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Voltage Controlled Filter






MGB - Ladder Filter



2 January 2004




Once the MGB project grew from companion circuits to a companion synthesizer, the ability to implement duophony, or independent Note Assignment for each of the synthesizers was one obvious side effect.

To ensure maximum independence of the companion synth, an independant Voltage Controlled Filter was required for the MGB oscillators.

In keeping with the conceptual MG-1 model, the MGB VCF implements as a Moog-type Ladder Filter.  While ladder filter designs abound upon the web, this specific design diverges from the MG-1 (and many alternative designs) as follows:
  • DC Coupling:

    • No coupling capacitors exist within the audio path.  This is done to extend the bass response and provide generality of use, even for DC signals;


  • Constant Volume regardless of Resonance:

    • Typically increasing Resonance causes a reduction of audio amplitude except at the resonant frequency.  By summing the Resonance circuit with the main filter output, audio level remains the same.


  • Constant Timbrality:

    • Ladder filters typically do not track well over a large frequency range.  Additionally, Resonance that does not cause self-oscillation at low frequencies, can self oscillate at high frequencies.  By feeding a portion of the frequency voltage to the Resonance VCA (after inversion) the affect of resonance can stay very constant across frequency ranges.  The inclusion of a high frequency tracking adjustment, careful exponential generator design, and Resonance "leveling" provide for a resonable Constant Timbrality.


The physical design of the MGB VCF includes two VCA sections.  One of these sections provides for Voltage Controlled Resonance, the alternate section performs as the main or Final VCA.

MGB VCF-VCA Implementation

The Final Audio Output VCA and the Voltage Controlled Resonance implement using a SSM2164 integrated circuit. MGB VCF/VCA


Resonance and Output VCA control voltage response is linear thanks to Mike Irwin's "servo'd linearizer".

The figure left indicates the block level implementation of the VCF/VCA combination.  As shown, patching is provided to allow the output of the VCF to be combined with the MG-1 VCF in addition or series configuration.

Unfiltered Audio and the output of the VCF and feed to alternate inputs of a Differential amplifier.   This results in a Hi-Pass characteristic as an alternative to the typical Low-Pass output.  This increases flexibility for the filter alone or in combination with the existing MG-1 filter.



MCB VCF/VCA Schematic
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