Moog Minimoog model D analog monophonic synthesizer

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Last Update 2014-12-14


Why do Minimoogs sound different?
Vintage Sound - Why?
Production Variations
MIDI Retrofits
Clones

The Minimoog is a landmark classic instrument in several ways.  It was the first portable monophonic synthesizer aimed at the performing musician.  It set the path for other imitators and is the standard against which other monophonics (and polyphonics) are measured against.  The sonority is similar to its big brother modular synthesizers at a fraction of the price (and the headache of patching it).  Back in the early 1970s a studio keyboardist couldn't get a job unless they had a Minimoog.  Most onstage keyboard rigs included a minimoog.  It held the record for the longest production product life for a keyboard instrument (1970-1981) until the its modern replacement Moog Voyager surpassed it (2003 - present).  Although it fell out of favor during the 1980s (back then I turned one down for $200), today it enjoys vintage status and has returned as a favorite once again.  It does not command total loyalty as many cursed its inability to stay in tune.  To be fair, this is not a fault of the instrument itself as age has not been kind to minimoogs.

Back in the early 90s I have had an arsenal of Moog synthesizers - but never owned a minimoog.  I decided it was about time I owned one of these classics.  In shopping around (this was way before ebay) I found this very early minimoog - it was a little more $$$ than market value but I decided to take the plunge.  Mind you this was shortly before minimoog values started skyrocketing so my timing could not have been better.  When it arrived three things struck me.  The front badge read "RA MOOG" which was unfamiliar to me.  The left hand controller panel was unusual.  And the blasted thing would not stay in tune for five minutes.

I've seen a few minimoogs in person, but the left hand controller panel was unusual on my specimen.  One, the mod and pitch wheels were clear plexiglass.  Instead of rocker switches for decay and glide, this one had mini toggle switches - I later learned these were not original as they left the factory with red momentary pushbuttons.  Small wonder that the pushbuttons were replaced.  There are also no jacks for decay/glide footswitches.  You can see the differences in the images at the top of this page.  There is a TRS jack that is not original but it is unwired.  Also not stock is the XLR jack although it does appear to have been installed at the factory as the black dymo label is typical for custom work at the factory.  This XLR jack was never wired but I installed a direct box transformer on it to give it a balanced output.

Upon receipt of this very early specimen I was dismayed to learn that it drifted so bad that it would not stay in tune for five minutes.  My EE experience would have a challenge to correct this.  I located a service manual and proceeded to search for sources of drift.  My unit has the 3046-based oscillator board which has a reputation of good tuning stability so I focused on the control voltage processing circuit.  Through piecemeal component substitutions I arrived at a stable solution.  This involved replacing selected 741 opamps with modern BiFET opamps that have better tempco and offset parameters.  741s suffer from large offset errors that also vary with temperature.  This was the best that technology had to offer when the minimoog was designed in 1970.  Today the modern BiFET pin-for-pin replacement is much less vulnerable to temperature variations.  In addition to opamps, I redesigned some other circuits and modified the power distribution.  Not only were the minimoog power busses wired in a "daisy chain" as opposed to the "star" configuration, but the oscillator cards had their own power rails that were offset from the busses at the tuning pots.  My work experience with system engineering played a part in correcting this.  No wonder the 1970s musicians cursed when their minimoogs drifted onstage!

My modifications focused only on the control circuits as none of the audio circuits were altered thus retaining the original sonority of the instrument.  When I finished the tuning improvements I gave it the ultimate test - the stage.  I gigged with this minimoog in clubs in the mid-90s.  To my delight, it only required tune up at the beginning of the night and I never had to adjust it the rest of the night.

I later learned that "RA MOOG" was Robert Moog's original company name and was operated out of Trumansburg NY.  The rear badge confirms this.  The birthplace of minimoogs was an hour from where I lived!!  It wasn't until I met Roger Luther of Moog Archives when I learned the location of the original factory, which still stands.  Trumansburg is a quiet town with quaint little early 20th-century storefronts, hardly where you'd expect the blossoming of the modern synthesizer.  Bob Moog selected this location because of its close proximity to Cornell University where he was studying for his PhD in Engineering Physics, and his original products was not synthesizers but theremins.  Today the former factory is an italian restaurant, which serves excellent food.  Look behind the bar for a framed blueprint of a theremin schematic, the only hint of its historical significance.

