Doepfer A-101-6

A-101-6 is a new filter module that uses so-called opto FETs to control the filter frequency. Opto FETs are very similar to Vactrols but use light depending field effect transistors (FETs) instead of light depending resistors (LDRs). A opto FET is a combination of a light depending FET and a LED (light emitting diode) both put into a small light-proof case. The advantage compared to vactrols is the much faster response of opto FETs compared to LDRs. This allows much faster attack/decay times and even FM effects. The disadvantage compared to vactrols is that the FET behaves as a normal Ohm resistor only for small levels. With higher levels the FET begins to distort. Apart from this the notes mentioned on the Vactrol Basics page are valid. Module A-101-6 is made of six serial 6dB filter stages. Each stage can work as lowpass, highpass or one of two allpass types. The following schematics show one of the six serial stages in the four possible modes: