This article has been re-written at 11-11-2020.
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, which has a sinewave output. Btw, you can find that schematic in the 'files' section of the 'Synth DIY for non engineers Facebook Group'.
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.
I altered the feedback resistor (Rf on layout) from 10K to 15K to get the amplitude correct with the waveform standard of my synthesizer project which is 0 to +10Vpp. This had the effect that the +/-5V output got a negative offset voltage. What I should have done is change the other 10K that goes from the + input to ground into a 15K also, so everything is in balance again but I put a 1µF capacitor in series with the output of the +/-5Vpp sinewave. The negative pole of the electrolytic capacitor is facing the direction the signal is coming from because I had a negative offset voltage to deal with. Make sure you match the 10K resistors so they all have the same value and if you change the feedback resistor to a 15K make sure you change the other 10K to ground also. Match the transistors too. (Matching them on hfe is good enough). If after all that you still have an offset voltage on the output (unlikely) then you can put a 1µF cap in series with the +/-5V output.
The output amplitude on a dual 12V powersupply is +/-4.2Vpp or 0 to 9.4Vpp. For a dual 15V power supply it is +/-5Vpp or 0 to 10Vpp.
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.
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.
Btw, all the comments below upto August 2020 refer to the original Tri- to Sinewave converter article and not to this one. So please disregard those comments.