This is the linear opto isolator circuits that use a few parts and cheap. In circuit we use the opto compiler ICs number of MOC 5010 as based. [....]
This is Simple preamplifier circuit using transistors,low noise and easy to builds we have many circuit below, simple to difference projects [...]
This is a 4 transistor audio amplifier circuit. Which is a 4-transistors complementary push-pull amplifier, that shows the basics of audio amplifier design.