This simple tone control using two transistors was found in an Electronics Today International of the 90s. Transistors are general purpose NPN types, like the BC549 for greater gain.
This circuit is from an old manual, but can be easily assembled as the integrated one is still...
The simple transistor test shown in the figure can be powered by a 9V battery. The test is for...
This regulator can use BD'136 as the main transistor and as a reference next to a BC548 zener. The...