Building a Eurorack Envelope Generator

An envelope generator is a Eurorack module that can be used to shape the amplitude of a sound over time. It generates a control voltage that can be used to modulate the level of an audio signal or second CV, for example, to control the volume or brightness of a sound.

The envelope generator circuit outputs a series of control voltages that describe the shape of the sound’s amplitude envelope in response to an incoming gate signal. These control voltages can be divided into several stages, each corresponding to a different envelope phase. The most common stages are Attack, Decay, Sustain, and Release, giving us the familiar synth ADSR envelope generator.

Attack sets the time for the envelope to reach its maximum level. Decay the time it takes for the envelope to fall from its maximum to its Sustain level. Sustain is the level at which the envelope is held for the duration of a gate CV. Finally, Release describes the time it takes for the envelope to return to its starting level after the gate falls to 0V.

By manipulating the shape and duration of these envelope stages, the envelope generator can create a wide range of dynamic and expressive sounds, such as percussive hits, long evolving pads, and rhythmic patterns. An ADSR envelope generator is essential to any modular synthesizer system, allowing musicians and sound designers to shape and sculpt their sounds in real-time.

In this tutorial, we’ll build a simple ADSR envelope generator for Eurorack using the N8 Synth Eurorack breadboard paired with a 4HP Eurorack Control Deck. The breadboard and Control Deck, pre-drilled panel, and pin headers can be purchased together as 4HP 1×6 Eurorack Prototype kit.

photo of the completed Envelope Generator module
The 555 ADSR

The 555 ADSR circuit ( or, more correctly, 7555 as we use the CMOS version of the 555 timer IC) described here is certainly not original and is something of a classic, dating back to the early 1980s. Further examples, with excellent references to original circuits, can be found on yusynth.net and schmitzbits.de. The humble 7555 timer IC also features in the ADSR circuits of the amazing Arturia MiniBrute and MicroBrute.

ADSR Envelope Generator Schematic & Layout

Circuit schematic of a 7555 timer based adsr envelope generator
Eurorack ADSR Envelope Generator Schematic - click to expand
Diagram of the envelope generator on Eurorack breadboard
Eurorack ADSR Envelope Generator Layout - click to expand

The general operation of any ADSR envelope generator is that a gate signal ( let’s assume +5V ) arrives at an input and persists for some time, for instance, as long as a key is pressed on a keyboard. The change in voltage at the gate ( let’s say 0V to +5V ) causes the ADSR envelope circuit to enter its attack phase, and voltage at the output increases.

Eventually, the output voltage hits some predefined threshold ( let’s assume +8V ), and the circuit enters the decay phase. The voltage at the output now decreases until it reaches the envelope’s sustain level. Once the output level has decayed to the sustain level, while a gate voltage at the input remains, the output stays at the sustain voltage.

When the gate voltage is removed ( the key on the keyboard is released, for example ), the envelope enters the Release phase, and the output falls to 0V. The rate at which the output voltage changes and the sustain level can usually be adjusted by either a CV source or a potentiometer.

ADSR Circuit Overview

In our circuit, the charging and discharging of the capacitor C4 creates the change in voltage at the output, which becomes our envelope CV. Half of a TL072 ( U2A ) buffers the voltage across C4 so that the rate at which it charges and discharges isn’t affected by the impedance of wherever we connect to the output. The switching from Attack to Decay is handled by a 7555 timer IC ( U1 ) configured as a mono-stable timer. The 7555 compares the voltage across C4 ( at its threshold pin 6 ) with 2/3 supply voltage (8V for Eurorack ) to determine when C4 is charging ( attack ) or discharging ( decay ). 

A gate voltage above +1V at the input (J1) will trigger the envelope. Three NPN transistors (Q1 – Q3) buffer the incoming gate and generate a trigger pulse to reset the 7555 timer. Diode D1 protects Q1 from input voltages below 0V, while D2 allows C3 to discharge when the gate voltage is removed.

Attack

With 0V at our gate input (J1), the voltage at the collector of Q3 will be around 12V. When a positive gate voltage is applied, Q1 switches on, lowering the voltage at its collector and closing Q2. The voltage at the collector of Q2 is now ~12V ( our buffered gate ), causing Q3 to open and the voltage at its collector to drop from +12V to 0V. However, C3 charges rapidly, shutting off Q3, so the voltage drop at the collector of Q3 is short-lived ( ~500uS ). This short trigger is applied to the trigger input of U1, which causes its output to switch on. The output of U1 charges our timing capacitor via the potentiometer RV1, which sets the attack rate. More resistance equals a slower charge resulting in a longer attack.

