[Ynze] has built an audio ... the amplifier itself. [Ynze] is using a large toroid transformer to drop his local 230V mains down to +25V and -25V. The amplifier circuit itself is simple ...

but again it’s a simple circuit to guess a resistor value for. A 1 kΩ collector resistor and a 330 kΩ base resistor has never failed me for a small-signal self-biasing audio amplifier ...

By using op amps powered from a bipolar supply and ground-referencing the sensor signals, the op amp’s output will be at zero ...