INTRODUCTION TO EURORACK
I’ve been attracted to analog synthesizers since the 1960’s when Bob Moog began shaking the world with voltage controlled oscillators, filters and amplifiers. I saw Keith Emerson in concert five times in the 70’s and was always mesmerized by his talent, showmanship and walls of Moog mahogany boxes filled with custom modules and patch cables. So it is with great reverence for all this technological history that I present the HoT introduction to Eurorack modular synthesizers.
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| Eurorack portable case; top = 1U frame, middle and bottom = 3U frames |
Today’s Eurorack synthesizers sit firmly on the shoulders of several technologies and brands that came before. Arp, Korg, Roland, Moog and others collectively led us to where we are today. The Eurorack packaging grew out of an IEEE standard, industrial-control hardware specification called “Eurocard” and a German company best known as “Doepfer” helped refine it to where we are today.
Eurorack is an open standard, as a manufacturer, one can adhere to what “it” is or you can innovate and do your own thing, hoping the world might notice and pick up on your innovation. Eurorack synths can be standalone boxes (small and/or large) or they can adhere to 19” rack mount standards which let the components mount right next to audio processors like you find in recording studios.
Eurorack systems are not limited to musical instruments; the openness of the standard allows all sorts of custom processing modules to be assembled together to provide exciting new capabilities.
EURORACK BASICS
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| Click to enlarge |
Patch Cables - Eurorack systems typically utilize two-conductor, unbalanced, shielded cables terminated with 1/8” (3.5mm) plugs on both ends. These cables are used to connect between 3.5mm jacks on the face of the modules installed in a Eurorack system. By using shielded wire a common pool of patch cables can be used to connect control signals and audio signals to their destinations with minimal hum and noise interference from nearby sources.
Control Voltage is used to control pitch, create filter sweeps, control volume, modulate pitch or volume and much more. Control voltages can either be unipolar or bipolar:
Unipolar voltage usually ranges from 0-volts to 8-volts;
Bipolar control voltages are typically 5-volts peak to peak (-2.5 to +2.5)
The most common pitch standard for keyboard control voltages is 1-volt per octave, so 10 volts can represent 10 octaves of pitch range.
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| Midi to Control Voltage Module, Click to enlarge |
Keyboards, Midi and Performance Controllers: Most modern Eurorack instrument systems use midi keyboard controllers; when a key is pressed the midi keyboard generates a digital midi message which is converted into Control Voltage(s) by a dedicated Eurorack module installed in your system, then the Control Voltage outputs from this module can be routed to various other modules to create musical mayhem. Not all Eurorack systems are controlled by keyboards; an instrument can use other controllers like pitch-wheels, ribbon controllers, Theremin, drum pads or a sequencer to initiate events or sounds. Your imagination and creativity combined with a little DIY experimentation can lead to something new and exciting through the world of Eurorack.
Triggers and Gates: Moog’s modular instruments used “Triggers” to signal when a key on the keyboard was pressed down and subsequently released. In modern Eurorack instrument systems Triggers can come in different form factors and have expanded duties; triggers can cycle an envelope generator, start/stop a sequencer, represent a drum hit or maybe a footswitch or sustain pedal pressed. There are basically two types of Triggers; Voltage Triggers (aka V-Trig or Gate) and S-Trig (aka S-Trig or Switch Trigger).
Switch Triggers - aka S-Trigs, are the most simple trigger, created by shorting two wires, closing a switch or pressing a foot pedal on-off or momentary button. Geeks sometimes refer to S-Trigs as a dry-contact closure. An S-Trig can be created by closing the contacts in a mechanical relay or a transistor acting as an on-off switch. S-Trigs were commonly found in Moog modular systems, they were used to tell an envelope generator to operate or represent a key down on the keyboard controller. S-Trigs are found on Behringer modular systems (which are themselves, tributes to the pioneering modular systems by Dr. Robert Moog).
