OTA as waveshaper from tri to sine to square? Schem?

[varialbender]

I've tried searching, but havn't been able to find a schematic for an OTA that takes a triangle wave and shapes it.
I've heard quite a few descriptions, but it always helps to see it on paper.
I know Paul Perry always recomends this type of circuit, so I figured I'd check it out.
Thanks a lot

[petemoore]

  IIRC it was discussed to not great avail in terms of simplicity, the prospect of getting a sine from a Tri or Sq wave, in previous experiments.
   implementation of a 2 channel mixer, have sine on one side, then have a Tri/Square generator on the other ...
   Actually, I think 'it was.. "hard to get a sine out of guitar as source"...so getting a decent sine from guitar might be a hurdle to focus on first?...
 

[StephenGiles]

Sine from a guitar.......how many more times??


Tough about the CA3094s though!

[varialbender]

I'm just looking for the schematic that Paul Perry often talks about.
I think it's just one OTA, probably with the output fed back into the intput like a normal opamp distortion, with a gain control.
I'm pretty sure I could figure it out, but I'd love to see a schematic for it.
Thanks again

[R.G.]

Diffamps in general and the diffamp in an OTA in particular have a soft rounding on both top and bottom distortion. If you feed a triangle wave of ...exactly... the right size into such a diffamp, you get an approximation of a sine wave to within a few percent of accuracy. The exact size of the triangle is dependent on the exact devices used in the diffamp. It is critical to trim the duty cycle of the triangle to exactly 50% and the size of the triangle for lowest distortion.

The OTA is a nice way to do this since it unwinds the differential output of a diffamp into a single ended output.

[varialbender]

Any chance you have a schematic of that R.G.?

[brett]

Hi
there's a signal generator project at Silicon Chip magazine that uses a 13600 to generate square, triangle and sine waves at 10 Hz to 100kHz, and 10mV to at least 1V.  It includes inputs for AM and FM.

If you are interested, and can't locate it at Silicon Chip, get back to me.
cheers

[calpolyengineer]

I'm not quite sure if this is what you mean, but the Boss TR-2 Tremolo has a wave shaping pot that goes from tri to square (and presumably passes sine in between). Here's a schematic from freeinfosociety.com http://freeinfosociety.com/electronics/schematics/audio/bosstr2.pdf. Hope this helps.

-Joe

[gez]

Any chance you have a schematic of that R.G.?

Paul has linked to it many times.  Use the search function to look for his posts.

You haven't said how you intend to use it.  Unless you find a way of keeping amplitude of the triangle constant, this approach isn't suitable for waveshaping a guitar signal. 

It is suitable for LFOs though, which is the context in which Paul usually mentions it, but even then you'll need to regulate the supply for the reasons mentioned by RG.

[StephenGiles]

That circuit I posted, the front end of the EH Guitar Synth provides an almost perfect sine wave from a guitar.

[StephenGiles]

Repeated post so deleted

[Paul Perry (Frostwave)]

I've mentioned it plenty of times but I cant remember linking. I do know that one has to tweak it to get the best sine (textbooks say within 2%). One possible problem is, the gain of an OTA varies with supply voltage (not a problem for ME, because I always use regulators, but if you are trying to work off a 9v battery it could be awkward).
Actually I googled just now for    ota triangle to sine   which gives a LOT of examples, I can't choose, maybe you can!
NOTE: the triangle has to be at a constant amplitude. Again, no problem if your triangle is from a triangle generator... but if you are trying to do signal procesing, or you are tying to work with a square wave, again that's hard, because while integrating a square gives a nice triangle, the triangle amplitude is frequency dependant, unfortunately. No free lunch tonight......

[varialbender]

I don't know where the idea of using guitar signal came in, but I guess I havn't shot it down and havn't been specific, and the word waveshaper may sound like that, but...
I'm using this as an LFO for a trem/ringmod.
Thanks for the info on the circuit Paul.
I'm using +/-15V regulated, so that's not a problem.
The LFO I've been using so far is integrated square I think.
I'll have to hook it up and check out amplitude's frequency dependance.
It doesn't need to be perfect, it's not so much a sine wave I'm trying to nail, but more of a nice waveshape knob to get you from tri to square with something sine-like and smooth in the middle.
What kind of precision would the OTA demand for something like that?
Thanks

[Paul Perry (Frostwave)]

Well, all that is happening is, you have an OTA arranged as a simple amplifier, and you use a pot to vary the level of the triangle sent to it. As you apply a higher input level you go from triangle to sine to flattened sine to square.
So if you are prepared to change the drive pot setting whenever the input signal changes, there is no problem at all. But, check your LFO circuit.. it might well be constant amplitude. All the ones I have seen are. It's just when you are integrating a variable frequency square wave from a separate source, (eg a Blue Box front end) that you get problems.

[calpolyengineer]

Well, now that I know what your after, I must once again point out that this exact control is found in the Boss TR2 Tremolo. A schematic for it is at freeinfosociety.com

-Joe