n.b. I would have assumed that anyone interested in building one of these would have a reasonable understanding of electronics in general and the use of operational amplifiers in particular.
One correspondent didn't realise that the op-amps require a dual supply of ±15V.
My mistake for not mentioning it? Not guilty. Read the first line again.
This circuit relies on the fact that the phase difference between the output of a Wein oscillator and the junction of the CR in the series feedback is constant, regardless of the oscillation frequency.
Phases are derived from adding appropriate magnitudes and signs of voltages obtained from these two. It is my version of a circuit I came across in commercially built equipment.
The original had a few pre-set pots so one could adjust the amplitudes of and phase differences between the outputs. The pots have been eliminated by using two resistors in series in a couple of places.
Due to a bit of lost motion between the shaft and rear section of the dual amplitude-setting pot, there is a very small variation in the amplitudes and phases of the outputs as the direction of rotation is changed. This would have made a total nonsense of using the afore-mentioned pre-sets. It is of no consequence in the application for which it was built, but is a bit disappointing in a pot of this pedigree (Bourns).
The one I built (see photos) was switchable between 50 and 400Hz.