A simple parametric EQ should not be this hard.

[robthequiet]

Thanks for posting that -- my first impression is that the values of some of the components are not what I'm used to, such as the 713K input resistor seems a bit large, and maybe the 10p caps could be a higher value. I'm guessing that you want to make sure that the rest of the circuit does not affect the frequency picks in your selector. Do you have the FETs biased to about 1/2 of Vcc at the drain?

[stonerbox]

Everything in that schematic is there for a reason trust me, even the 10pF.  The issue lies within the Wien bridge circuit. I have plugged a guitar straight into the filter and it still oscillates badly when R8 is present.

[samhay]

What type of op-amps are you using?

[stonerbox]

A plain old RC4558 (U1) and a fancy pants OPA2134 (U2).

[Kipper4]

try this Stoner

http://www.diystompboxes.com/smfforum/index.php?topic=117511.0

[stonerbox]

Much obliged Klipper! I've been searching out large parts of this forum in hunt of a good Parametric and somehow missed that one. Thanks for giving me hope back again.

On another note, I noticed that the other parametric I've been working a lot on does not keep its gain constant. The higher the frequency the higher the gain (or perhaps Q?).. very, very strange. I have breadboarded it over and over again and I am positive that every component/pot is connected correctly but still it persists to act this way. Too bad, it looked like a really nice circuit.