Voltage Triggers - referred to as V-Trigs. A V-Trig is a positive voltage (typically 5 volts) and 0-volts represents no trigger present. V-Trig is the same as what we call a “gate” in modern modular jargon.
Gates - are another form of V-Trig. Gates, Triggers and Clock signals are 0-volt to 5-volt pulses used for timing and other event signaling. These are the most commonly found type of trigger signal on modern modular systems. Gate signals make connecting modules together easier, you can use the same type of cables for gates, control voltages and audio.
Since Eurorack is an open standard, a manufacturer is free to innovate and bend these specifications to their own design needs. This means we often find other interpretations in the market place. A basic handheld DVM (Digital Volt Meter) is not a necessity but it can be a friend when you’re exploring new or strange Eurorack hardware :-)
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| Eurorack Portable Case, Click to enlarge |
Mounting Hardware and Eurorack Cases: The most common Eurorack hardware form factor is two mounting rails spaced to occupy 3U/5.25” of vertical rack space.
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| Eurorack mounting rails between formed metal rack ears - Click to enlarge |
Eurorack mounting rails can be placed between two formed metal brackets (rack ears) which allow the system to be installed with other audio gear in racks or road cases. Alternatively the mounting rails can be extra long (or short) and installed directly in wood, plastic or metal enclosures.
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| Eurorack case with drilled rails, Click to enlarge |
Eurorack mounting rails typically have threaded holes drilled in the rails or they are formed of extruded aluminum which has captive nuts which can be slid back and forth to align with mounting holes in the modules installed. Sliding nuts let you move the threads of the mounting nuts to exactly where they need to be and you have a possible option of replacing nuts that accidentally get stripped or cross threaded. Predrilled holes don’t give you any option for replacement in the event of cross thread damage.
Some manufacturers space the two mounting rails to occupy 1U/1.75” of vertical rack space which creates a smaller piece of equipment. If you’re delving into the 1U form factor it’s a good idea to be sure your mounting rails will line up with the manufacturer’s dimensional specifications.
Power Supply in Eurorack - Getting power to modules in a Eurorack system can be easy or difficult depending on the case you choose. Cases or “Skiffs” come in almost as many shapes and sizes as Eurorack modules.
Doepfer and Behringer (two German design companies) favor having a circuit board which goes across the rear of the case and offers outlets for modules to plug in, generally the connectors have contacts which are spaced 2.54 mm (0.1 inches). This bus board design is rigid and mounted firmly to the case; they usually have 16-bus lines which are +12V, Ground, -12V, +5-volts, CV and Gate.
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| Flexible power bus ribbon cable, Click to enlarge |
Some manufacturers favor distributing the 16-bus lines on flexible ribbon cables instead of rigid circuit boards. This allows open rack mount frames and simple “skiffs” to be offered without any power supply.
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| 4-HP power supply in tabletop extruded aluminum rails with slide nuts, Click to enlarge |
Add 4-HP power supply to unpowered Eurorack: To power up installed modules in an unpowered rack or frame, you mount a power supply module just like any other Eurorack module, these are typically a 4-HP module with a front mounted power supply input socket which is driven by a wall-wart or power supply brick. The power supply module breaks out the power to drive the flexible ribbon bus-lines which power any modules installed.
Not all power supply connections inside the Eurorack have nice neat sockets, many of the DIY and 1U Eurorack modules just have pins sticking out off the rear of the circuit board, it’s up to the Eurorack builder (that’s you) to BE SURE the correct pins are connected in the correct order to the distributed bus-lines (failure to connect this right can be fatal to your module - you’ve been warned).
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| Eurorack power supply connections, Click to enlarge |
Power Supply Current Draw in Eurorack- Modules which plugin to the distributed bus in the Eurorack case or frame consume current measured in mA (milliamps). When designing your Eurorack, be sure the power supply has enough power to drive the installed modules. It’s a good idea to reserve at least 10% of your total power supply capacity for peak demand headroom.
