Hi Sean,
Your right-most opamp is just a unity-gain buffer. The two 10K resistors should work as a passive summer. And C3 will make a 1st-order lowpass at about 0.34 Hz.
Referring to the famous AN-31 and AN-20 op-amp application notes, from national.com, note that a non-inverting op amp summer is typically implemented slightly differently (i.e. with two more resistors, one from out to - input and one from - input to gnd).
Regarding your core oscillator circuit:
If you lower the caps to 22 uF, then 6.8k for each of the three resistors will make it go at 0.388 Hz, and 3x 680 Ohms will put it at 1.73 Hz. You will have to raise the R1 feedback resistance value (a lot), probably. [I guess the caps might need to be bipolar. For that, you could just put two 47uF in series, with either both - leads connected to each other or both + leads connected to each other, and have a bipolar cap of 47uF/2 = 23.5 uF nominal.]
Also, it should work better if you connect the bottom of R10 to ground and use the output of C11 as your feedback signal, with an additional series resisistor before the neg opamp input; probably something like 1K, if you have 330K or more for R1 (270k is probably the bare minimum).
If you use a decently-robust opamp, you can probably control the amplitude by just hanging a small resistor to ground, from the opamp output. I simulated it with, just as an example, 10 Ohms to gnd (with an LT1057A opamp), and the amplitude stabilized at 400 mV p-p. The resistor was passing +/-20 mA peaks and its average dissipation was only 2 mW, while the opamp's avg dissipation was only 54 mW. I was using +/-17.5v supplies.
The oscillator does seem to take quite a while to get its output amplitude up; about 50 seconds, in my simulation.
For your JFETS: You should use the linearization trick that was mentioned. i.e. Use two equal-value resistors, one in series with the gate and one from drain to gate. You can probably use any value from 10k to 100k [use as low as your circuit will handle, for less distortion]. That should enable you to use anything from about -6v to 0v on the gates, with good linearity and minimal distortion, for low-level signals.
- Tom Gootee
**broken link removed**
Your right-most opamp is just a unity-gain buffer. The two 10K resistors should work as a passive summer. And C3 will make a 1st-order lowpass at about 0.34 Hz.
Referring to the famous AN-31 and AN-20 op-amp application notes, from national.com, note that a non-inverting op amp summer is typically implemented slightly differently (i.e. with two more resistors, one from out to - input and one from - input to gnd).
Regarding your core oscillator circuit:
If you lower the caps to 22 uF, then 6.8k for each of the three resistors will make it go at 0.388 Hz, and 3x 680 Ohms will put it at 1.73 Hz. You will have to raise the R1 feedback resistance value (a lot), probably. [I guess the caps might need to be bipolar. For that, you could just put two 47uF in series, with either both - leads connected to each other or both + leads connected to each other, and have a bipolar cap of 47uF/2 = 23.5 uF nominal.]
Also, it should work better if you connect the bottom of R10 to ground and use the output of C11 as your feedback signal, with an additional series resisistor before the neg opamp input; probably something like 1K, if you have 330K or more for R1 (270k is probably the bare minimum).
If you use a decently-robust opamp, you can probably control the amplitude by just hanging a small resistor to ground, from the opamp output. I simulated it with, just as an example, 10 Ohms to gnd (with an LT1057A opamp), and the amplitude stabilized at 400 mV p-p. The resistor was passing +/-20 mA peaks and its average dissipation was only 2 mW, while the opamp's avg dissipation was only 54 mW. I was using +/-17.5v supplies.
The oscillator does seem to take quite a while to get its output amplitude up; about 50 seconds, in my simulation.
For your JFETS: You should use the linearization trick that was mentioned. i.e. Use two equal-value resistors, one in series with the gate and one from drain to gate. You can probably use any value from 10k to 100k [use as low as your circuit will handle, for less distortion]. That should enable you to use anything from about -6v to 0v on the gates, with good linearity and minimal distortion, for low-level signals.
- Tom Gootee
**broken link removed**