Age is not kind to minimoogs as pots and switches fail and connector contacts oxidize.  Often a pot can be restored by repeated sweeping between extremes, this was the case with the filter emphasis pot on my unit.  Same with switches, sweep 'em back and forth and they're good as new.  Contacts have to be restored manually and is best left to a qualified repair tech.  Minimoogs aren't hard to work on and with the exception of the ua726 all the components are standard and readily available.

The tuning calibration in the service manual is incorrect.  There is an OCTAVE trimpot that must be nulled while calibrating the oscillators and the service manual neglects this.  The correct procedure is to set the oscillator range all the way to "2" as this will null out the OCTAVE trimpot.  Once the oscillators are calibrated, then the OCTAVE trimpot can be adjusted.  The complete procedure is posted here on the Analogue Heaven archives.

The original RA Moog company of Trumansburg ran into financial trouble and in 1970 it was bought by an entrepreneur who moved the company to Buffalo and renamed it "Moog-MuSonic".  The predecessor MuSonic company designed and produced the Sonic-5 to exploit the growing popularity of synthesizers, but slow sales convinced the owner that name recognition through acquisition of Moog's company would improve his bottom line.  The Sonix 5 ultimately became the Sonic 6 during the Moog era.  By 1974 the company was purchased by the huge music corporate conglomerate Norlin (who also owned Gibson guitars, Lowrey organs, Maestro effects pedals, Pearl drums, etc) who renamed it "Moog Music".  This corporate entity continued until 1986 when Norlin dropped all the music businesses, and Moog Music became part of EJE Electronics in Buffalo.  EJE/Moog Music's only musical product was a guitar amplifier (Rockie amps), and its only function in any keyboard entity was warranty services.  By the early 1990s EJE/Moog Music was history.  Analog synthesizers were no longer desireable.

Why do Minimoogs sound different?  Which oscillator card sound better?

There is much debate over which oscillator card sounds better.  Few people realize that the oscillator card is not the main contributor to differences in sound - the filter is.

I became aware of this on my first visit to a NAMM show.  I was invited to this NAMM show to help set up a display on many wonderful vintage keyboards, including the first three minimoog prototypes and production unit numero uno.  The minimoogs were wired up and making noise.  The numero uno unit is very similar to my unit except this one had the original discrete oscillator board.  It sounded very similar to mine.  Another colleague had a later minimoog set up at the Big Briar booth with the same oscillator card as mine.  I was shocked at how different it sounded.

To paraphrase Sherlock Holmes, the game is afoot!  Why did two early minimoogs with different oscillator cards sound identical while a later unit with same oscillator card sounded radically different?  Later Roger Luther acquired an early minimoog of his own and THIS one had the third oscillator card - the UA726 version.  THAT minimoog sounded like my unit too!  Therefore I have played three RA Moog Minimoogs each with different oscillator cards, and they sounded EXACTLY the same.

I found minimoog schematics from the RA Moog days.  My suspicion zeroed in on the filter schematics.  The filter is a configuration of five transistor pairs.  A little sidenote specified that all the pairs had to be matched.  When I referred to the newer service manual, the sidenote revealed that only the top and bottom pairs were to be matched.  Here was the secret!  Since the middle pairs did not have matched transistors this impacted the sonority of the filter in that the poles (formed by the RC product comprised by the transistors and filter caps) are no longer equidistant.  Therein lies the answer - later minimoog filters did NOT have 100% matched transistor pairs and thus this contributed to sonic differences from one unit to another.  Because the early minimoogs (RA Moog and some early "Moog-MuSonics") had 100% matched transistor pairs in the filter, the sonic quality was identical from one unit to another.

When I was getting the NAMM display ready I played around with the model C prototype.  This had no PC boards at all, the circuits were manually laid out on vector boards.  The filter sounded radically different from ANY minimoog and the resonance was very sterile.  They had traced the circuit and I got to examine the schematic.  I found that the filter feedback was radically different in that instead of BJT transistors in the production models, the prototype used JFETs.  The two transistor technologies yield vastly different sonics.

There were seven minimoog prototypes in all.  One model A was built from existing modules from the modular system.  Two model B units (one was lost in a pawnshop fire years ago) used circuit boards from the modulars but used a single control panel.  Four model Cs were built which were close to the final production model (one of them is unaccounted for).  The Audities Foundation has the prototype model A, B, and C as well as minimoog numero uno.

