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This is the first vactrol based module in the a-100 system. a vactrol is a combination of a light depending resistor (ldr) and a led (light emitting diode) both put into a small light-proof case. increasing the led brightness causes a decrease of the ldr resistance (e.g. the light controlled voltage module a-179 is an application of a ldr). this effect can be used in circuits that require variable resistors to obtain the desired function (e.g. vcfs, vcas, vc phasers, vc trigger delays, vc slew limiters, vc envelope generators, vc lfos, vcos and many more). the advantage of vactrols compared with other solutions (e.g. otas, diodes, transistors or fets as variable resistors) is that they behave like a normal ohm resistor and can handle high voltages without causing distortion. this is probably the reason why the sound of vactrol-based circuits usually is described as soft or mellow. disadvantages are the expenditure and consequently the expenses and the not exactly defined coherence between led current and ldr resistance. for each vactrol the ldr resistance decreases when the led current increases but mechanical and electrical tolerances cause that each vactrol will behave a bit different (e.g. because of the differences in alignment and distance between led and ldr, different brightness of the leds at the same current).