About

The first FOXYSTEM format module for the first proof-of-concept rack of the said format, Oscillators is a triple VCO module. Its heart is the CD40106 hex inverter module, and the design is a more parts-efficient and improved version of the SFP21 40106 Dual Oscillator.

Each oscillator has the frequency range from about 8Hz to about 16KHz, and tracks exponentially. The FREQ. knob sets the initial frequency, and it can be modulated exponentially with the attenuverted CV1 and unattenuated CV2 inputs. Each oscillator is calibrated so that CV2 tracks 1V/Oct for 6-7 octaves inside the range: not perfect, but insanely part-efficient. The FM input/attenuator is AC-coupled, linear frequency modulation. It will not work well with CVs, and is intended for use with audio, e.g. the other oscillator's output. Each oscillator has two outputs, both about 12Vp-p: saw and 'triangle'. Triangle is quoted because its shape is not perfect triangle, and becomes clicky at the very low frequency end, but is trianglish enough to be happy about it. Saw stays reliable over the entire range.

These VCOs are very easy to build and extremely cheap parts-wise, yet the performance is quite robust and they easily become the main sound source of the system.

Schematic

Yet again, a 40106 VCO. This time, i made it simple again, yet it tracks better than its preceeding design, the SFP21 40106 VCO. This design is improved in part count and uses one TL074 and 1/6th of the CD40106 - you can build six on the chip, but i don't recommend building more than 3 due to crosstalk issues. You can read in-depth on how the basic design works on the SFP21 page as well, as the core mechanism is the same.

Briefly on why it oscillates: assume C2 is discharged and voltage at the inverter input is 0. Then, the inverter output is logic 1 (about 12v). The schmidt trigger quickly charges C2 through the D1, until the voltage at the C2/inverter input junction hits the up-going threshold. The inverter output flips down, and C2 discharges through the T1/Q1 setup until the voltage at the 40106 inverter hits the down-going threshold. The 40106 inverter output flips up again, almost-instantly charging the capacitor to the inverter's up-going threshold, and so on. The process repeats again and again, forming a heavily offest down-going sawtooth at C2/D1S/40106 inverter input junction.

The NPN transistor T1 acts as a path to discharge the capacitor C2, and the PNP Q1 is an exponential current sink. Additionally to that, using a two-transistor setup like this compensates most of the thermal error, meaning you can live happily, although still imperfectly, without the tempcos.

This offset sawtooth gets DC-decoupled with C4 and amplified by IC2C - a good, centered, 12Vp-p saw appears on the IC2C out. The fourth op-amp is used to shape a 'triangle' by inverting one half of the sawtooth triangle. This comes at a price: such shaper fails on very low frequencies and still goes a bit sawtoothily clicky, and in general, the triangle it creates is far from a perfect one. But for such low part count, this is a nice easy shaper that stays the same for most of the range. Moreover, you've seen countless VCOs with pure triangles and sinewaves, but what about your very own crappy, sawtooth-infused one?

Finally, this design improves on the CV mixing stage. The unattenuated CV is now amplified about 2 times for good 1v/oct tracking tuning possibility. The attenuated one is now attenuverted, using a little hack over the double-inverting op-amp summator: see 'matrix mixer' on the Doepfer DIY page. A V/O trimmer has been included to try and tune the VCO to respond to the one volt per octave standard. With it and my arturia keystep, i managed to get 6 octaves of very nice response in the reasonable musical range, and about 1.5 more of crappier response.

This design is a way more part-efficient, clean and easy to build version of the SFP one, and making three of these on a breadboard is a no-brainer. Yet, by building this, you get three workhorse sound sources with everything you need to start having fun.

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