An audio amplifier which can be used for a wide range of experiments and demonstrations. The unit is powered by any laboratory power supply (9-12V d.c). Signal input is via 4mm sockets on the front panel, which includes a volume control. Impedance is 500k ohms (approx.); as such the unit can be driven directly from a crystal microphone, signal generator, radio headphone output etc. The unit is internally protected against incorrect input voltage polarity. Signal output is from the internal loudspeaker or via 4mm output sockets, which allows amplified signals to be displayed on an oscilloscope or used to drive a separate speaker; a two-position switch is provided to select internal or external output.