Hello,
Eddy did one: Eddy Bergman.com: Synthesizer Build part-18: A REALLY GOOD AS3340 VCO DESIGN!! just my ten pence worth.
Cheers
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Thatās from the VCO Maximus too, pretty much the same as the above. I referenced that design for the VCO Iāve been working on and it seems to work very well. I think that general tri to sine designās been around for a long time. The Kassutronics VCO 3340 uses a different tri to sine design based on a JFET no longer available as a THT package; I tried it with whatever THT JFETs I did have in my stash and none gave me results I liked. Decided to go with the one based on 2Ā¢ THT transistors instead of a 42Ā¢ SMD JFET and Iām happy.
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I used this from Hal Chamberlinās book for my sine shaper - works pretty well.
I couldnāt get any 2N3819ās, but a J113 worked in its place. I fiddled with the reference voltages - theyāre +/- 8V on my VCO, and the diodes were just 1N4148s. The op-amp was just a spare unit in a TL074.
Cheers
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Thatās very much like the Kassutronics:
and J113 was one of the JFETs I tried without good results. But this version lacks the sine symmetry trimmer, maybe that was why.
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I went to post a link to a page on tri to sine conversion and Discourse told me itād already been posted, back in June 2020 ā and I see I responded to it ā I think this discussionās going in circles, albeit 2-year ones!
These two methods correspond to the one from the VCO Maximus and the Chamberlin/Kassutronics ones.
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Which way round do the 50k trims go?
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In which circuit? There are several posted here. If you use the Reply button at the bottom of the post in question instead of the one at the bottom of this topic, itāll link that post to your reply so it becomes clear which one youāre asking about.
Anyway, I rarely care about orientation of trim pots. By their nature you usually use them only once or rarely, and if they work opposite the direction you expect, no big deal, you turn the wiper the other way and then forget about it.
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@analogoutput Oh ok, so if I put them in āupside downā the knob just works the other way around? That actually answers my question.
This is the one i was talking about though.
Also, next question, how do you tune things right without a scope? I plan to get one EVENTUALLY but Im mostly scrounging parts rn because money is tight.
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For the approach advocated by Thomas Henry and Ray Wilson see VCO 3340 Sine wave - #35 by analogoutput ā and for my disgruntled reaction, see same.
If you have magic ears that hear and understand what theyāre talking about, use 'em. Otherwise I think a scope is about the only way to go.
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I doubt whether a really pure sine wave is important for synthesizer purposes. For a piece of test equipment, yes, pure tones can be essential for some tests. But not for synths.
In a Synth, a sine wave will come from an LFO or audio frequency oscillator (e.g. a VCO).
An LFO generated sinewave will be used to modulate some other parameter, and so any slight distortion or harmonics will be unnoticable.
At audio frequencies, a sine wave is about as uninteresting as you can get. Put a sine wave through a fllter, and all the filter will do is amplfy or attenuate a pure sine wave.
Low amplitude harmonics on a sine wave will be essentailly inaudible.
So I wouldnāt worry about slight distortion or harmonics on a sine wave.
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Hi, the 18k resistor should be connected to -12v.
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thanks Iāve just create an account for asking this and just saw your answer.
Should two transistors must be thermally connected ? as the 390 resistor ?
Iām gonna try this build.
Iāve put 3 trim attenuator on my others waveforms before the following summing mixer and gonna try to increase master gain before VCF
thanks for help
anyone have try the layout from Sound Bender and can confirm me it work ?
If you mean the one here itās quite similar to what I used in my 3340 based VCO, which is based on the VCO Maximus tri to sine section shown above.
No need to thermally couple the transistors, that would be more for a V/oct exponential converter circuit where you need very good thermal stability, which isnāt the case here.
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Hi and welcome to the forum 
The schem is not mine, I found it on the internet I donāt know where, (therefore no source), I would rather advise you to test to build an already verified version like the link of @analogoutput.
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Ok thanks for your fresh answers!
I tried yesterday for the first time a +/- 12v power supply (made on stripboardā¦) which seems to work perfectly on the multimeter. when I plugged in my vco and its sine converter (which Iām trying for the first time too) I have the right voltages on each pin of the 3340 but when I insert the chip, it heats up a lot and no sound.
I followed the stripboard posted by dud (maximus vco) by moving the 18k resistor to V-. the only change I made is to take the signal from the triangle after the buffer and not from pin 10 (so the triangle goes through two buffers before being converted?). could this lead to my problem?
Or itās a psu problĆØm ?
Iām beginner to synths and any help is really appreciate !!
Likely to be a short or wrong connection. Not the place the signal comes from.
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Not very clear what youāre doing and what the problem is, but if whatās happening is you have a working 3340 VCO, and youāre adding the tri to sine, but when you power it up the TL072 in the tri to sine heats up and no sound is produced:
Pretty much the only possibilities are (1) The TL072 is defective or (2) one of pins 1, 4, 7, or 8 is shorted (or connected via a very low resistance) to a voltage rail (the wrong one, in the cases of pins 4 and 8) or ground. Pin 4 should connect to -12 V, pin 8 to +12 V. Pine 1 and 7 should have at least a few k ohms to ground. Of course there should be large resistance between the +12 V and -12 V rails, and between each of these and ground. Check those with power off, then with NO chip plugged in turn power on and verify -12 V at pin 4, +12 V at pin 8; you probably should see 0 at pins 1 and 8. (Iām assuming you know how the pin numbering works.)
If all those tests pass Iād try a different TL072. Maybe one from a different source if the first came from say AliExpress or eBay.
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Again, thanks for your rapid repply. I just read your post.
Iāve check everything maybe ten times and just found thĆ© tiniest short Iāve ever seen between pulse out and V- .
Iām gonna try an audio test, Hope Iāve not Fried my cem 3340
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And sorry I didnāt mention that I was trying for the first time the whole ensemble Iām trying to build a stand alone synth