VCV Rack Modules

CF100

The CF100 is a highly detailed analog model of the classic cascade filter found in the early Prophet synthesisers which used the SSM2040 filter chips. This model is based off the Discrete OTA schematic created by Jürgen Haible, with some extras added including multiple filters responses, and extra input and output saturation options. It runs without internal oversampling over the full range of frequencies. Oversampling can be optionally done using VCV Racks sample rate settings, and is recommended for patches where large amounts of drive or stronger sounding high frequency resonance is required.

Input Controls:

• DRIVE (0 to 10) – Exponential input gain amount from -inf dB up to +12 dB. Voltage controlled from the DRIVE CV input port.
• FREQ (8hz to 30khz) – Exponential cutoff frequency of the filter. Can be controlled by the 1V/Oct, FM1, and FM2 CV input ports.
• RES (0 to 10) – Linear resonance control, self oscillates at around 7.5. Voltage controlled from the RES CV input ports.
• DRV Mod Depth (0 to 10 CV depth)
• FM1 Mod Depth (0 to 10 CV depth)
• FM2 Mod Depth (-5 to 5 CV depth)
• RES Mod Depth (0 to 10 CV depth)

Input Ports:

• DRIVE (CV in)
• FM1 (CV in)
• 1V/OCT (CV in)
• FM2 (CV in)
• RES (CV in)
• IN (audio in)

Output Ports:

• NPN2 (audio out) – twin 2 pole notch filter, with one notch an octave above the cutoff, and another an octave below. This response has lots of phase shift and sounds a bit like a phaser.
• HP2 (audio out) – 2 pole high pass
• BP4 (audio out) – 4 pole band pass, with a 2 pole high pass and two pole low pass both centred at the cutoff.
• LP2 (audio out) – 2 pole low pass, with lots of character and drive
• LP4 (audio out) – 4 pole low pass, with tamed and deeper character and drive

Right click options:

• Boost Input – Off, +6dB, and +12 dB. This changes the input level just like moving the DRIVE knob but allows for extra gain
• Boost Output – Off, +6 dB, and +12 dB. This places gain between the output of each of the filters stages and an Op-Amp that clips to the voltage rails. Increase this for post filter distortion.
• Circuit – AC JH, DC JH, DC Datasheet. The AC version is for audio, the DC versions can be used for either CV or Audio. The JH is the original CF100, and the Datasheet version is based on the schematic shown in the SSI2140 datasheet.
• Noise Level – Off, Input Only, 50%, 100%, 150%, 200%. Analog modelled thermal hiss. 100% is true to the analog circuit.
• PNP BF – 40, 400, 4k. Forward beta value of the PNP transistors used in the circuit. 40 gives more 2nd order harmonics, 400 is the default, and 4k reduces 2nd order harmonics.
• Oversampling – Off, x2, x4. The oversampling multiplier to increase the sample rate used by the module internally, which reduces aliasing and gives stronger resonance
• Knob LED Brightness – Off, 25%, 50%, 75%, 100%

Credits:

• User interface design: Pyer
• Original JH circuit design: Jürgen Haible
• SSI2140 Datasheet circuit design: Sound Semiconductor
• Analog modelling, design input: Andrew Simper