From FB group Building Transistor Radios
Both versions have pros and cons. I’m not trying to take sides just make some observations.
#1 (Willie’s) has the 2N3904 and 2N3906 for output Transistors. They can handle only about 100 mA and they easily go into thermal runaway, especially when it gets warm. The 3904 can be replaced with a PN2222A, the 3906 by a PN2907A.
The #2 (Pedro’s) connects the 1.8k bias resistor to the +9V through the speaker. This bootstraps the bias resistor so the output Transistors don’t become current starved as the output approaches +9V. #1 doesn’t have any bootstrap. One disadvantage of #2 is that the speaker must be connected in order to supply bias current. It won’t work with a piezo earpiece.
The excessive idle current in #2 can be reduced by putting a 4.7 to 10 ohm resistor in series with each emitter. The BC337 and BC327 can handle much greater current than the other amp, and are good choices. The C8050 and C8550 are another pair that handle high currents. If you want even more, use the BD135 and BD136.
I’ll post another schematic next.
