Minimoog Model D (1980)

This is one of the last Minimoogs made. I bought it in 1981, when Moog Music announced that they were stopping production. Here's a picture of its birthplace, in a suburb of Buffalo.

The Minimoog was designed in 1970 by Bill Hemsath and Robert Moog, and was introduced to the world at the 1971 NAMM show. Over the course of ten years, Moog Music produced over ten thousand units.

In July, 2016, Moog Music announced that they were resuming production, using mostly the same circuit boards, after a 35-year hiatus.

The new ones are going for about the same price (after adjusting for inflation) as this one did in 1981. It can be argued that this is an absurd price to pay for an antiquated, monophonic, analog synth, as instruments with many more features can be purchased at lower price points. However, there is a lot of nostalgia surrounding this iconic instrument, and Moog Music reportedly has a massive backlog of orders.

Figure 1. Minimoog Model D control panel, front.

Principles of Operation

The Minimoog is designed to be a portable performance instrument. Unlike modular synthesizers, its architecture is fixed and there are no patch cables required to make connections between the modules.

The front panel can be visualized as a system block diagram, with signals passing (generally) from left to right. The keyboard produces two signals: a control voltage, which represents musical pitch corresponding to the key that is pressed, and a trigger, which represents the duration of any key-press.

The control voltage produced by the keyboard covers a range of three and a half octaves. The control voltage is routed to the first section, labeled "CONTROLLERS" on the front panel. Here, the overall tuning of the instrument can be set, and the "glide" or portamento between pitches can be be adjusted.

The control voltage is then routed to the second section, labeled "OSCILLATOR BANK" on the front panel. The voltage-controlled oscillators (VCOs) generate audible pitches, with the frequencies varying exponentially with the control voltage. The first oscillator is considered the nominal pitch, and the tuning of the other two oscillators can be adjusted relative to the first. Each oscillator can be set to produce pitches in one of five overlapping octave ranges, which are labeled in "feet", similar to organ stops. Each oscillator can also operate in a sub-audio low frequency range. Each oscillator can be set to produce one of six different waveforms, each with a different harmonic content.

In the next section, labeled "MIXER", the outputs from the three oscillators, an external audio source, and an internal noise source are mixed together to produce a single audio signal.

The mixed audio is then passed through a distinctive voltage-controlled low-pass filter (VCF) and a voltage-controlled amplifier (VCA). These circuits give each note its characteristic tone and articulation. The VCF and VCA are each controlled by control voltage envelope waveforms (called "contours" on the Minimoog) which are triggered by the keyboard, with attack and decay times which can be adjusted independently from the control panel.

From the VCA, the signal goes to the "OUTPUT" section, where it can be routed to an external amplifier and/or to headphones.

Features

Limitations

The Minimoog, by modern standards and, in particular, in comparison to modular synthesizers, has a number of deficiencies.

The biggest limitation, which applies to all purely-analog synthesizers, is that there is no way to save the settings to re-create a specific sound. You have to either write down the settings of every knob and switch (blank forms are provided for that purpose), or take a photograph.

The Minimoog is quite sensitive to technique, as it can only react to one key being pressed at a time (the lowest note is given priority). Minimoog players have to develop a light staccato touch, so that notes are given proper duration and so that the trigger opens and closes with each note. When playing legato, the trigger signal is continuously active, so the envelopes do not re-trigger between notes. The sensitivity to technique makes the instrument somewhat intimidating to inexperienced players.

Construction

The electronic circuits in the Minimoog are constructed on an aluminum chassis, using completely manual assembly techniques. The printed circuit boards are all single-sided, through-hole technology. Crossovers are made with jumper wires.

The potentiometers and switches on the front panel all connected by wiring harnesses to four printed circuit boards, entirely through edge connectors. Each printed circuit board can be removed simply by removing two screws and pulling the board from the edge connector. Unmarked holes in the back cover of the chassis, each lined with a rubber grommet, provide screwdriver access to the trimmer potentiometers on the oscillator and filter boards, so that the instrument can be calibrated without opening the chassis.

Figure 2. Minimoog Model D control panel, back, with circuit boards removed.

Oscillator Board

Figure 3. Oscillator board, front.
Figure 4. Oscillator board, back.
Figure 5. Oscillator board, front, backlit.

Controller Board

Board No. 2 contains three independent circuits:

Figure 6. Controller board, front.
Figure 7. Controller board, back.
Figure 8. Controller board, front, backlit.

Noise/Voltage Regulator Board

Board No. 3 contains four independent circuits:

Figure 9. Noise/Voltage Regulator board, front.
Figure 10. Noise/Voltage Regulator board, back.
Note the corrosion on the back of Board No. 3. This may have been caused by fungus on the board, due to humidity and excess solder (rosin) flux.

Figure 11. Noise/Voltage Regulator board, front, backlit.

Filter/VCA Board

Board No. 4 contains four independent circuits:

The transistor-ladder voltage-controlled filter is Bob Moog's most famous circuit, which earned him a place in the National Inventors Hall of Fame. It uses the IV characteristic of a bipolar transistor to implement an exponential voltage controlled current source. In the Minimoog version (a subtle variation on the circuit in the patent), varying the current through a stack of differential transistor pairs varies their transconductance, changing the time constants on all four poles of a low-pass RC filter.

I built a copy of it in 1976 as my sophomore PCB lab project. It worked, first try, even with crudely-matched transistor pairs.

Interestingly, two differential pairs in the VCA, Q13/Q14 and Q15/Q16, are glued together with thermally-conductive cement, but none of the differential pairs in the VCF have been given the same treatment. My guess is that keeping the differential pairs balanced is critical for the VCA to "close" (i.e., to go down to zero gain) as much as possible, and less vital for the filter.

Figure 12. Filter/VCA board, front.
Figure 13. Filter/VCA board, back.
Figure 14. Filter/VCA board, front, backlit.

Power Supply

The power supply is a conventional step-down transformer with full-wave bridge rectifier inside the chassis. Voltage regulator circuits are on "Board No. 3".

Figure 15. Power supply, connectors for Boards 3 and 4.

Inspection Tag

Boards are rev 11-79. This unit was constructed in March, 1980, and sold in late 1981, after production had stopped.

Postscript: The 2016 Re-issue

In March, 2016, Moog Music announced that they had started pilot production of a limited number of Model D units. In July, they announced that they were commencing production of an updated Model D. While it is constructed using most of the same circuits as the original, the re-issue addresses quite a few of the problems and limitations listed above. It's different enough, I think, that they should have called it a "Model E".

In particular, the re-issue has the following new features:

In the re-issue, the linear power supply transformer/rectifier has been removed and replaced by an external switching power "brick". This is not only more efficient, but contributes to greater thermal stability, since there is less heat being generated within the chassis.

David Thomas
22 July 2016