>   maybe there are two distinct problem:
> 1. the offset of the OA is not compensated
> 2. the output is AC coupled through a capacitor and there is no
> resistive load to ground behind this capacitor. The effect is a
> variable DC component passing by through the capacitor untill
> full charge.

http://home.clear.net.nz/pages/joecolquitt/tremolo.html

The circuit is in the bottom left. Quite happy with it as a
LFO for a tremolo unit. A good smooth volume control
with the MC3340 VCA

Top left is a very old guitar tremolo as you can tell by the
germanium transistors. Classic oscillator that works just
as well with newer npn, but square wave output though

Top right is an old one that uses a light bulb. OK, but
chews through the power and works by brute force - shorts
the input to ground through the pot

Bottom right is the Wein bridge using an RA53, which had
got expensive the last time I looked

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