This is a 3 stage discrete amplifier with gain control. Alternative transists such as BC109C, BC548, BC549, BC549C may be used with little change in perfmance. The first stage built around Q1 operates in common base configuration. This is unusuable in audio stages, but in this case, it allows Q1 to operate at low noise levels improves overall signal to noise ratio. Q2 Q3 fm a direct coupled amplifier, similar to my earlier mic preamp.Input Output Impedance:As the signal from a dynamic microphone is low typically much less than 10mV, then there is little to be gained by setting the collect voltage voltage of Q1 to half the supply voltage. In power amplifiers, biasing to half the supply voltage allows f maximum voltage swing, highest overload margin, but where input levels are low, any value in the linear part of the operating acteristics will suffice. Here Q1 operates with a collect voltage of 2.4V a low collect current of around 200uA. This low collect current ensures low noise perfmance also raises the input impedance of the stage to around 400 ohms. This is a good match f any dynamic microphone having an impedances between 200 600 ohms.