Minimoogs had three generations of oscillator cards, each with improvements on tuning stability.  The majority of analog synthesizer - Minimoog included - are volts-per-octave (v/oct) oscillators.  V/oct oscillators require an exponential converter to conver linear control voltages to exponential currents.  Tuning stability is a challenge because the transistors used in the exponential converter has a temperature factor in the transfer equation.  As temperature changes, this changes the scaling and the oscillators will drift and/or the scaling will change IE pitch will not track across the keyboard.  This temperature factor must be nulled out.  A common technique is to use a "tempco" resistor that has a known variation with temperature.  The first generation is a 100% discrete design with no opamps and a single tempco resistor shared among all three oscillators.  This wasn't entirely successful.  These were installed in the first few hundred minimoogs up to serial #1299, which was superceded by the "3046" design that introduced opamps and had independent "tempco"s per oscillator.  The 3046 cards were better but still not great.  By serial #12175 a 3rd oscillator card was installed which used the UA726 heated transistor pair to compensate for temperature variations.  The UA726 version is the one that commands the highest prices on the vintage market, although the RA Moog units are quickly closing the gap.  It was not uncommon for early oscillator cards to be replaced with later ones.  You can often tell a later unit from the nine trimpot access holes on the back of the unit, on the right.  If you count less holes it either has the earlier card or the owner upgraded to a newer card without making more access holes.

The minimoog oscillators were BEEFY.  Two were usually enough to do the job, three can be overkill (in a good way).  The minimoog oscillators were special in that fine detuning led to a pleasant sweeping of harmonics as the oscillators "beat" against each other.  This is a trait that is rarely duplicated and is the most appealing feature of the minimoog.  Unfortunately hard sync is not available.  The keyboard is single trigger low priority, crude compared to today but effective.  The famous Moog filter is the 24dB lowpass class and it has a wonderful resonant quality.  At high resonance it can accent the sweeping harmonics of multiple oscillators and can eventually break into self-oscillation, creating a sine wave at the frequency of its cutoff.  This instrument can produce some of the fattest sounds in the industry.  Even though derivative moog products were introduced, they failed to duplicate the fat sound of the minimoog.  Only the Moog Source and Memorymoog came close.

If three oscillators weren't enough, Moog offered a custom unit built to order: the Dual VCO expander. This unit had the equivalent of two UA726 based oscillators in a 2U rack format with independent VC inputs and audio outputs.  Minimoogs had to be modified to supply the CV voltages to this unit. It also offered variable pulse width which the minimoog did not have.  This is a rare unit and is not often seen on the market.

Vintage Sound - Why?

Why can't modern synthesizers or softsynths duplicate the creamy fat sounds of the Minimoog?  Cloning the VCOs didn't get it, cloning the VCF didn't get it.  Some people think it's the filter, some people think it's the oscillators. 

The real reason is the Minimoog's discrete VCAs.  VCA fidelity when the Minimoog was designed in the late 1960s was pretty poor and distortion and noise was a necessary evil.  The VCAs in a Minimoog (there are two) were not high fidelity low distortion audio devices, in fact if you pump CD material through a wide open Minimoog filter that program material will sound terrible. That was the best that 1960s technology had to offer at the time.  In fact, the VCA was a simple differential amplifier right out of an EE college textbook.  As technology advanced through the years VCAs got cleaner and quieter.  Musicians noticed that the new synths were missing a certain high frequency "sheen" that vintage synths like the Minimoog had, especially when you open the VCF cutoff all the way.  The signal path in the Minimoog is driven a bit hot, driving the VCAs into subtle distortion and creating that creamy fat sound that we all know and love.  This hot signal was a design error not spotted until they were well into production at the RA Moog works, but in their wisdom they left it intact.  Imperfections can be useful!

The same VCA distortion was responsible for that big Oberheim sound on their early polyphonic synthesizers.  VCAs contribute more to the "vintage sound" of synthesizers than designers care to admit.  When Moog Music set out to re-issue the Taurus I bass pedal synthesizer in their Taurus III, they even duplicated the dirty sounding VCA in key circuits.  Taurus enthusiasists all over the world were delighted at how close the Taurus III sounded to the original.

