"Living VCOs "

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I already have a lot of different VCOs, from the ultra precise MOTM-300 to the lush, but not so well-tracking VCOs of my EMS-Synthi clone. Each VCO has it's benefits, the CEM3340's (partly under autotune control) are the "workhorses" in the OB-8, the Prophet 5 and my own JH-3, the Yamaha CS-oscillators are more on the "temperamental" side, but still playable without autotune (the 4 ones in my CS-50 stay better in tune than the 8 ones in my CS-60 ...). The precision of the MOTM VCO is invaluable for complex audio range modulation patches. I've never heard better drones than from three EMS VCOs running at almost the same frequency, and beating against each other in an ever changing pattern.
My goal was to build a set of VCOs that have the untamed bass range power of early EMS and Moog VCOs, but which are tracking a keyboard voltage over 5 or more octaves nevertheless. I found that "untamed" Beating in the bass range and controlled beating in higher octaves is not possible with standard exponential 1V/Oct oscillators. A good part of that special sound of early Moog and EMS oscillators is not because of any "randomness", "unstability", "instability" or "noisyness", as so often is said. A good deal of their behavior is because of that, but it is not the whole story. There are also some very deterministic factors in these old circuits which have been unpleasant side effects for the designers back then, but which are worth a closer analysis when we're designing a musical VCO today. This is implemented in form of three "linear detune" potentiometers on the JH-5A VCOs.

Analysis: What certain VCOs have in common
Now as my goal was to implement that feature in a standard, exponential V/Oct VCO, I could model that bias current effect of the VCS3, using a FET-input opamp (almost zero bias current), plus a variable bias current source. So with the turn of a potentiometer, I can apply as much "linear detuning" effect as I want. (I have published a linear detuning circuit many years ago, whcih works on Oberheim SEM-style VCOs that use passive integration and a voltage follower; buit it's much easier to implement on an inverting opamp integrator - it just takes 3 resistors.)

Drift and Noise and all that stuff

Every few years there seems to be a heated debate how "stable" a good-sounding VCO should be, or shouldn't be.
I never quite understood how one can make an almost religious question out of this. IMO, the situation is quite easy: Find which factors cause these random changes in a VCO (there aren't many possibilities!), and then decide if you either
  1. want to get rid of them, or
  2. add them externally, or
  3. deliberately keep them in your VCO.
VCOs with design philosophy 1 are more expensive, can be used in applications where stability is important, and you can always do Number 2.
I could have built a VCO like this, and added the linear detune feature there. But I decided to go for Number 3 in the "Living VCOs" project. Asuming those who buy them want to play animated 3-VCO-sounds in the first place, I kept everything as simple and unexpensive as possible, and I've chosen the same noisy 4-transitor exponential converter that EMS used in the VCS3. I'm not overly scientific about it - I just like the sound of it.
To demonstrate the wandering frequency, I've made a short video, taken from the oscilloscope (external link).
The setup was like this:
Using only two VCOs, nearly in unison at a rather high pitch, one going to channel 1 of the scope, the other to channel 2. The scope is triggered from channel 1, so one of the VCOs seems to stand still. (Of course in reality it doesn't but as others have shown, it's quite difficult and possibly very misleading to try and show the wandering pitch of a single VCO with a scope.) So when you see the wave of  VCO 2 wandering, you don't see the momentary pitch of VCO, but you see it relative to VCO 1. Which is all that really matters for the sound, IMO.)
I should also mention that this effect is more important for the animation of 2-VCO-Sounds. When using 3 VCOs, there are very complex beating patterns even when the individual VCOs are perfectly staying at one frequency. So I think in my 3 VCOs + Oscillator Driver Setup, that subtle built-in random modulation is a nice add-on, while the real interesting feature is the low range animation - the "linear detuning" stuff.

Audio Demo 1 13.5 MByte mp3 (First dry: just VCOs thru VCA. Then VCOs thru SSM2040 filter, VCA and Reverb.)

CS-80-Style PWM

Like the Yamaha CS-80, the Living VCOs have a limited pulse width. Even with strong pulse width modulation, you never "loose" the VCO signal, because you never get down to 0% or up to 100% pulse width. What may sound like a limitation at first, actually opens the possiblitity of "overmodulation": You can modulate the pulse width with a triangle LFO of rather slow rate, and with a depth that would normally be bigger that 100% modulation index. As a result, the modulation is clipped, becomes trapeuoid-shaped. Musically, this is like a periodic "push" of the modulated oscillator's pitch - something that sounds less detuned than ordinary PWM and allows a sonically very rich modulation.
Single-VCO-PWM-demo (One VCO thru wide open filter + VCA + reverb)


Bill of Materials


Component Overlay (component values)

Component Overlay  (reference designators)


Schematics, Page 1

Schematics, Page 2

Schematics, Page 3

Schematics, Page 4

Wiring diagrams

What I provide is just the PCB and some ideas how to hook it up. Whether you want modulation input attenuators or not, individual pulse width setting or one setting shared among 3 VCOs, is up to you. Same for output attenuators. The waveform from the 3 VCOs are roughly +/-1.2V to be compatible with Moog systems. The board also contains 3 Amplifiers to boost thsi up to the MOTM (and others) standard level of +/-5V. The rightmost pin of the SAW_n connectors carries a 0 ... 10V saw from the VCO core; there you can connect waveshapers for triangle and sine and so on, if you like. Because of the many possible choices the following options are just given as examples. Of course you can combine ideas from different options, and make your own personalized version.

Wiring, Option 1

Wiring, Option 2

Wiring, Option 3


Calibration is quite similar to what you have on other VCOs.
S - Scale. Here you adjust the 1V/Oct tracking. Apply a CV from your Keyboard or Midi->CV Converter. Connect a guitar tuner or frequency counter to the VCO output and adjust "S" until 1 octave on the keyboard is exactly 1 octave of VCO frequency.
F - Frequency. Here you can adjust the absolute frequency (not the scale). You can set it to something like 16Hz (or whatever else you prefer) for the ccw end position of the 10-Turn front panel "Frequency" control.
H - High frequency tracking. Honestly: i didn't even adjust this in my prototype - I just left it in mid position. If you need to, you can fine tune the octave tracking for higher frequencies here.
PW1(2)(3)Adj - Pulse width adjust. With the front panel Pulse Width knob in 12 o'clock position, adjust the trimmer for 50% pulse width.

Copyright J. Haible (C) 2009
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