I am continuing to add DIY equipment to my home workshop. Audio oscillator is a very useful tool that is needed when we test audio projects. I already have a similar tool but it is just an oscillator without a frequency counter. I decided to use the schematic described in this page: http://www.redcircuits.com/Page82.htm.
It is a standard Wien Bridge oscillator but uses a very interesting way to stabilize amplitude - optocouple of photoresistor and LED.