- The Minimoog Topology
- Eurorack Modules
- Patching the Synth
- Testing the System
Recently I’ve found some time to take on projects I haven’t previously had a chance to try. I love my Eurorack system and I love using an exorbitant amount of patch cables to make wild and crazy sounds, so I had an extraordinary idea recently. What would be required to make a classic Minimoog design using only Eurorack modules? I know all the components are available — the schematics are all over the internet — and I am fortunate enough to have one of the Minimoog reissues to test things out on. So, while I have ample time (read quarantine time) to focus on productive things, I figured why not? It could be a fun experiment re-creating the signal flow and different circuits with my Eurorack rig and telling you all about it.
II. The Minimoog Topology
Let’s start with a little background on the Minimoog. It was the first synthesizer to bring the sound of the earlier Moog modular synthesizers to a simple, portable musical platform. It became an industry standard — a go-to instrument onstage and in the studio. The Minimoog had a collection of components laid out in a simple workflow with an intuitive user interface that made a very effective instrument. These features became the foundation on which most modern synthesizers are designed. Here is a very basic signal flow of the Minimoog.
I quickly found out this is extremely oversimplified after really delving into the functionality of the controls. The design is super deep. The front panel is simple, but all that intuitive control requires lots of multing, mixing, and tweaking to get things right.
The first thing I figured out, based on what I had to work with, was that I couldn’t make a clone. But I did my best to create something that had the functionality and type of components of the original. The audio signal path is as follows:
- The three VCOs (voltage-controlled oscillator), an external input, and a noise source route to a mixer
- The mixer then goes into the LPF (lowpass filter)
- The LPF travels to the VCA (voltage-controlled amplifier)
- The VCA goes to the main outs
The control and modulation needs are as follows: A keyboard with pitch and modulation wheels and CV (control voltage), Gate, and Modulation outs provides the controls. While there is not a dedicated low-frequency oscillator (LFO) on the original, Oscillator 3 could be turned into one with the flip of a switch. Two envelope generators are used to control the LPF and VCA.
III. Eurorack Modules
Let’s break down what Eurorack components were used to create a Minimoog-type synthesizer. Let’s start with the audio modules.
1 – Audio Modules
The Intellijel Dixie 2+ is an excellent VCO that can provide all the waveforms needed to re-create the classic Minimoog sound. It includes all the features of the Moog oscillator. It even features an LFO mode, so — like the original — the third oscillator can be used as an LFO. Three were needed.
The Minimoog has a switchable white/pink noise generator, so the Roland 555 is a good choice. It has both white and pink noise and some other options to experiment with later. The two noise sources are on separate outputs and will have to be combined and made to be selectable, so we will use a mixer and attenuate them as needed.
The Pittsburgh Modular Lifeforms 2+2 Mixer is a great 4-channel mixer to combine the three VCOs and the noise source. It will also function as dual 2-channel mixers. This was perfect to combine all those modulation sources. To get the job done, four mixers were needed for all the ins and outs.
In addition to the Pittsburgh Modular mixers mentioned above, I needed a mixer to combine the noise generators into one signal. I chose to use the 2HP Mix because I was trying to conserve some space to keep the form factor small.
There are a variety of filter options on the market, but I went with a filter that is inspired by the ladder topology of the Moog filter. The Doepfer A–108 filter is very flexible with a bandpass filter and several different filter-slope outputs, but I stuck with the 24dB per octave lowpass to stay true to the design of the Minimoog.
My choice of VCA was the Intellijel Quad VCA. It was needed for two VCAs and a level attenuator. This thing is super flexible. It mixes, it attenuates, and it provides four VCAs to boot!
Last in my signal chain was an output module. The Make Noise Rosie provided a line-level out/headphone jack. It also features a send/return in case you want to add effects to the output. This is a necessary module to hear what is going on.
2 – The CV/Control Modules
To play and control the synth, a keyboard with a pitch-bend wheel, a modulation wheel, and CV, Gate, and Modulation outputs is needed. There are several options that would fit this bill, but I chose the Arturia KeyStep. It’s a keyboard controller that has a great step sequencer for additional programming capabilities.
A great choice for an envelope generator is the Tiptop Audio Z4000 NS. Two envelope generators are needed for a Minimoog setup. These are ADSR (attack, decay, sustain, release) envelopes instead of the Minimoog’s ADS (attack, decay, sustain) envelope, but they will easily substitute and provide a little more control.