Today high fidelity VCAs are readily available and they are easy to implement in softsynths.  What they missed out is the distortion of the classic VCA which is a direct contributor of that "vintage sound".  By the time the Oberheim OB-8 was designed, they exploited new technology with cleaner VCAs and Tom Oberheim lamented that "something" was lost in the sound.  Modeling dynamic distortion in softsynths is a big challenge because the exact mathematical model is not easy to derive and it is a major number cruncher to implement on a microprocessor.  The challenge to modeling VCA distortion is that it varies with the frequency spectrum (IE triangle vs ramp vs all variations of pulse width) and how hard you drive it.  Many owners of Moog Voyagers have noticed that when you opened the filter all the way, it was missing that Minimoog high end "sheen".  I was able to confirm that the Minimoog VCA was the contributing factor by routing the Voyager output to the Minimoog external input which put the Voyager through the Minimoog VCA and - wala - instant Minimoog "sheen" on the Voyager.  The Voyager VCAs are too clean! top

Production Variations and Modifications

Sometimes a minimoog is found with different color rocker switches.  This isn't any great mystery as Moog simply ran out of their stock and substituted what they had on hand.  I have seen minimoogs with all black or all white rocker switches, sometimes a mix of colored and white.  Early units had handsome stained walnut cases while later ones were painted basswood.  RA Moog and early MuSonic minimoog panels were anodized aluminum while later ones had an overlay on top of the aluminum base.  The lettering font was different between the panels.  Early units did not have a mains voltage selector and operated on US110VAC only.  Moog offered an optional replacement left hand controller that replaced the pitch wheel with the pitch ribbon found on the Polymoog, Micromoog, Multimoog, and Liberation.  Over the years there were minor circuit differences such as input impedance to the filter's audio input and a trimpot to calibrate the filter resonance.  The last 25 minimoogs had lights under the clear mod/pitch wheels and a plaque on its front identifying it as one of the last 25.  The very last minimoog was given to Bob Moog.

Some outfits such as Rivera Music Services on the west coast offered customization of minimoogs.  Modifications included hard sync, a dedicated LFO, multiple trigger for the keyboard, multiturn tuning pots, tunable distortion, chromatic transpose of each oscillator, modulation from filter EG, beat indicator for ease of fine tuning the oscillators, etc.  RMS had the most extensive modifications available, but they are very rare on the market.

MIDI Retrofits

When MIDI came along, fans of the minimoog wanted their units retrofitted to respond to MIDI.  This isn't trivial as a MIDI to CV converter must be designed.  But the Minimoog did have CV and trigger inputs on the rear panel which facilitated adaptation of these MIDI to CV converters.

Rudi Linhard of Lintronics offers an excellent MIDI retrofit for the minimoog, known as the LMC.  Back then Bob Moog's company - called Big Briar then - offered Linhard's retrofit as a modification or a kit for user installation.  Big Briar eventually was renamed Moog Music when Bob got his namesake trademark back.  I paid a visit to Big Briar and they showed me the Lintronic retrofit (they were a small operation then, they have not yet introduced the moogerfooger pedals or even the Voyager).  It didn't take long to sell me.  I told them I had my unit with me and they were interested in seeing it.  When they spotted how old it was, they called Bob to check it out.  Bob was quite taken aback as he hadn't seen one of his Trumansburg minimoogs in a long time.  After a little digging through my archives I uncovered a photo of this visit.

Anyway, the Lintronic retrofit is superior to other MIDI to CV interfaces in a couple of ways.  The minimoog has CV and trigger jacks on the rear panel that can easily interface to a MIDI to CV interface.  However this approach has two shortfalls - the CV is NOT processed by the glide processor, an important component of the minimoog sound.  Also the keyboard is still part of the CV circuit - since the S&H tends to droop over time the keyboard CV will slowly drop, resulting in flat pitch.  One must periodically press a key to restore the S&H and thus correct the flat pitch.

The LMC overcomes this by directly shoehorning into the keyboard circuit.  You get glide processing and you no longer have to press a key to restore the keyboard S&H.  It also extends the range so you have full MIDI control of the minimoog from a 64 note keyboard.  The LMC implements many useful MIDI functions such as MIDI sustain pedal to enable final decay (IE release), MIDI pitch wheel, MIDI mod wheel, MIDI velocity which is assignable to filter or VCA.  MIDI velocity to filter cutoff is my favorite modulation as it leads to funky basslines.  The LMC also offers a standalone LFO so you don't have to sacrifice the 3rd oscillator for this chore.  The LFO offers all the waveshapes of the 3rd oscillator plus S&H.  At power up the LFO rate defaults to 6hz but this can be changed over MIDI.  The interface can respond to any of the 16 MIDI channels.  It evens offers some unusual stuff like reversed keyboard tracking - as you traverse from low to high MIDI keys, the pitch traverses in reverse from high to low (this really messes with your head!).  All configurations can be changed over MIDI using program change messages.  Custom configurations are not programmable but with today's sequencers you can store MIDI strings with your sequenced songs so this is not a big deal.  This is much more than a standard MIDI to CV interface can offer.  All this is done with no modifications to the front panel, only a MIDI jack and mini toggle switch is added to the rear panel.

