Diode Superladder Variable Slope VCF - 1U (Issue 2)

Discrete four stage diode ladder low pass and band pass filter

This latest version of the classic Oakley Superladder voltage controlled filter (VCF) has been re-designed from the ground up to sound good using diodes. The circuit is considerably different from all previous designs and has its own tonal characteristics. It features a new switch to select the number of diode rungs in the ladder. The user can now choose between four and five pole filtering - with the five pole mode giving a steeper roll off and smoother tone.

The Diode Superladder VCF features both voltage control over frequency and resonance. Unique to any diode ladder filter is that it has a variable shape output. This incorporates a feature first seen on the Oakley Orbital prototypes. A single pot allows you to vary the output from one (or two) pole low pass to four (or five) pole low pass via band pass in the middle.

The Diode Superladder's output is what we call 'Q compensated'. This means that the output volume will not drop significantly when you turn up the resonance pot. This design uses two inbuilt VCAs to achieve voltage controlled resonance without the hassle. At high levels of resonance the Diode Superladder will self-oscillate across most of the audio band and thus can be used as a sinewave voltage controlled oscillator.

The filter core uses transistors wired as diodes which means it does not clip the signal abruptly but saturates smoothly. This allows the Diode Superladder to sound very different depending on the input signal level. When used with signals below 2V peak the Superladder will sound clean and bright. However, take your signal levels above 2V peak and it will start to take on a more strident tone particularly at higher levels of resonance. Coupled with the variable slope control, this sensitivity to input level means the Diode Superladder can produce a wonderful array of different tones.

The VCF may be built as a 1U filter core or as a complete 3U filter system. In the 1U 'Filter Core' format the module can be used as a filter module on its own. Users need to make use of external mixers to control CV and audio levels going into the filter. In this way, you will be able to have a collection of space saving 1U filter cores that can be used with any generic mixer module. The Oakley Multimix is an ideal choice for a handy mixer module.

http://www.oakleysound.com/super-d.htm