Modifying a 1KHz Low Distortion Audio Range Oscillator

[enut11]

This audio signal generator design is sold on eBay as a 1KHz Low Distortion Audio Range Oscillator. Preferred power supply is +/-15v although it will work down to +/-9v or less. At 15vDC it outputs up to 7vRMS. A 2K pot on the output is all that is required for variable amplitude.

It can be purchased as a kit or fully assembled. Performance is excellent with distortion <<0.01% and if fed from a regulated supply provides a very stable amplitude.

[enut11]

My application needs a variable frequency up to about 20KHz so I was playing around with changing the values of R6/R7 (ganged 10K pot) and C1/C2.

Frequencies lower than 1KHz were easy to achieve but the high frequency limit seems to be about 6KHz after which the signal becomes unstable in both frequency and amplitude.

I suspect this may have something to do with the amplitude control circuitry around TL071 but I do not understand how this part of the circuit works.

Any suggestions on how to proceed?

[Renate]

I think you have to decrease C3/C4 proportionally to increase speed of response and retain stability.

[enut11]

thanks @Renate. Changing C4 to 0.047uF and C1/C2 to 0.0047uF allowed me to push the frequency up to 12KHz before instability. Lower frequencies were not affected by this mod.

[Vovk_Z]

R1, R2, R3 values influence stability too. So you may play with them.