Six Stage Opto FET Voltage Controlled Filter (multi mode)
The A-101-6 is a new filter module that uses 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). An 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 resistor only for small levels. With higher levels the FET begins to distort.
The A-101-6 is made of six serial 6dB filter stages. Each stage can work as lowpass, highpass or one of two allpass types. These modes can be set with jumpers on the pcb (set to Low Pass by default).
The brightness of the Opto FET LEDs and consequently the filter frequency can be adjusted manually (Frequ. control) and controlled by means of an external control voltage (CV) with attenuator. The LED at the front panel reflects the LED brightness inside the opto FETs.
The resonance is controlled by the Feedback control up to self oscillation. By means of a trimming potentiometer the maximal feedback can be adjusted. High feedback values can be used mainly in the allpass mode to obtain very extreme self oscillation sounds.
External feedback signal can be used instead of the internal feedback (FB In socket). The Mix control is used to pan between the original and effect signal. In filter mode (LP/HP) this control is usually set fully CW. In the allpass modes phasing sounds are achieved at the centre position or "pure" allpass sound in fully CW position.