A buffered mult is essential to get all the control voltages accurately to their destinations. This module is a huge deal for this project. I like 2HP, so I went with the 2HP Buff. I learned a little while ago that when trying to keep consistent CV levels, a buffered mult is imperative. An unbuffered mult will change the voltage as it splits to the different patch points. While this is fine for a lot of applications, it will not work for what I am doing. We are going to need four of these. We have a lot of routing to do!
One thing that is not in this design is switches. I figured using mixers and removing patch cables would do the same job and would allow for some cool mods and tweaking after I was done. So, to be clear, this isn’t a totally accurate clone of a Minimoog. It is a Eurorack adaptation with some added flexibility that requires a little bit of work to get it to act like a Minimoog. One more note about this design: I did not incorporate an external input. While this is easy to implement, this would have required another channel on the audio mixer; and while there are several options out there that have more inputs, I wanted to stick with a 4-channel audio mixer.
3 – All Racked Up
Those are all the pieces to the puzzle. Then I figured out how I wanted to put it together. I trial-and-errored this for a little while. While others might have done it differently, this made the most sense to me.
IV. Patching the Synth
Just for the fun of it, here is the full patch layout.
This picture looks a bit nuts, so I’ll break it down in sections. A nice byproduct of this approach is seeing different parts of the Minimoog and being able to implement them in our own later designs. So, even if you don’t have all these modules, you can certainly use these patches for other purposes.
1 – Audio Path
The first thing I did was establish the audio path. I plugged VCO 1 and 2 into IN 1 and IN 2 on the Audio Mixer. I addressed VCO 3 later, as it was a little more involved. The output of the Audio Mixer is then routed to the IN of the Lowpass Filter. Next, I plugged the Lowpass Filter (24dB/oct) into IN 3 on the Quad VCA. I did this because of the way the CV control works on the Quad VCA. The Quad VCA can be used as a mixer as well as four VCAs. If I have a CV input to control VCA 1, it cascades to the following channels unless there is a CV control input on said later channel. I reserved IN 1 on the Quad VCA to use as a level control later on. Next, I took the OUT 3 of the Quad VCA and plugged it into the Channel A IN of the Rosie module.With the first channel of the Audio Mixer up, the Frequency control of the LPF opened all the way, and the VCA Level of OUT 3 turned up, I turned the crossfade on the Rosie all the way counterclockwise to Channel A, turned up the Master Volume on the Rosie output module, and heard a sine wave. Success! I had a solid audio path.
A note about the oscillators. These oscillators have multiple waveform-output patch points. As I don’t have a switchable waveform selector on these VCOs, I had to move the patch cable to the different waveform outputs when changing waveforms. For this demonstration, I just used the sine-wave output.
2 – Oscillator 3
I then turned down the Master Volume of the Rosie module and moved on. With the sine-wave Out of Oscillator 3 (OSC 3) plugged into MULT 7, I routed OSC 3 to two different locations. The first thing I did is patch MULT 7 to IN 3 on the Audio Mixer. Next, I set up OSC 3 to be an LFO. By patching out of MULT 7 to IN 1 on the OSC 3 Noise Mix, OSC 3 can be mixed with the noise source in the next step.
3 – Noise Generator
The Roland System-500 555 has a lot of cool features. Out of those, I used the white and pink noise generators. They have independent outputs. Since the Minimoog had a switchable white/pink noise generator, I needed to be able to select between the two noise sources. I didn’t use any switches, so I used the 2HP Mix to combine the two signals. So, I patched the PINK noise Out of the 555 into the first input of the 2HP Mix and the white noise of the 555 into the second input of the Mix. The top patch point on the Mix is the output. It is patched into the first point of MULT 6. Be prepared, we are going to use a lot of mults! I routed the second point of MULT 6 to IN 4 on the Audio Mixer. Now the audio path is complete! I patched the third point on MULT 6 to IN 4 of the OSC 3 Noise Mix. FYI — IN 3 and IN 4 on this Pittsburgh Modular 2+2 mixer is used as a 2-channel mixer. I will use the output from this mixer at a later time.