Minimoog clones

There have been some attempts to clone the minimoog.  One of the first was Stage Electronics out of Buffalo that comprised the remnants of the original 1970s Moog Music.  In the early 1990s they announced a reissue minimoog in the form of two rack units - one had the sound engine while the other was a remote controller with the panel full of knobs and switches.  The remote unit looked a lot like the Studio Electronics SE-1 that was to appear almost ten years later.  It had MIDI and was programmable.  It never got into production and no report of any of these units has surfaced.  A shame because it certainly had promise.

Studio Electronics got their start by cannibalizing minimoog guts, adding a MIDI retrofit, and stuffing them into a rack format dubbed the MIDIMOOG later known as the MIDIMINI.  The difference is the MIDIMINI used SE's custom designed oscillator card to replace the original.  I have recently heard a friend's MIDIMINI and can attest that it is a dead ringer for a minimoog.  SE also did "racktrofits" of Prophet-5s, Oberheim two voice SEMS, Oberheim OB-8, and Roland TR-808 all with MIDI retrofits.  As minimoogs went up in value customers became reluctant to cannibalize their minimoogs so SE dropped the racktrofit business and started their own successful design of rackmount analog synthesizers.  Not many MIDIMOOGs or MIDIMINIs are around but they command a high price on the market.

The next clone attempts came from Welsh UK and from Cincinnati Ohio.  When Norlin folded in the mid-1980s, the moog trademark lapsed in the UK by early 1990.  A welsh brit by the name of Alex Winter snatched it up through the legal system with the intention of making reissue moog products.  Likewise in the US, the trademark was snatched by Don Martin with the same intention.  The former had better success at cloning what was known as the "Welsh Minimoog".  Because Martin owned the moog trademark in the US, the UK company never had any distribution in the US.  Both companies announced grand plans to reissue the modulars, the Taurus pedals, and other moog products.  At the time Moog fever was building so this wasn't a stretch of marketing.  Unfortunately cloning was easier said than done as some of the critical components are no longer available.  The Welsh Minimoog was actually made in respectable numbers.  The "Martinmoog" operation was less than successful.

Don Martin was a businessman, not an engineer.  His cloning efforts failed to redesign the instrument to replace obsolete component with modern ones.  The late great Kevin Lightner had one of these clones in his shop and I got to examine it.  The clone was an exact duplicate of original circuit boards, but where the unavailable-at-any-price UA726 was supposed to be present was an empty socket.  The oscillators make no sound without the UA726!!!  Besides the missing UA726s, the "Donimoog" looked good on the outside but it was a poor clone owing to horrible construction.  They simply did not employ qualified staff to make a bonafide clone.  However Don Martin had been accepting deposits from customers wanting the new minimoogs.  As time progressed, Don couldn't make his minimoog but he kept accepting deposits.  After a couple of years customers started getting upset.  Internet boards everywhere became peppered with posts from angry customers.  Soon Don stopped answering his phone and did not return letters inquiring when orders would be ready.  At one point somebody visited the physical address of "Martinmoog" and discovered that it was a storefront for MailBoxEtc.

In a classic case of trademark name confusion, angry customers found Bob Moog at Big Briar and demanded to know when their minimoog would be delivered.  Bob had no idea of the Don Martin business. He filed a lawsuit against Martin and petitioned the US trademark office to revert the moog trademark back to its rightful owner - Bob Moog.

Don Martin ultimately was forced to declare bankruptcy.  But Don Martin was a crafty fraudster with no listed personal address and the court server was forced to sit at the doors of the storefront waiting to deliver papers.  The bankruptcy court liquidated the assets and awarded the trademark to Bob Moog, which enabled him to rename Big Briar to Moog Music.  Nothing was ever heard from Don Martin again and the customers lost out on their deposits.  Few "Donimoogs" have surfaced along with some model 9500 modules for modular systems, which have been built better than the Donimoogs.  An Ohio university lent their vintage Moog modular synthesizer to Don Martin for study of cloning operations but they never got it back and it is still missing to this day.

The Welsh Moog?  He never made more than a few minimoogs before shutting down his operation.

By 2003, Bob had released the Voyager Minimoog to great fanfare.

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