The Mini has a three and a half octave keyboard. It plays one note at a time. If more than one note is played, the lowest note sounds ("Lownote priority"). To the left of the keyboard is the "left hand controller" section. It consist of a pitch bending wheel, a modulation wheel, and two switches, labeled "decay" and "glide". The PB wheel has a center detent, and usually is kept centered. The Mod wheel usually is kept at 0. Neither wheel is spring loaded.
It is significant to note that the PB wheel directly controls all of the oscillators. There is no center "dead band" as there is on most modern synths. (The "dead band" would be an area of travel at the center where the wheel has no effect.) This makes it possible to do accurate pitch bending that sounds good. It is also possible to use a very light touch on the PB wheel to introduce a slight phasing or a vibrato, which is not possible on wheels with a "dead band".
The electronics consists of three voltage controlled oscillators (VCO) and a white/pink/red noise source, which feed into four inputs of a five channel mixer. The fifth mixer input comes from an external input on the top of the case. The output of the mixer feeds into a 24db/octave voltage controlled variable resonance lowpass filter (VCF). The output of the VCF feeds into a voltage controlled amplifier (VCA).
There are two identical envelope generators in the Mini. One controls the filter cutoff frequency, and the other controls the loudness of the VCA. Each envelope is an ADSR with three knobs labeled Attack Time, Decay Time, and Sustain Level. The release time is always equal to the decay time. Both release times can also be set to zero by turning off the "decay" switch on the left-hand controller section of the keyboard.
The ADSRs are only triggered when a note is played on the keyboard while no other notes are down. If a note is down, hitting another note may change the pitch (only if the second note is lower than the first), but will not retrigger the ADSRs.
The ADSRs are of the non-return-to-zero variety. That is, each attack starts from the current voltage of the EG, not necessarily from 0. Combined with the non-retriggering nature of the keyboard, this makes phrasing possible. To do phrasing, the player must be careful to lift the last key of a phrase fully before playing the next note, and to play the notes within the phrase legato. For instance, setting a slow attack and decay time on the VCF EG will make the filter tend to open up more and more as successive notes of a phrase are played. This a pleasing effect which "rounds out" the phrase, and can also make quicker, more intense passages have a brighter tone.
The mod wheel controls the amount of modulation which is sent to both the VCOs and the VCF simultaneously. A knob labeled "modulation mix" controls whether the modulation source is to be the output of VCO 3, the noise generator, or some mixture of the two. Two switches, one at the left of the oscillator section, and one at the left of the filter section, can entirely disconnect the output of the mod wheel from that section.
Each of the three VCOs is a one volt per octave analog VCO. Each VCO has an octave switch labeled "low 32 16 8 4 2" which can be used to change the octave of an oscillator in performance without retuning. Oscillators 2 and 3 also have a tuning knob, which can be used to tune (or detune) the oscillator relative to VCO1 over the range of about +/- a 6th.
Each VCO has 6 waveforms: triangle, triangle/sawtooth mix, sawtooth, 1:2 square, 1:3 retangular, and 1:4 rectangular. VCO 3 has a reverse sawtooth instead of the tri/saw mix. Only one waveform of each VCO can be used at a time. VCO3 has a switch that can disconnect it from keyboard control, which is useful when it is being used as a modulation source instead of an audio source. This switch also has the effect of increasing the range of the detuning knob to about 6 octaves.
The noise source is represented by a switch in the mixer section labeled "white/pink". With the switch in the "white" position, white noise is sent to the mixer and pink noise is sent to the modulation mix knob. With the switch in the "pink" position, pink noise is sent to the mixer and red noise is sent to the mod mix knob.
All routing in the Mini is accomplished using switches and knobs. No patchcords are used. The Mini has no memory, so all changes to parameters must be made in real time.
One characteristic of many analog synths which is also present in the Mini is that of tuning instability. The Mini will be flat if it is cold. The unit comes up to within a quarter tone of its final pitch after being on for about 10 minutes, after which it will slowly drift up the other quarter tone over the next 20 minutes or so. It is recommended that when preparing to use the Mini in the studio or at a gig, the Mini be powered on before the rest of the equipment is set up.
If the temperature changes during performance because of cold weather or because the stage lighting comes on, the pitch will be affected. The instrument is easily tuned by a master tuning knob on the front panel. It's not a bad idea to keep a tuning machine plugged into the headphone jack when playing live.
The Mini has four qualities which make it a viable contender for solo line and studio work, even today: fat sound, the pitch wheel, the ASDR phrasing, and the ability to change all aspects of the sound easily during live performance.
The "fat sound" results from the type of filter used, and from the tuning instabilities. The pitch wheel feels good because the detent is weak and because there is no "dead band". The ADSR phrasing was discussed above. Finally, each adjustable parameter is (necessarily) controlled by a dedicated knob or switch on the front panel.
Earlier Minis use discrete components (transistors) for the VCOs. Later Minis use ICs. Although the ICs improve tuning stability somewhat, they don't sound quite as "fat".
It is interesting to note that the Prophet 5's front panel and architecture was modelled after the Mini. However, the Prophet doesn't have that "fat" sound, probably mostly due to differences in the filter design (the Moog filter was patented.) Even now, after the filter patent has expired, there are no synths that (to my knowledge) sound as "fat" as the Mini. This is probably due to the use of "better" VCOs, which track each other closely and do not drift as much as the old transistor ones did.
Every couple of years, the oscillators need to be tuned. Inside the case there are 7 trimmer pots used for tuning. Each VCO has two, labeled "range" and "scale". The seventh pot is used to calibrate the octave switches.
The VCOs are tuned by first tuning the "range" pot so that an extremely high note is in tune with an external pitch reference. Then the "scale" pot is tuned to adjust the frequency of a low note. Since adjusting the scale affects the range, tuning is somewhat of an iterative process. Each VCO is tuned independently.
I tune VCO1 exactly, using a strobe tuner. I then tune the other two VCOs to it by ear. I stretch tune VCO2 a little by tuning the high end to VCO1 exactly and then tuning the low end so that it is a little flat, beating against VCO1 to a pleasurable degree. I tune VCO3 exactly to VCO1.