4 – Keyboard to the Buffered Mult
Now that the audio path is complete, I wanted to get the keyboard CV Out set up. On the Minimoog, it has two functions. The obvious one is controlling the note value of the oscillators. The other provides key tracking for the filter. So, as you play higher notes on the keyboard, the filter frequency moves up. To send to both places, the keyboard CV output goes to MULT 4. I route from the second patch point of MULT 4to the A-170 Dual Slew Limiter module.This gives me portamento when playing the keyboard. The Dual Slew Limiter output goes into the first input of MULT 3. Let’s get back to MULT 4. Patch the third point on MULT 4 to IN 1 on the Filter Mod Mix. This CV mixer combines the modulation sources to the LPF’s cutoff frequency. I am going to add more modulation sources to this later.
5 – Keyboard Buffer Mult to the CV Mixers
MULT 3 distributes the keyboard CV to the OSC Mixers. They also combine this CV with the pitch modulation for OSCs 1, 2, and 3. I patched MULT 3’s points 2–4 to INs 1–3 on the three respective OSC Mixes.
6 – Keyboard CV Mixers to VCOs
After this step, we can test the keyboard CV output. I patched MIX 1+2 of OSC 1 Mix to 1V/OCT on VCO 1. I then patched MIX 3+4 of OSC 2 Mix to 1V/OCT on VCO 2. Then, I did the same with MIX 1+2 of OSC 3 Mix to 1V/OCT on VCO 3. The CV/octave from the keyboard is now patched to the Oscillators. Turning Level 1 up all the way on all the OSC Mixes now gets close to a full 1V/octave to the VCO’s pitch input. I turned up the Master Volume on the Rosie and heard the pitch tracking.
It took some time to get a true 1V/octave pitch on the oscillator. With Level 1 on the OSC Mixes turned up all the way, I used a chromatic tuner and, on the oscillator, tuned the lowest C note I was going to play. Then, I played two octaves above that and adjusted Level 1 on the OSC Mix until it was in tune. I went back and forth a few times and finally got it dialed in. I repeated this for the other two oscillators. After all the modulation routing was completed, I did this again. It was pretty stable at that point.
7 – Gates
The two ADSR envelopes in this synth are routed to the filter frequency and the audio VCA. They are both simultaneously triggered by the Gate Out of the keyboard. Once again, we are going to use a mult. From the keyboard Gate Out, I patched into MULT 8. MULT 8 then patches into the GATE input of ENV 1 and ENV 2. I route ENV 1’s output to IN 3 on the Filter Mod Mix. So, by adjusting the Level for IN 3 on the Filter Mod Mix, you can change the amount of envelope affecting the filter. I patched the OUT of ENV 2 to the Quad VCA’s VCA 3 CV input. This is for the audio path. I could then directly affect the amplitude of the overall audio signal.
8 – Modulation Routing
The modulation routing was a lot of fun to figure out. Once I got all the patches in place, it still took some tweaking and experimenting with the mixer and VCA levels to get everything working correctly. In my design, because I am not using on/off switches, controlling whether the modulation is routed to the filter or to the oscillator is done by disconnecting a patch cable or turning down a level on a mixer. I used the mixers for everything. The modulation slider on the keyboard controls the amount of LFO (OSC 3 Noise Mix) sent to the filter’s cutoff frequency and the oscillator section’s pitch modulation.
8a – Filter Modulation
First things first, I patched the Modulation Out of the keyboard to the CV input on the Quad VCA’s VCA 2. This way, I can control the modulation amount of the OSC 3 signal. This doesn’t control the frequency, just the amplitude of the OSC 3. The frequency is controlled by the VCO’s frequency knob, or if you have the Keyboard CV Out turned up on OSC 3 Mix, that will also control it. Anyway, I patched the output of OSC 3 Noise Mix into the IN of VCA 2 on the Quad VCA. I routed the OUT of VCA 2 to MULT 5. I used MULT 5 to send signal to the Filter Mod Mix and the oscillator section. We will get to the oscillator part. I patched MULT 5 to IN 2 of the Filter Mod Mix. Finally, the output of the Filter Mod Mix is routed to CV1 (the Frequency control) of the filter. Now, all my modulation sources could control the frequency of the filter. I know that’s a lot!
Here is the basic breakdown. The keyboard Modulation Out goes to the CV input up on VCA 2. OSC 3 Noise Mix (LFO) goes to the input of VCA 2 and then out to a mult. The mult then goes to the Filter Mod Mix. This signal, combined with the Keyboard CV Out (Key Follow) and ENV 1 (ADSR), are mixed at the Filter Mod Mix. Filter Mod Mix goes to CV 1 (Frequency) of LPF.
