Here’s a simple circuit you can build for your own low power home audio project. It only uses a NPN transistor rather than the most conventional OPAMP versions. It is simple and easy to build using an universal board. It can be powered by 9v or 12v single power supply.
Transistor can be any general purpose transistor such as the popular 2N2222 or equivalent. R6, R7, R8, R9, C4, C3, C5 and C6 sets the crossover frequency. The crossover frequency is calculated by formula f = 1/(2*pi*R*C). Those resistors and capacitor have same values. If you will calculate the crossover frequency of the circuit below, R = 10k and C=0.1u, frequency will be 159Hz which is close enough on the simulation result. Why not accurate? Because we didn’t consider the emitter resistor.
Simulation result
This time I used LTSpice for simulation instead of the simetrix that I usually use. It is a bit harder to learn than simetrix but at least it is completely free unlike simetrix which is only free for personal and educational use with limitations on the circuit size.
Kicad schematic diagram
