All about it
Guess we really don't like using capacitors in our filters. Not the conventional kind anyways.....
Turns out that if you connect a diode backwards, it becomes a capacitor whose small-signal capacitance is (more or less) exponentially controlled by a biasing voltage. These "varicaps" or "varactors" have been used for electronic radio tuning for quite a while, but it's the now-obsolete ones designed for AM radio tuners that are the most interesting, because they have a relatively large capacitance AND a relatively large tuning range-over 4 octaves of tuning. And they actually sound great in a 4-pole lowpass filter!
Taming a device which needs direct CV application of up to 11 volts through the audio signal path was no mean feat, but we did it! We also added a range switch so the filter can cover frequencies up to 5kHz (about the same total tuning range as a Korg Delta VCF)
The capacitor tuning can respond to very fast signals, and has plenty of interaction between the tuning and the signal, for fun, funky FM.
Control range tuning is accomplished by the illuminated pushbutton in the centre of the panel controlling a digital 4-pole switch, hmmm, now there's an idea....
Temperature stability of the control path is non-linear, so this module should not be relied on for creating sinewaves in precise intonation...
+5V: Not required
Eurorack standard, 10HP width
38mm depth below mounting rails with connected power cable
In development-watch this space
Watch this space