Decay

The gate voltage is holding the Reset pin (4) of U1 high, so its output will remain on until the voltage across its Threshold pin (6) reaches 2/3 of the supply voltage ( ~8V ). C4 is connected across the threshold pin, so once the voltage across it reaches ~8V, the 7555 will switch off its output. C4 now discharges via RV2, our Release potentiometer. Again greater resistance equals slower discharge resulting in longer release time. 

Sustain

RV3 and R3 form a voltage divider, allowing us to set the sustain level between 0V-8V. This voltage is buffered by the op-amp, U2B, and opposes the voltage across our discharging C4 capacitor. As a result, C4 can only discharge down to the sustain level during the Decay phase and will hold at this level as long as the gate voltage is present.

Release

The release phase occurs once the gate voltage is removed, allowing C4 to discharge through RV4. As Yves Usson notes, there is some interplay between the decay and release times in this simple design, and adding the FET per his design to counter this is a simple modification.

Diodes D3, D4, and D5 ensure C4 charges and discharges through the correct potentiometer. R5, R6, and R7 set the minimum Attack, Decay, and Release, respectively. C1 is the recommended 10nF capacitor to ground the unused control voltage input of the 7555. Operationally the 7555 timer is identical in operation to the famous 555 timer IC but is a CMOS low-power device.

Bill of Materials

We’ll build the Eurorack envelope generator module using a 4HP Eurorack Prototype kit. The kit contains a Eurorack Solderable Breadboard and Eurorack Control Deck, a pre-drilled Eurorack panel, and the pin headers used to connect them. The Euroack breadboard provides power to our ADSR module, and the Eurorack Control Deck makes mounting our controls a breeze.

The 4HP Eurorack Prototype kit, jacks, and pots required for this module are available from the N8 Synth store. The remaining components are widely available and relatively inexpensive. If you are just getting started building modular synths, stock up on these components, as they are ubiquitous in the schematics you’ll find online.

CategoryRefValueNotes
4HP 1x6 Eurorack Prototype Kit--Includes Eurorack Breadboard, 4HP Control Deck, 4HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J3.

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HardwareJ1-J2PJ 3001F3.5mm vertical mounting jack socket

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PotentiometerRV1, RV2, RV4A100KA100k (Log) 9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style ).
Swap for A1M to increase attack, decay, and release durations.

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PotentiometerRV3B10K9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

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Hardware--4x Davies 1900H style knobs, white

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ICU1ICM7555General purpose CMOS timer
ICU2TL072TL072 Op amp
CapacitorC31nFCeramic 50V
CapacitorC110nFCeramic 50V
CapacitorC2, C5, C6, C9, C10100nFCeramic 50V
CapacitorC4, C7, C810uFElectrolytic. 50V, 5mmx11mm
DiodeD1-D51N4148Small signal diode
Hardware--2 x 8 pin DIP IC socket for U1 and U3
ResistorR1,R2,R4,R8100K1/8W or 1/4W 1% metal film
ResistorR5, R6, R7100R1/8W or 1/4W 1% metal film
ResistorR91K1/8W or 1/4W 1% metal film
ResistorR35K11/8W or 1/4W 1% metal film
TransistorQ1-Q32N3904General purpose NPN silicon transistor

Constructing the Eurorack ADSR Envelope Generator

Building the breadboard ADSR circuit

Our envelope generator is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should be right at home, as the layout is basically the same. If you need a quick overview of the design and features of the N8 Eurorack breadboard, the product page provides a diagram of the layout and connections.

As with any PCB construction, we want to build low to high. Starting with the wires and then adding components in reverse order of height. Beginning with the lower profile components makes our life easier, giving us more room to manoeuvre without the taller parts getting in the way.

1. First up, we add the wires. Strip around 3mm of insulation from each wire end and feed the conductor through the appropriate holes. It can be helpful to work in sections and use masking tape to hold several wires in place before flipping the board for soldering.

2. Next, we solder in our resistors and diodes, starting with those that lie flat on the board. Again it can be helpful to hold components in place with masking tape.

3. Add the two chip holders and three transistors, taking care to orient the transistor correctly.

4. Capacitors. Solder in the small ones and then the big ones!

5. Solder in the 2×8 pin header (J3), our power connector. The power connector can be mounted on either side of the breadboard, but for a 4HP module like this, mount the power connector on the component side. 

6. Finally, solder the single-row, 90-degree, 40-pin male header to the left edge connector. Because we are using 4HP Eurorack Control Deck, the pin header is mounted on the bottom of the breadboard.

