I kept all the waveforms of the original Elektor Formant, but changed the circuitry significantly. The only thing i added a symmetrical square out, that i found useful for sync. For converting the saw wave to triangle a precision full wave rectifier is used, followed by a level shifter. The originally used matched Germanium diodes are hard to obtain these days. Here you only have to match two resistors. The switching glitch is not audible and can be suppressed with means of a capacitor of some 100pF if wanted. Don’t choose the capacitance to high, it rounds the edge of the triangle.

Converting saw to triangle

For triangle to sine conversion i used a well known circuit that works better then the two diodes approach from the original. This circuitry can be found at various places in Electronotes and others. The circuitry is very sensible to the used values and input voltage. If you use another input voltage you probably have to change type of FET and resistor values.

Convering triangle wave to sine wave

Screenshots:

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine

Screenshot: saw to triangle to sine