Minimoog - Design

Design

At its most basic, the Minimoog control panel can be broken up into 3 sections:

• The signal generators (the 3 VCO's or Voltage-controlled oscillator and pink or white noise)
• The filter (the VCF or voltage-controlled filter)
• The amplifier (the VCA or voltage-controlled amplifier)

The Minimoog is monophonic (only one note can be played at a time) and its three-oscillator design gave it its famous fat sound. Four prototypes were made over the years before a final design was decided upon to release as a commercial product. The Minimoog Model D adapted some of the circuitry (such as the filter section) from earlier modular instruments, but designed other circuitry (such as the oscillators and contour generators) from scratch. To produce a sound, the musician would first choose a sound shape to be generated from the VCO(s) and/or the type of Noise (White or Pink). The VCO provides a choice of several switchable waveforms:

• triangle wave
• Reverse sawtooth/ramp wave
• Sawtooth/Triangle(only in oscillators 1 and 2/sawtooth wave in oscillator 3)
• square wave
• 2 different width pulse waves

The signals are routed through the Mixer to the VCF (Voltage Controlled Filter) where harmonic content can be modified and resonance added.

The filtered signal is then routed to the Voltage Controlled Amplifer (VCA), where its contour is shaped by a dedicated ADS (Attack, Decay/Release, Sustain) envelope generator. Part of the appeal of this instrument over the early modular Moogs was the fact that the Minimoog required no patch cables; its signal and control voltage path is hard-wired, or "normalled". While this imposed the signal flow limitation outlined above (VCO → VCF → VCA), there are ways to tweak the sound. For example, in reality, the Minimoog has six sound sources. 5 of these sound sources pass to a mixer with independent level controls:

• 3 voltage-controlled oscillators (see above)
• A noise generator
• An external line input

And the VCF can itself be made to oscillate, thus providing the Minimoog's sixth sound source.

The voltage-controlled filter (VCF) and voltage-controlled amplifier (VCA) each have their own ADSD envelope generator (or Attack-Decay-Sustain-Decay). Musicians who are familiar with more modern synthesizers might expect the last letter to be R for "Release" (as in ADSR). However, on the Minimoog, the envelopes are ADSD as the Decay setting also sets the time for what's regularly known as Release. In other words, there are 3 knobs to control 4 sections of the sound (most modern synths have 4 knobs, one for each section) - a "shortcoming" that doesn't seem to diminish the Minimoog's popularity in any way. There is also a switch above the pitch and modulation wheels to engage the final decay stage as well as a switch for engaging the glide circuit.

The VCF is of transistor ladder type, a famous, even envied design patented by Moog (US 3,475,623). Rumors that Moog had to go to court over the patent seem to be nothing more; 'differences' with ARP at one point were settled amicably.

The output of the third oscillator and/or the noise generator can also be routed to the control voltage inputs of the filter and/or oscillators. The amount of pitch or filter modulation thus realized is controlled by the modulation wheel, which is the right 1 of the 2 plastic disks located to the left of the keyboard. In this way the third oscillator is frequently used as a low-frequency oscillator to control pitch (oscillator modulation) and/or harmonic content (filter cutoff frequency modulation).

The Minimoog can be controlled using its built-in, 44-note keyboard, which is equipped with modulation and pitch-bend wheels or by feeding in an external one-volt-per-octave pitch-control voltage and triggering the envelope generators with an inverted Switch trigger (S-Trigger in Moog terminology).

External pitch control does not pass through the glide circuit; nor is it presented to the VCF tracking switches - the external inputs were not designed for external keyboard control. The lowest note played on the keyboard determines the pitch, a condition that is referred to as low-note priority. The envelope generators do not retrigger unless all notes are lifted before the next note is played, an important characteristic which allows phrasing. The modulation and pitch-bending wheels were an innovation that many instrumentalists found to be extremely playable. The pitch-bend wheel is on the left of the modulation wheel. It is normally kept in the centered position. It is not spring-loaded; the player must return it to the centered position to play in tune. There is a delicate detent mechanism to help the player find the center position tactually. In sharp contrast to later synthesizers that also have pitch-bend wheels, there is no deadband near the center of the wheel's travel; the wheel produces minute changes in pitch no matter how slightly it is moved in either direction. The wheel can therefore be used to introduce slight vibrato or nuance, as well as accurate pitch changes. However, Moog later recommended adding a deadband mod and published this mod in their factory service notes. The detent mechanism can be adjusted somewhat in its strength.