Looking to modify this circuit for 30Hz operation

To change the frequency, you have to (I think) change the values of R5 and 6, simultaneously. Double both of these and the frequency should fall to about half.

If you want to make a variable oscillator, you need a ganged pair of pots that track each other well.

Thanks John,

Any idea of the approx values to get this near to 30Hz? I could just make it up with sockets and keep plugging away, but I wondered if the math was relatively simple.

• It acts as a fine frequency trim only.

Makes the frequency just on 500Hz.

F = 1 / 2.pi.R.C

Gives you about 29 Hz.

... Phil

If I recall, the basic formula is f=1/(2*pi*R*C) for each of the RC pairs in the positive feedback loop, so with C being

Any RC pairs that have an R*C product of 1/(2*pi*30)=0.00531 seconds would produce 30 Hz. For instance

That's great thanks - I ordered some caps close to your value, and a ganged 200K dual pot - I should be able to get pretty close now.

The frequency of oscillation of a Wien Bridge is: f = 1/(2*pi*(R5*(R6 + R9)*C1*C2)^1/2).

The R's and C's are the same sub numbers as your schematic. There is no requirement that R5 = R6 or C1 = C2. That's just a convenience. When those components are equal, the above formula simplifies as can be seen and was posted. When the components are equal, the gain must equal exactly 3. The purpose of the feedback network with the LED is to allow the gain to automatically adjust to the right value for oscillation.

If the R and C values are not equal, a different gain than 3 will balance the bridge for stable oscillation. The gain is critical, too much and the output slams into the amplifier limits and too little and the oscillation quickly dies out. The LED's help stabilize the gain by lowering the gain when the drive gets too high at which point the LED's conduct more.

Because the LED's conduct at the top (and bottom) of the sine cycle, this method of stabilization adds some distortion to the sine wave which may be ok depending what you intend to do with the signal.

From about 10 Hz up to 200 Hz or more.

..... Phil