From FB group Building Transistor Radios 2018-08-24 – The discussion was about the OTL complementary symmetry audio amplifier that Mr. Barnett has posted several times to the group, which uses a 2N3904 and 2N3906.
Willie Barnett
Let’s talk about current. Suppose your amp has a 9V supply. When the output transistor is turned fully on, the peak current can be as much as 1 amp through the 8 ohm load. The absolute maximum rating for the 2N3904 or ‘6 is 0.2 amps. So the peak current is *five times* the absolute maximum rating for those transistors. Now do you understand why I have said that you should not use the 2N3904, 2N3906 or the BC547 or BC 557. These small signal transistors will go into thermal runaway. If they don’t, you’re just lucky.
The better choice is to use the PN2222A / PN2907A or the BC337 / BC327 or the C8050 / C8550. These were made for this amplifier.
