This article has been re-written at 11-11-2020 and updated in Dec. 2025.
After I had finished the VCO I wanted to add a Sinewave option to it. The first design I had posted here was a bit sketchy so I now present a new layout here. This layout has been made using the schematic of the Thomas Henry CEM3340 Deluxe VCO.
This circuit needs the Triangle input wave to be +/-5 Volt peak-to-peak. You can input a Trianglewave of 0V to +10Vpp but then the input must first go through the 1µF electrolytic capacitor to take away the offset voltage. The Triangle to Sinewave converter will not work properly if you input a 0/+10Vpp Trianglewave without first filtering out the DC offset voltage.
Here is the new stripboard layout. This converter offers a +/-5Vpp output and a 0/+10Vpp output.
Here is the schematic drawing. I did not include any de-coupling capacitors but if you want to include them then just add two 100nF ceramic capacitors to the voltage rails as close to the chip as possible. One going from +15V to ground and the other from ground to -15V.
If you only need a bipolar output (+/-5V) then you can leave out the two opamps on the right and just tap the sinewave off at pin 1 of IC1-A as shown below, and leave out the 100K resistors and the opamps D and C.
Updated schematic 2025:
(There's also a KiCad version of this schematic further down this article.)
Here are two pictures from the oscilloscope. One without offset from the +/-5Vpp output and one with offset from the 0/+10Vpp output. If you look closely at the pictures you see that the scope is set to 2V per division and therefore that the amplitude of the sinewave is 8V. But now that I changed the feedback resistor Rf, that has changed to 10V (even a tiny bit over):
As you can see they are beautiful sinewaves and you can set the symmetry and distortion very accurately with the trimpots on the stripboard.
It will be easy enough to mount this little stripboard on one of the M3 bolts used to mount the print of the 'Really Good VCO' and thus add a Sinewave output to that VCO. You can tap the Trianglewave straight from pin 10 of the AS3340 (or CEM3340) chip or from pin 12 of the TL074 quad opamp chip. I think that will be even easier. On those pins the Trianglewave is not yet given a +5V offset voltage so it is still +/-5Vpp and therefore doesn't need to go through the 1µF electrolytic capacitor on the layout of the Triangle- to Sinewave converter.
Okay that's the new version of this article done. If you have any questions please put them in the comments below or on the EddyBergman Facebook Group page.
UPDATED VERSION OF THE SCHEMATIC Oct. 2025:
Here's the KICad version of this circuit which I used in the Eurorack PCB version of the 3340 VCO of project 18. It works like a charm and it has an extra 1K trimmer at the top of the transistor pair which lets you trim away any offset voltages. (I updated the stripboard layout with this feature too). It has a small range though so you need to make sure the 10K resistors are well matched to eachother. The transistors themselves must also be matched to get the best results.
Here's a screenshot of the sinewave it produces. A really beautiful symmetric sinewave:
Btw, all the comments below upto August 2020 refer to the original Tri- to Sinewave converter article (which is now deleted) and not to this new updated one. So please disregard those comments as they are not relevant to this article.