This is a very quick little circuit that I made. Partly I wanted to experiement with controlling the gain of a differential amplifier by changing the voltage on the constant current sink in the tail of the diff-pair.

The circuit uses an astable-multivibrator to drive a longtailed-pair differential amplifier with an input waveform. The tail of the differential amplifier has a transistor acting as it's constant current source, but the current source is not constant, the base of the transistor is driven by a waveform with attack and decay. This results in the gain of the differential pair changing and the output waveform being rather 'chimey'. The attached PDF expains more fully.

Nice Tone Generator (pdf ~400k)