The first stage is an input stage which amplifying can be set by the Gain 
potentiometer. The next stage is the frequncy doubler. And finally the third 
stage works as a difference amplifier, which amplifyes only the difference 
between the first and second stage, and mixes the two sources together.
                                                                              

If you feel the Output volume control is too inactive in the beginning of the 
scale you can add the 47kohm resistor I've marked in the schematic.
                                                                                

This circuit works best if the voltage is between +- 9 to +- 15 V.
The OP-amps can be for instance one LF351 or TL071 (single) and one LF 355 or 
TL072 (double).