Photo of the completed breadboard layout of the ADSR circuit
Eurorack Envelope Generator breadboard layout - click to expand

Top Tip: It can be tough to strip the insulation from the short wires, like the ones connecting the power rails to the TL072 op-amp. Instead of stripping 3mm of insulation from each end, remove 6mm from one end of the wire and then cut the wire to length. You should then be able to slide the short piece of insulation along the conductor, leaving 3mm at each end.

The ADSR Envelope Generator Control Deck

N8 Eurorack Control Decks have logical front and rear sides. The front has screen-printed boxes indicating where pots, jacks, and switches are mounted. The rear doesn’t have these boxes. Components can be mounted on either the front or the rear.

In this ADSR Envelope Generator module, we mount our jack sockets and pots on the front of the Control Deck, the side with screen-printed boxes.

Check out these guides for further details on mounting components on your Control Deck.

4HP Eurorack Prototype Kit - click to expand

1. On the rear of the Control Deck, solder the short wires connecting the “d-bus” to the spare horizontal C pad below each position where the 3.5mm jack sockets (J1-J2) will be mounted. 

We are using the d-bus to create a common ground on the control deck so that we don’t need to add a ground connection for each jack on the breadboard. This saves space on the breadboard for our core circuit.

2. Our schematic shows that pin one & two of our Attack, Decay, and Release potentiometers ( RV1, RV2, and RV4 ) are connected. We can make these connections on the control deck, saving space on the breadboard. 

On the rear of the Control Deck, connect the vertical B & C pads in positions JSP2, JSP3 and JSP5, where the Attack, Decay, and Release potentiometers will be mounted. In our diagram, these are short purple wires. In the photograph, these are short yellow wires.

3. Dry-fit the potentiometers on the front side of the Control Deck. If the pots have metal support tabs on their top and bottom edges, they should be tucked under the pot’s body, as illustrated here and shown in the photographs of the module on this page.

4. Dry fit the jacks on the front side of the Control Deck. If in doubt, check out this guide for correctly positioning jacks on the Control Deck.

5. Attach the pre-drilled Eurorack panel using the hex nuts provided with the jacks and pots, checking that the jacks are centred in the holes and that each component is seated on the Control Deck. 

6. Leaving the panel attached, flip the Control Deck, and solder the pots and jacks into position. The input jack (J1) needs its switch pin connected to ground. We achieve this by soldering its switch pin to its ground pin on the rear of the Control Deck.

7. Position the 40-pin female header on the rear and solder it into place from the front. Hold it in place with masking tape if needed, solder a pad at either end and check it is perpendicular before soldering the other pads.

8. Connect the finished Control Deck to the breadboard using the pin headers.

Testing, Testing, check one two.

Before you power up your envelope generator synth module for the first time is good to do some basic tests. While not extensive, these help keep the magic smoke in the components where it belongs.

Visual inspection

  • Compare your module to the diagrams and schematic in this article. Do all the components and wires look like they are in the right place? Anything missing?
  • Inspect the solder side of the Eurorack Breadboard and Control Deck.
    • Are any of the pads shorted by solder splashes or untrimmed component leads?
    • Are all the component leads soldered? 
    • Have solder bridges indicated on the diagrams been made?

Continuity

We want to ensure there is no continuity between +12v, ground and -12v rails. We’ll do this using a multimeter.

Put your multimeter in continuity test mode, then, with your module unpowered, check the continuity between the following points on the circuit:

  • Connect one of the multimeter’s test leads to the +12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to ground. There should be no continuity
  • Connect one of the multimeter’s test leads to the -12V rail and the other to +12V. There should be no continuity

Power Up

If your new module passed the continuity tests and visual inspection, it’s time to power it up. If you have a bench power supply, it is good practice to use this for the first power-up of a DIY synth module so that it is a minimum safe distance from your other modules.

The Eurorack power connector format is sadly a little open to interpretation, and many a module has lost its life to the specification’s vagueries.

N8 Eurorack prototype boards follow the most common convention. A white stripe is printed next to the -12V end of the power connector. Typically this is where the red stripe of the power cable should be aligned. BUT not every manufacturer follows this convention, and this is a DIY synth tutorial, so chances are you made your cables, right?

Always check that your power supply is supplying -12V at the red stripe before connecting power to your synth module and that the red stripe is connected to the -12V pin on the module.

Making your ADSR look beautiful

The final step is to make your shiny new DIY Envelope Generator look the part next to those commercial Eurorack modules. The good news is you can do this with nothing fancier than an inkjet printer and some sticky-back plastic.

We have a complete guide to making labels for Euroack panels here. If the finest green is your thing, you can download our finished label, ready for printing.

Download the Simple ADSR Envelope Generator label template.

We’d love to see your build. Share your pix with us on Facebook and Instagram.

Photo of the completed envelop generator panel

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