Sample MP3sFilters modulated pulse input.
The MFOS Quad Timbre Bank began as a fixed filter bank but evolved into more. The board houses four 12dB/Oct resonant VCFs and four VCAs. Front panel controls for each VCF/VCA pair include cut-off frequency, resonance and output level. As shown here the board routes the input signal into four parallel VCF-VCA circuits. I liked the filter from the Echo Rockit so much that I used the design in this module. The exception is the filter capable of high-pass, lo-pass, and band pass. Another op amp was needed in that circuit to realize the highpass function. The low pass output is available from this filter as well on the PC board (at circuit point X37). In addition the cut-off frequency of each VCF and the output level of each VCA is able to be modulated independently with control voltage. The external CV modulation is linear in nature and not logarithmic so the filters are not intended to be 1V/Oct tone generators in sine oscillation mode.
Resonance can be increased to the point of oscillation. The level controls permit distortion at higher settings for unusual tonal generation. The cleanest sine waves are output when the BAND/LO-Pass filters are in low pass mode and in high pass mode for the fourth filter. In band pass mode all filter's output slightly distorted sine waves.
With some imagination the VCFs and VCAs can be routed in a variety of ways (independent, serial, par/ser) If I find myself with the time to experiment with this I'll document it here. In the mean time I hope benders and technologists out there give it a try.
The unit can be constructed without including the VCF and VCA control voltage input jacks if fixed filter bank functionality alone is desired.
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