Here are some simple power supply examples to help you understand current requirements:
Example 1. You have a power supply capable of 1,000 mA and modules which draw 100mA each; you could install nine of these modules and have a small bit of headroom; consider only eight modules at 100mA each leaving 200mA for greater headroom.
Example 2. You have a power supply capable of 1,000 mA and modules with the following current draw each:
Module 1 70mA
Module 2 30mA
Module 3 60mA
Module 4 40mA
Module 5 10mA
Module 6 45mA
Module 7 135mA
Module 8 20mA
Total mA 410-total used with these eight modules, leaves 590mA for headroom and more modules.
Some modules have a bit of a surge in current draw when you turn on the power supply so more headroom is better.
Not all case power supply systems have symmetrical power available; as an example, some cases have 3000 mA @+12V split across three sections, but only 1000 mA @-12V split across three sections and only 1000 mA @ 5V split across three sections. Read your specifications carefully.
Some modules draw unevenly from the three power supply rails of +12, -12, and +5 so you’ll need to be careful (real-world is not as simple as my examples). A rhythm programmer I’m currently installing draws 335mA on the +12 rail but only 105ma on the -12v rail; do your engineering homework and check the “Specifications” page for each product you’re adding to your system.
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| Click to enlarge |
What if you don’t have enough power? You can install an additional 4-HP external power supply as was previously discussed above and divide your modules across two or more power supplies. Some of the larger prebuilt Eurorack cases come with multiple power supplies installed from the manufacturer. The only real drawback to adding a module this way is that it takes up space that could be another sound or utility module.
Modules for Eurorack have horizontally shaped metal panels with width specified in “HP”, which stands for “Horizontal Pitch”. Typically Eurorack module width is simply stated in HP. Here are some popular size conversion examples:
One HP = 0.2-inches
Two HP = 0.4-inches
Four HP = 0.8-inches
Five HP = 1.0-inches
Ten HP = 2.0-inches
12 HP = 2.4-inches
16 HP = 3.2-inches
52 HP = 10.4 inches
70 HP = 14-inches
72 HP = 14.4-inches
80 HP = 16-inches
84 HP = 16.8-inches
104 HP = 20.8-inches
112 HP = 22.4-inches
140 HP = 28-inches
If a manufacturer provides a number of HP, simply multiply that times 0.2-inches to determine the width in inches.
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| Tabletop rack and power supply, Click to enlarge |
Mounting Modules - Eurorack modules usually don’t have enclosures around their electronics, the rack you install them in becomes the enclosure. This means sensitive components are exposed on the rear. Sometimes these circuits are very sensitive to dust, static electricity and short-circuits from conductive things touching places they should not. Use caution with open frame mounting and simple table top kits. Try not to touch sensitive circuit board components with your hands or fingers and be careful how you put pressure on things. As a rule-of-thumb, digital circuits are more sensitive to static than analog types.
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| Making bus connections, Click to enlarge |
Turn Off the power and unplug the power supply before fiddling with adding modules to the power supply bus inside your Eurorack. If you’re using an inexpensive open frame dual rail table top rack, use great care and be sure the ribbon cables don’t short out on anything metal. The outlets on a Eurorack power supply bus have exposed male pins, an errant foil gum wrapper or loose piece of uninsulated wire like a paper clip can make quite a spark and do a lot of damage when it shorts out a 1,000 mA power supply rail, not to mention the connector will most likely be ruined.
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| Beginning a Eurorack build with dual rail rack mount frame, Click to enlarge |
EPILOGUE
Eurorack can be a lot of fun but it can be a dark hole of money; just when you think you have it figured out another module pops onto your radar; then it turns out you need one more module to support that other one, then you need a larger case and then a larger power supply, and on and on it goes, when will it end? Never?
On the other hand putting together a Eurorack system can be a great learning experience and the versatility offered by a modular system can’t be beat. If you’re ok with not having preset sounds and you enjoy experimenting with music and sound then Eurorack could be your ticket to paradise.
Thanks for reading High on Technology, Good Music To You!
©2024 by Mark King, it’s not ok to copy or quote without written permission.
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