Voltage controlled modular analog synthesizer by Moog


Don’t be impressed too much by this complicated headline: It’s only one possible attempt to describe the technical abilities of this keyboard, introduced to the public at the late sixties by Robert (Bob) Moog which revolutionized not only pop-, rock- and even classical music but also contributed to make groups like Emerson, Lake and Palmer, Yes or Camel known to a world-wide audience.


But what was new? The oscillators and filters of the Moog-Synthesizer had a special ability: They were voltage-controlled, which means that all the parameters which contributed to their special sound could be controlled by external control voltages. By this, all output voltages of the different modules could be used to control the parameters of other modules, which made it possible to create new characteristic sounds unknown until that time.


The basic principle is simple: A keyboard creates different DC-voltages which depend on the key you press and  which control a special Oscillator (VCO = voltage controlled oscillator) in its frequency. By this, already a simple low pass between keyboard and VCO-control-input creates a sound  which is very typical for analog-synthesizers: the so called “glissando” effect, where the notes are gliding continuously between each others.


Because it’s possible to mix different control voltages, you can add a periodic signal of a very low frequency oscillator (LFO) to the VCO control voltage to achieve a frequency modulation which for example makes the VCO sound like police siren.


Mixing the signals of two parallel independent VCOs creates additional new sound effects like parallel intervals, e.g. the same frequency (creating nice chorus or phasing effects like heard in the solo at the end of "Lucky Man" by Emerson, Lake and Palmer), the fifth or an octave. By changing the waveform of the different VCOs, the user can also influence the degree and structure of the harmonics of the created sound (triangle, ramp, symmetric rectangle and pulse).


All these elements belong to the first part of the synthesizer: The part which is responsible for the creation of the sound and which delivers the “raw material”. The second part with its voltage controlled filter (VCF) and voltage controlled amplifier (VCA) cares for influencing and shaping the created signals.


It’s also possible to deactivate the filter and only use the pure ramp- or triangle signal. In this case the VCA makes sure that the loudness of the sound is fading out slowly or quickly after releasing a key.



The use of the filter is recommended either for sounds with a high number of harmonics or for changing a triangle signal into a sine. The Moog-Synthesizer and also the synthesizer described on this website use a 24 dB/octave filter where the cut-off-frequency can be changed by a programmable envelope (ADSR) which is triggered after pressing a key, but I must not forget to mention that the typical original Moog-VCF (not used in the circuit described on this webseite) has a sound quality which, as far as I know, cannot or hardly be reached by other comparable circuits. Anyway: By a VCF, conventional instruments can be imitated (among limits) and new sounds can be created. All these abilities and others make the analog-synthesizer to an individual instrument with a unique sound.




Facing all the enthusiasm about analog-synthesizers, we must not forget that they have some handicaps that should be mentioned:


On the original Moog-Synthesizer as well as on the synthesizer presented here, its not possible to play an chords but only single notes like e.g. on a flute. Theoretically this would be possible, but then we had to build up a complete device with all its modules for each single note played at the same time (which means at least four complete synthesizers), controlled by a microcomputer. You do not need much fantasy to imagine the large amount of space, time, patience and last not least money to build up an analog-polyphone synthesizer of that kind. Because of this modern polyphone synthesizers work completely digitally.


Second: The amount of different sounds that clearly can be distinguished is limited to less than (let me estimate) approximately two dozen. This fact is related to the principle of the given sound creation which is mostly realized by filtering the harmonics of the VCOs by an envelope controlled low pass filter. An additional creation of harmonics (like for example at the fm-synthesis) is not given here.


Nevertheless it is worth to build the device introduced here, especially because if you do it yourself instead of buying a used Minimoog via the web will save a lot of money. The estimated costs for the spare parts I used were about five hundred Euros (excluded the keyboard, which was 200 Euros extra).


The synthesizer described here should only be built by experienced hobby electronics with practical experience in building up electronic circuits, manual skills, experiences with operational amplifiers and a feeling for music too. The work is not done by soldering the spare parts shown in the circuit: After this, all the circuits and operating elements have to be connected. A lot of potentiometers have to be tuned exactly - and possible wiring or soldering errors have to be detected and corrected, which is often much more complicated than only soldering the components on the boards. More than thousand soldering points have to be made with precision to avoid bad contacts which later won’t be detectable very easily and will lead to disturbing sounds and noise.



Playing melodies on the synthesizer won’t be possible without a keyboard of course, the only external part of the project and which I bought from the German distributor Doepfer. The keyboard, no matter where you get it from, must create a keyboard-output-voltage (KOV) of one volt per octave (1 V/octave), which is a default value. The trigger pulses which occur when a key is pressed can be between 5 and 10 volts.


If you press an additional key after the last key is still pressed, the keyboard must send a small interruption pulse (legato).



By using a special interface, it’s also possible to use a default midi keyboard for these purposes. Because I didn’t try this option until now, I cannot give any comments about this alternative.