8b – Pitch Modulation
Let’s pick up pitch modulation at MULT 5. Patch point 3 of MULT 5 needed to be sent to the IN of VCA 1 on the Quad VCA. I did not patch anything into the CV input on this VCA. I used this as an attenuator to control the amount of pitch modulation for the oscillator section. I took OUT of VCA 1 to MULT 1. I patch the remaining 3 outputs of MULT 1 into IN 2 on OSC 1 Mix, OSC 2 Mix, and OSC 3 Mix, respectively. All our patching is now done!
V. Testing the System
To be clear, I tested each step as I went along. I actually tested each step separately with a basic one-oscillator audio signal flow just to be sure I got all the routing right. It would have been a nightmare getting to the end and trying to troubleshoot — figuring out what was and wasn’t working. Also, setting the mixer values to full does not always create a desirable result. It can cause distortion in different places, both in the audio and CV paths. The only place I would start at full value is the keyboard’s CV inputs into OSC Mixes.
Here is the basic breakdown of testing the synth. I started by getting a simple sine wave through the audio path to the output. I turned up the Level 1 on the Audio Mixer, opened up the filter frequency, turned up the VCA 3 CV control knob on the Quad VCA, and brought up the Channel A In Level on the Rosie. At this point, I had a constant drone. I then went through the tuning process again. I tweaked the controls of the slew generator until I got my subjectively desired results. Next, I tested ENV 2. I turned the CV control for VCA 3 on the Quad VCA and adjusted the Envelope and CV controls on the VCA until I got a nice, fast but bubbly attack, a long sustain, and a medium release and made sure the VCO audio closed all the way. Once again, it took some tweaking of the controls to get this right.
Now for the real fun: testing the modulation routing! Let’s start with what I found to take the most finesse in configuring — the filter modulation. First off, I made sure OSC 3 and Noise were turned down on the Audio Mixer. I flicked the LFO switch on OSC 3. Then, I turned up Channel 3 Level on the OSC 3 Noise Mix. I set the Frequency value on the LPF to a low setting, but not totally closed. I turned up the emphasis a bit just to make the filter not so soft. On the Filter Mod Mix, I turned up Level 2 (input for the OSC 3). Now, with the mod slider on my keyboard all the way up, I adjusted the CV control on Channel 2 on the Quad VCA to get the max level of Filter modulation I wanted. It took some additional tweaking of the CV controls on the Quad VCA and Level 2 on the Filter Mod Mix to get the amount of modulation I wanted.
Now for Key Follow on the filter, I adjusted Level 1 on the Filter Mod Mix until I got the amount I felt was appropriate. On the Minimoog, there are two switches that change the amount of Key Follow applied to the filter. This same effect is accomplished with the Level 2 control on the Filter Mod Mix. I played with this a little while. I had fun getting some pretty drastic changes. I have never really played around with Key Follow like that. Then, I turned up Level 3 on the Filter Mix Mod to adjust the amount of ENV 1 applied to the filter. I adjusted ENV 1 ADSR values until I got something I liked. Of course, filter modulation is subjective to the particular patch you are creating. I just took some time to prove it worked and to see how it responded as compared to the Minimoog. All the controls are there. They do the same things, but with the Eurorack, you can get much more drastic with changes.
Finally, I looked at pitch modulation. I began by turning down Attenuator 2 on the Filter Mod Mix. This removed OSC 3 from the filter. I turned Level 2 on OSC 1, 2, and 3 Mixers halfway. The keyboard’s mod slider controls the CV input of VCA 2, which controls the depth of OSC 3’s modulation. VCA 2’s output feeds VCA 1 by way of a mult. VCA 1 only acts as an attenuator for the amount of modulation going to the oscillator section. With the modulation slider pushed up all the way, adjust the level on VCA 1. This will change the amount of pitch modulation going to the oscillators. Once again, it took a little while adjusting each stage of the signal until I got a desirable result. There you have it!
To be clear, with the use of Eurorack modules, the exact mix values used in the Minimoog may be hard to dial in, but you can get close to a Minimoog “type” of sound and control. While it may look like I just plugged it in and ran with it, it took a lot of trial and error to get to the final outcome. The filter and pitch modulation routing was the biggest challenge. I did my best to follow the schematics of the original Minimoog, and I believe I got pretty close. I hope you enjoyed this journey. This was a lot of fun for me, and I look forward to creating more intense Eurorack patches in the future.
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