What it is
The combination of the very "fat" sounding VCOs with the (rightly) famous filter is extremely musical. Because of its warmth (cumulatied even harmonics) the mini is quite "dirty", which can lead to wonderful sounds. Its focus is to be seen rather in the interaction of the three VCOs - like tunings - and filter gradients. It can also produce quasi-ring-modulated sounds (frequency modulation), if Osc3 is used as the modulation frequency for Osc1 and the mod wheel is open. Sounds reminding to sreeming electric guitars can be heard when the parameter "Amount Of Contour" is encouraged - by the specific behavior of the filter, the Amount also acts as a sort of "Drive". The return of the signal into the audio input can even generate more extreme, distorted sounds. True Dirt!
This synthesizer exerts a strong magic and it is hard to escape this magic. A synthesizer for any kind of forceful melodies, but also for more abstract sounds. Abstraction is mainly caused by detuning of the 3 VCOs, filter-curves and through the use of the modulation wheel. Filter-emphasis (resonance) and Contour Amount are essential parameters that complement ideally the sound of these VCOs.
The Minimoog Model D is a conglomerate, also in its design: everything has to do with everything! The tuning procedures (6 trimmers which have to be matched among each other) is symbolic for this instrument. The lack of complexity, it compensates for it´s richness in the sound. The Minimoog has a very distinct character and an extremly high recognition value!
Extensibility
The Minimoog Model D is not so good for expandings as it´s successor Voyager Old School. Pitch, cutoff frequency and volume are provided for external control. Up to 3 pedals (Moog 958) may be connected.
Today, very hard to find is this pretty joystick XY controller (Moog 959). It gets power from the Mini and has two outputs (cables which can be patched into the appropriate inputs).
I have never seen the Sequencer System (960 sequencer, 961 Interface, 962 Sequential Switch, possibly in the appropriate cabinet) - and at least it seems to be not so widespreaded nowadays.
I will examine in the near future, whether as the Doepfer A174 joy-stick can be adapted to the Minimoog (information welcome!) and look forward to the musical experiences yet to come. More about Moog at a later date.
Here is a short clip (before restauration):


