Input is a Eurorack module designed for pre‑amplifying audio signals from instruments such as guitars, microphones, and more.
This module is an old, unreleased project.
Its design is straightforward: two different‑sized mono jack sockets (3.5 mm and 6.35 mm) accept audio signals from various instruments or microphones, while a potentiometer sets the amplification gain – adjustable from 5 up to 130. Input impedance is 1 MΩ.
The resulting low‑impedance, amplified signal is available at the 3.5 mm output jack.
Circuit description
The electronic circuit itself is simple: two op‑amps are connected in series, both in non‑inverting configuration. This topology provides a very high input impedance.
The first stage has a fixed gain of 5, while the second stage, controlled by the Gain potentiometer, can vary between 1 and 26, resulting in a total gain adjustable from 5 to 130.
The input is protected against overvoltages by two clipping diodes connected to the power rails. A resistor to ground sets the input impedance to 1 MΩ, and a simple low‑pass filter (a series resistor with a capacitor to ground) completes the input circuitry.
Input 1 is normalled to input 2, meaning that when no jack is plugged into input 1, the signal from input 2 is routed to input 1.
On the output side, a low‑value series resistor protects the final op‑amp stage from unwanted incoming currents. A capacitor in the feedback loop prevents parasitic oscillations when long cables are used.