A simple circuit with an optocoupler creates a “tube” sound

This simple scheme is based on the circuit shown below (Figure 1) in my recent article [1].

Figure 1 Using common transistor optocoupler to achieve a tube-like sound.

It is known that the main feature of tube amplifiers is the presence of small distortions. In these distortions, the second harmonic prevails. Some optocouplers have a pass-through characteristic close to the pass-through characteristic of the electron tube.

An improved circuit with an optocoupler type TLP621 is shown in Figure 2.

Figure 2 A circuit with an optocoupler (TLP621) which generates higher (even) harmonics that can be added to the original signal to achieve a tube-like sound.

Transistor Q1 creates a current through the optocoupler U1 LED corresponding to the input signal. The collector of the optocoupler phototransistor is connected to the emitter repeater on transistor Q2 and the signal from it is fed to the output of the circuit. The circuit parameters depend on the parameters of a particular optocoupler instance.

To adjust the gain, R1 (3-8 kΩ) is selected, and R3 is selected for the current of the operating point of the optocoupler 1. The value of this current is usually 0.670-0.750 mA.

With an input signal of about 200 mV RMS, the output signal spectrum has the one shown in Figure 3.

Figure 3 Output spectrum of the optocoupler circuit in Figure 2 with an input signal of 200 mV RMS.

As one can see, the second harmonic of a 5 to 15 W tube amplifier dominates the spectrum, and EL34 type lamps are used in the output stage.

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Reference

Garcia, Robert. “Tube sound: about the benefits of the second harmonic.”habr. 4 Nov. 2022, https://habr.com/ru/articles/697508/

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