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DIY Synth Kit Page 20 User's Guide

How to use the summing inputs

Each voltage controlled parameter (e.g. VCO frequency or VCF frequency) has at least one

control voltage (CV) input available. If the number of CV inputs is not sufficient more inputs

can be added. For this we have to take a look at the input circuit used for all types of control

voltage summing applications. It's nothing but the standard inverting operational amplifier

circuit:

The voltage appearing at the output is:

- Uout = U1 (Rx/R1) + U2 (Rx/R2) + U3 (Rx/R3) + …

This circuit can be used to add several voltages (U1, U2, U3 …) with selectable sensitivity

(defined by R1, R2, R3 …). Provided that all resistors have the same value (e.g. 100k) the

output voltage is nothing but

Uout = -(U1 + U2 + U3 + …)

This type of circuit is used whenever control voltages or audio signals have to be added (or

mixed) in the DIY Synthesizer. In addition the summing node (i.e. the inverting input of the

operational amplifier) is available as a separate terminal. This allows to expand the number

of CV or audio inputs if the available inputs are not sufficient. For this the additional input has

to be connected to the summing node via a suitable serial resistor (R4, R5 …). Do not

connect a voltage directly to the summing node ! This may destroy the operational amplifer.

In the tables JP2…JP7 that explain the functions of the pin headers the value of the required

serial resistor is mentioned. If the input in question has to be more sensitive the value of the

serial resistor has to be reduced (e.g. 51k or 47k instead of 100k for doubling the sensitivity).

If the input in question has to be less sensitive the value of the serial resistor has to be

increased (e.g. 1M instead on 100k). A typical application are several tuning inputs for the

VCO frequency. A course tune control may be connected e.g. via 100k to the summing node,

a fine tune control via 2M2. In this case the fine tune is about 20 times more sensitive than

the coarse control.

Uout

R2U2

R3U3

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DOEPFER A-100CGK Guía de usuario Pagina 20