is an entirely passive addition (no extra power required) but is currently not available in standalone form.

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 500mA, 2.1mm centre positive) to the bipolar +/-15V internal power. A 90-250VAC worldwide power pack is included with interchangeable plug socket adaptors.

The COFilter is covered by a 2 year parts warranty.
Price - £250 (+VAT in EU - £300) + shipping

Demo Sounds: Best enjoyed on a decent STEREO system - for further details of what you're hearing, click through to SoundCloud.

Added on Monday 21 July, 2014.