BJT audio amplifier for 8 ohms 0.025 watts speaker
Can someone please explain to me, why is that there is a resistor after c1?

With this amplifier topology the global gain is defined by \$A=\dfrac{R4}{R1}\$, provided that the open loop gain from the transistors is much higher than that.

In that the base of Q1 will act as virtual ground for the AC signal. Notice that for DC the base is biased to approximately 0.6V. With R1 at (virtual AC) ground, your input impedance equals R1.

A simplified opamp circuit would look like this (notice that it will not work in practice due to low voltage).







simulate this circuit &ndash; Schematic created using CircuitLab