Sine wave VCO

[uneumann]

I use a sine wave VCO for trem effects (no thumps or ticks). I'm currently using the circuit in Fig 35 of the LM13700 spec/app notes from TI. It works well. The range (~1Hz -10 Hz) is good and the amplitude is fairly constant. The only issue is that the part count is higher than I'd like. Does anyone have a pointer to a simpler alternative?

[Tom Phillips]

I use a sine wave VCO for trem effects (no thumps or ticks). I'm currently using the circuit in Fig 35 of the LM13700 spec/app notes from TI. It works well. The range (~1Hz -10 Hz) is good and the amplitude is fairly constant. The only issue is that the part count is higher than I'd like. Does anyone have a pointer to a simpler alternative?

My past searches have never found a clean low Freq sine oscillator circuit of the range you achieved (~1Hz -10 Hz).
Attached is a copy of the circuit you referenced. The main issue I have had is achieving both a properly shaped sine wave AND a speed control of useful range that maintains sufficiently constant amplitude output. Those parameters do seem to require a much more complex circuit commonly used for a guitar amp the LFO.

Unfortunately, the app note that you referenced does not include a theory of operation description for the Fig. 35 circuit. Can you show the actual circuit you used for your 1-10Hz variable control?

[uneumann]

Tom - Hi, I struggled with the same issues. Freq range and constant output sine waves are just not easily obtained.
I'm OK to drop the VC feature. I could use a pot.
The ota approach is inherently VC, so I went that route. It was a freebie.

My circuit is below... I've used it for a while now and I'm happy with it - other than the fact it takes up so much board space.
You can see where I've changed the values. I use a 34v supply. The output is bottom-left from a 100k pot.

An IC solution would be ideal, but I have not found one yet. I can imagine a small uP that's programmed to do this. I'd be open to that too.
I've looked at the pedal-related sites and circuits, but it seem they keep their secrets close. I can't find anything useful so far.

[Greg Robinson]

GIC oscillator is relatively few and standard parts, and produces a low distortion sine wave.

[loudthud]

Shouldn't R27, R38 and R45 go to -15V instead of ground ?

[x-pro]

Shouldn't R27, R38 and R45 go to -15V instead of ground ?

The power supply is unipolar. The midpoint is formed by U4A.

[uneumann]

GIC oscillator is relatively few and standard parts, and produces a low distortion sine wave.

Thanks for that! It looks promising. Love the symmetry.

[Tom Phillips]

Thanks for that! It looks promising. Love the symmetry.

It would be interesting to know the parts values that would be required to get the operating frequency of the GIC oscillator low enough to be useful as a tremolo LFO.

[uneumann]

Just to followup... I've spiced and proto'd some GIC attempts. I've not given up yet, but results are mixed.
It's straightforward to settle on values for a given freq (say 1 Hz). It is a nice simple circuit for that and the waveform is clean. When you try to vary freq by a factor of 10 or so, problems arise. The amplitude varies, oscillation may stop, or wave shape deteriorates (or all of above). It's also tricky in that simulations and prototypes don't behave the same. LTspice isn't perfect, and this circuit reveals that. So, don't trade in your favorite trem osc yet...

[Tom Phillips]

Just to followup... I've spiced and proto'd some GIC attempts... don't trade in your favorite trem osc yet...

Thanks for doing that. Your results show the unanticipated issues with a simple, or at least reasonable, LFO design for use in a guitar amp tremolo circuit.