Doepfer A-101-6 Opto FET VCF

A-101-6 is a new filter module that uses so-called "Opto-FETs" to control the frequency. Opto-Fets are similar in structure to Vactrols , but use is made of photosensitive field-effect transistors ( FETs ) instead of photosensitive resistors (LDRs). Opto-FETs incorporate a photosensitive FET and a light emitting diode in a common, light-tight housing. The advantage over Vactrols is that Opto-FETs are much faster than LDRs and thus very short attacks / decays or FM effects are possible.