Guin Guin MME

Guin Guin MME

Introduction Part of my “Big Moog” setup is a Behringer Model D, which occupies the space where the third bank of 921A/B VCO’s are in the replica Moog IIIP. This added the Minimoog sound which Klaus has used for many years and it avoids the setup issues in the Behringer VCO’s. In 2022 the Model D is being moved and replaced with a bank of AM901 VCO’s, with the MME acting as main lead Moog, whilst the Model D sits on top of the “Big Moog”.

The plan is to add a SCI 700 Programmer (3U) replica above the “Big Moog” to enable preset patches with the Model D. and traditional ADSR envelopes, to broaden the sound. However this approach needs separate VCO CV inputs, which the Model D does not have. I did consider adding a patch bay to the right of the Model D but then I came across the 60 HP Guin Guin MME.

The MME replicates the Minimoog circuit with THD components, except the power regulator chips and 3046 which are SMD. It has CV inputs for the VCO’s, VCF and VCA and is a perfect match for the 700 Programmer. It also enables me to make component changes. The Model D sounds very good and uses SMD PPS capacitors in the filter, so it will be interesting to compare.

I ordered a panel and PCB set in October 2021, and watched them travel across to the UK from France. Very excited to get this project started!

The Build Buying most of the components looked simple initially, especially as Tayda had nearly everything in stock. However the SMD LDO’s were not in stock anywhere, and with 1 year delivery timeframes. However I did find them at UTSource (which I have success with in the past), so I ordered them. The required 38x 2N3392 transistors were also hard to locate, I found 15x at UTSource and the rest from eBay. I  later found that Futerlec have loads at a low price!

The Micro Match connectors, which connect the Control Board to the Jack Board proved difficult to source, with very long lead times for some parts. In the end I used a pre-built cable from Farnell to finish the build. The connectors are small and the plugs are difficult to close over the cable, I used a vise.

Original Moog VCF

Component Choices The Minimoog went through a number of revisions during its manufacturing life. The MME PCB’s are designed with the 3046 based VCO’s which makes it an “old” version produced from 1972 to 1978 and I am making my replica as a 1972 version. I have used 5% carbon resistors in the build, except for where 1% metal resistors are needed for precision. Some resistors would have been carbon composite in 1972, but there is no space on the board for them.

I have matched all the transistor pairs in the ladder filter and VCA, which was typical of an original 1972 Minimoog. Later versions reduced the hand matching in the filter to save production costs. The capacitors in the VCF and VCA audio path were polyester in the Minimoog; either Tropical Fish in the early 1970’s or polyester box in the late 1970’s.

• C62, C65, C66, C67 = 68nF – VCF ladder
• C63, C64 = 220nf – VCF buffer
• C51 = 330nF – VCA input

I fitted 68nF Tropical Fish capacitors in the VCF ladder and upgraded some of the electrolytic capacitors. The Minimoog and MME make extensive use of the 741 Op Amp in the VCO’s to gain stability over the previous all transistor design. Today we have options about whether to upgrade some of these to further improve frequency stability, as the wave shaping remains all discrete. The VCO Buffer Op Amps at IC4, IC8, IC15 are obvious candidates.

Powering On By the end of November I had received all the components and populated the boards, it was about 5 days work to solder all the components in. The Micro Molex connectors and control knobs proved hard to find, but success in the end. I powered on the MME with all boards connected and set the operating voltages. All the SMD regulators worked perfectly.

Initial power on faults, with resolutions:

• VCO3 dead – Dual FET pins swapped
• Broken VCO2 frequency pot – bought new one
• VCO2 frequency too high due to above
• Noise level low – trimmed louder
• VCO1 PW goes to 0% and 100% – incorrect pot fitted
• No sound through VCF and VCA – reflow of solder joint
• No glide – incorrect pot fitted
• VCO3 scaling fails

The schematics do not come with the kit, but can be requested. I used the original Moog schematics to help fault find. The components are very tightly packed on the Analog Board and its very easy to make an unwanted solder bridge. I checked the top of the VCF ladder for -10V and with some re-soldering got the VCF and VCA working. I broke the VCO2 pot when fitting the panel, so I ordered a replacement. VCO3 is not oscillating, so I checked all the transistors and came across the problem – the dual FET had its legs crossed!

The noise level was be adjusted with the trimmer, if this isn’t sufficient I will replace the modern 2N3904 with an old 2SC828R which provides a loud and wide noise spectrum (it was used in the TR808). Sorting out the PW is more complicated, once I had all 3 VCO’s working I compared the wave shapes and checked the resistor values – all ok. Turns out I had put 5K pot in Glide and 1MA in VCO1 PW! Easily corrected! The glide pot should be really 5MA rather than 1MA, so I may switch this over to give longer glide times.

Calibration This is carefully explained in the build notes and the VCO’s scaled up accurately with only a few cents error over 5 octaves. I set my analog synths at C1 =32Hz = 0V. There are additional trimmers for the VCA which need setting. The MME uses the “old” Moog oscillator design with 3046 transistor arrays but I noticed no temperature or scaling drift once warmed up.

Outcome A very nice Minimoog clone with added features and a fantastic filter sound thanks to the Tropical Fish capacitors. The use of precision resistors has helped create an accurate set of VCO’s and I don’t think I will upgrade the buffer Op Amps. The build documentation is very good but some explanation of the new features would be useful (see below). It is a very large and challenging build, which requires lots of care in soldering, but delivers a very accurate Minimoog in 60HP!

MME Extra Features The MME has a very useful set of additional features over the Minimoog (and Model D) plus some omissions;

• VCO2 and VCO3 Sync to VCO1
• PWM on all VCO’s with external PWM inputs
• Modulation has a level pot
• VCO Mod and Wheel CV inputs
• Individual CV inputs for the VCO’s
• VCF audio input with level pot
• VCF and VCF CV inputs
• Envelope CV outputs
• No VCF input switches (use the level pots)
• No power on LED indication
• No headphone output or 440Hz reference tone