"Phil Allison"
The final design operate at three fixed frequencies ( 66, 998 & 5278 Hz )
it employs a single LM308 op-amp in a Wien bridge notch filter followed by
a TL071 x10 gain stage. The caps are all polystyrene, resistors all metal
film and fine adjustment in each leg of the filter is done with a pair of
200ohm, 10 turn WW pots.
** This schem has the same basic circuit.
http://www.tradeofic.com/uploadfile/...7284957652.gif
In my version, resistor values were kept below 50k ohms for low noise and
the op-amp can be the readily available a OP07 type. Cap values were 150nF,
10 nF and 1.8nF.
.... Phil