This simple circuit can support up to four string instruments with electrical pickups and works as a mixer, preamplifier, driver and distortioner all at the same time with adjustable gain.
As shown in the diagram, it is built around a single operational amplifier IC NE5534 (IC1) featuring low-noise, high-gain, low total harmonic distortion (THD) and the capability to drive 600-ohm loads. The circuit was originally designed for guitars and violins with magnetic pickups but proved to be useful for other string instruments and other types of pickups too.
The working of the circuit is simple. There are four input connectors (CON1 through CON4) for string instruments. The input resistance is around 1 kilo-ohm, adjustable mainly with the value of resistors R1, R2, R3, R4 and R5. This much input resistance is appropriate for most of the pickups but can be changed by changing the values of these resistors and capacitor C2.
The values of resistors R1, R2, R3 and R4 depend on the type of pickups used and can vary from 50 kilo-ohms to around 3 mega-ohms. Some active pickups will work with 10-kilo-ohm load resistance. There are magnetic pickups working with load resistance between 100 and 500 kilo-ohms. And finally there are ceramic and other pickups requiring load resistance of 1 mega-ohm and sometimes even more.
The bandwidth of the circuit depends mainly on the operational amplifier used. With NE5534, the bandwidth is larger than 20 kHz.
The total harmonic distortion of the unit with switch S1 open is very low and can be neglected. If switch S1 is open, the gain of IC1 could be regulated to approximately 27 with potentiometer VR1. When the gain is too high, IC1 will be overloaded and noticeable distortions will appear. Potentiometer VR1 can be adjusted to 100 kilo-ohms to achieve a gain of around 50.
The circuit has two outputs. Load-1 is non-regulated output and should be loaded with resistance greater than or equal to 600 ohms. Load-2 is regulated output and the regulation is achieved with potentiometer VR2. The load on this output is normally greater than 10 kilo-ohms but it will not be damaged if connected to a 600-ohm load.
Distortions are introduced when switch S1 is closed. The type of distortions depends on the type of diodes D1 and D2 and the position of the knob of potentiometer VR1. 1N4148 diodes (D1 and D2) can be replaced with 1N914 small power diodes.
The circuit works off a 9V battery. But you can replace it up to the maximum allowed voltage for IC1 and the other components. The supply current without input signals and load is typically below 10 mA. In practice, batteries are preferred but a well-regulated wall adaptor can also be used.
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect four 2-pin connectors (CON1 through CON4) at the front side for string instruments. Also connect two 2-pin connectors (Load-1 and Load-2) at the rear side of the cabinet. Connect the four string instruments to connectors CON1 through CON4. Connect appropriate headphones and other higher-resistance loads at Load-1 and Load-2, respectively.
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