Introduction:
This module is the fifth chapter of my Oberheim-SEM-Clone-2-SMD - activities, the VCF sub function, again exposed as stand-alone module. Again there was the challenge of finding out whether the VCF is clonable in SMT.
The TSEM VCF:
Yes, this feasibility study can be considered as success. I had to replace some components though:
- OTA CA3086 is LM13700
- All OpAmps are TL072
- N-Channel FET are MMBF4391
- All NPNs are BC847C
- All PNPs are BC857C
The original:
This VCF is basically a state variable filter. It creates highpass, bandpass and lowpass outputs in parallel. Following the original SEM my clone processes three input signal groups of two input signals (see schematic below). Each input group can morph between its input signals with a balance pot (in this stand alone version of my clone I use only one of them ("PULSE IN" <=> "RAMP IN")).
Each filter stage is implemented with a standard OTA cell.
An interesting aspect is the way the resonance function works: it works as damper, not as amplifier. That means that omitting the resonance circuitry means maximum of resonance!
Additionally the module processes three selectable modulation sources for voltage control.
Remark: The "NOTCH" filter function of the original SEM module (see above) is created by the VCA by addition of the high- and lowpass VCF outputs. My VCA clone of the SEM sub functions offers the same possibility with morphing between lowpass, notch and highpass.
The resonance adjustment is done manually and can't be voltage controlled.
Schematics:
The basic functions of the schema are the same as in the original. I added some enhancements though:
- Adjustable zero symmetry of the filter outputs (LP, BP and HP) for internal post processing (like being used in a complete SEM clone)
- Adjustable output amplitude of the filter outputs (LP, BP and HP) for internal post processing (like being used in a complete SEM clone)
Conclusion:
The SEM VCF is definitely clonable in SMT. Playing with resonance creates bitchy or smooth behavior (see demo video below).
TSEM VCF Hardware
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Offer: PCB of the TSEM VCF for Euro 25.00 | 
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YouTube
TSEM VCF Demo