frequency-central-system-x-filter

Frequency Central System X Filter

Roland System 100M 4-pole VCF clone with simultaneous 12dB and 24dB outputs

10 HP Eurorack module

The System X Filter is a Eurorack clone of the Roland System 100M VCF (model 121), built by UK DIY house Frequency Central as a kit module. The classic 24 dB/octave OTA-cascade lowpass — originally implemented in the System 100M with four CA3080 OTAs and FET buffers — is reproduced faithfully, with a 12 dB/octave tap added on the second pole that is 180° out of phase with the 24 dB output. Two audio inputs each have their own LEVEL attenuator and sum into the filter core; two CV inputs each have their own attenuator and sum into the cutoff control along with the FREQ knob. RES sweeps from clean lowpass through resonant peaks into self-oscillation at very high settings.

Features

  • Roland System 100M VCF (model 121) topology in 10HP Eurorack
  • Original revision uses 4x CA3080 OTAs with FET buffers (Roland-faithful); v2.0 uses 2x LM13700
  • Simultaneous 12 dB/oct (2-pole) and 24 dB/oct (4-pole) lowpass outputs
  • 12 dB output is 180° phase-inverted relative to 24 dB output
  • Two audio inputs each with its own LEVEL attenuator and pre-filter sum
  • Two cutoff CV inputs each with its own attenuator and pre-filter sum
  • Self-oscillation at high RES — usable as a tracking sine source
  • Build doc includes optional modifications for highpass / bandpass alternatives on the 12 dB tap
  • DIY kit form factor (now discontinued — replaced by XVCF TWO)
  • Available also as PCB+panel set or PCBs only for builders
  • 10HP, 65mm depth, classic Roland-style sound

Controls & Jacks (12)

  • Level 1
  • Audio In 1
  • Level 2
  • Audio In 2
  • 12 dB/Oct Out
  • Freq
  • 24 dB/Oct Out
  • Res
  • CV 1 In
  • CV 1
  • CV 2 In
  • CV 2

How-To Guides

  • Classic Roland 100M lowpass voice
  • Self-oscillating sine source
  • Stack two VCOs into one filter
  • Phase-difference pad from both outputs
  • LFO wobble with envelope keytracking

Explore the interactive System X Filter panel on BleepDex — tap any control to learn what it does.

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