Sine wave oscillator with easily variable frequency?

[StaticBuildup]

Hi everyone,

I am thinking of making a musical instrument with a sliding controller, similar to the Tannerin or Ondes Martenot. I want to use a sine wave oscillator for the sound, so I have been trying to find circuits that can produce a sine wave sound that can be varied in frequency with a single resistor, which would be the controller. I haven't managed to find any.

I don't want to use computers or anything digital, or even ICs if possible. The original Tannerin, Ondes Martenot etc. didn't use those, and I like that.

I have made a sawtooth wave oscillator which works well over a reasonable range of frequencies, so why are there no simple sine wave oscillators? How did those instruments generate the tone?

Any help would be much appreciated.

Lucia

[AlanP]

You could do a triangle oscillator, and stick a tri-to-sine converter on the output?

[JovianPyx]

The reason you don't see a lot of (or any) schematics for musical analog sine wave VCOs is that the sine is one of the most difficult to manage, especially for wide range use as music usually desires. The 8038 chip claims sine wave output, but it's not perfect as it comes from a diode based wave shaper that distorts the internal triangle waveform. Of course, digital works well these days with modern CPU chips and will be closer to perfection than the 8038. Another way to do it in analog is to use a self oscillating filter, but that has it's own set of headaches including changes in amplitude as frequency is changed and possible dropping out.

[blue hell]

I dont know about the Ondes and the Tannerin .. they both have a keyboard .. so I guess you'd want something tracking the keyboard well .. a good oscillating filter could do that .. there was some discussion in the chat room here suggesting a mankato filter might be up to that task .. but I'm just echoing here

for a theremin it would not be an issue really.

[LFLab]

Something like this? Wien bridge which uses a single pot rather than harder to get dual.
Do not build the vca's by the way, they do not work well (despite the fact that one can find this circuit on several places on the web)

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[StaticBuildup]

Thank you for the replies.

I will have a look at all those ideas.

[StaticBuildup]

I have been researching it more and it seems that the Ondes Martenot and Theremin use beat frequency oscillators to create the tone. I guess maybe an adapted Theremin circuit could work?

[flagada]

Another simple Wienbridge oscillator. R1 and R2 should be a stereo pot.

[flagada]

Also, the XR2206 function generator IC gives a sinewave.

[StaticBuildup]

Thank you for the replies. I might try to convert the sawtooth wave to a triangle wave and then possibly to sine, as that looks easier than making a sine wave oscillator. It would also give the options of sawtooth, triangle or sine waves.

[LFLab]

Tri to sine conversion is not hard, but a wien bridge oscillator creates a sine with much lower distortion than most "regular" VCO's.
If the distortion level is not important than yes, regular VCO's are easier use and more common to find.

[JovianPyx]

Saw to tri is notorious for having a glitch at either the negative or the positive peak. It never goes away completely with low pass filtering and it's harmonics are thus always present. It can be rather in your face because sine is supposed to have only it's fundamental harmonic and no others.. Triangle from a good triangle core, however can be converted to sawtooth without much "mess" and triangle is also fairly easy to convert to sine and there is no glitch to deal with. The sine won't be perfect but it won't have the usual saw-to-tri glitch. You may not care about perfection, and if so, take this with a grain of salt.

If you can find a way to create a glitch-free triangle from sawtooth - we all want to see it.

[blue hell]