Changing the sensitivity of manual controls, control voltage inputs and audio inputs

P1 is the manual control of the corresponding parameter (e.g. tune for a VCO, frequency for a VCF, manual gain for a VCA, manual phase shift for a phaser and so on). P1 generates the voltage U1. J1 is the (first) input socket for the external control voltage. P2 is the corresponding attenuator. The slider of P2 outputs the voltage U2. Additional CV inputs with our without attenuators may be available (e.g. two or more CV inputs for frequency control for a VCF). The dashed line in the picture is the common point in the circuit where all CV's are added. The output voltage of the circuit (output of O1) is used to control the corresponding parameter (tune, filter frequency, gain ...) of the module in question. The output voltage is defined by: R3/R1 * U1 + R3/R2 * U2 The relations R3/R1 resp. R3/R2 determine the sensitivity of the corresponding control (P1) resp. input (J1/P2). If for example all resistors are 47k (a common value in the A-100) the sensitivity is 1 for each input. Provided that R3 remains unchanged the resistors R1 and R2 determine the sensitivity of the corresponding control resp. input. Reducing the resistance of R1 resp. R2 increases the sensitivity of the manual control (P1) resp. input (J1/P2). Increasing the resistance of R1 resp. R2 reduces the sensitivity. To modify the sensitivity of a control knob (P1) or CV input (J1/P2) the corresponding resistor R1 resp. R2 simply has to be changed. Changing the resistance of R3 has the opposite effect and affects the sensivity of both the manual control and CV input.

The audio input circuit for most A-100 modules is similar but the manual control P1 is absent (a DC offset would not make sense for an audio input, audio signals are AC signals). Normally only one audio input is available but there are exceptions (e.g. VCA A-130 and A-131, signal processor A-109). To change the sensitivity of an audio input simply the resistor R2 connected to the slider P2 of the audio input has to be replace. A smaller value will increase the sensitivity and consequently lead to clipping/distortion for higher input levels. Especially for the first A-100 VCFs and VCAs (A-120, A-121, A-122 and first versions of A-130, A-131) the audio inputs have been designed to avoid distortion with standard A-100 signals (e.g. VCO). Lowering the input resistors will allow distortion for these moduls too. Even the input resistors of CV or audio mixers (e.g. A-138a/b) can be changed to allow "real" amplification (i.e. > 1). The factory values of the resistors in the mixer modules A-138a/b allow a maximum amplification of about 1 (which is not really amplification). Reducing the input resistors (R2 type) or increasing the feedback resistor (R3 type) will increase the amplification of the circuit. The factory values of the corresponding resistors (R1, R2, R3) for all modules can be found in the A-100 service manual. Normally they are in the 100k range (~ 47k...220k).