DIY Eurorack VCA Module

In the world of modular synthesizers, the saying goes, you can never have too many VCAs, so let’s squeeze two onto a 6HP Eurorack module using the venerable LM13700 operational transconductance amplifier chip.

A voltage control amplifier ( VCA ) is a synthesizer module that uses voltages from one source to control the amplitude of voltages from another source. For example, patching the control voltage output by your ADSR envelope generator to the VCA’s CV input and an audio signal from one of your oscillators to the VCA’s input will allow the envelope to modulate the amplitude of the oscillator. The length and shape of the envelope will enable us to create dynamic volume changes, such as the gradual swell of a pad or the sharp attack of a percussive sound.

Here we’ll use the N8 Synth Eurorack breadboard and a 6HP Eurorack Control Deck to build a DIY Eurorack VCA module. The VCA circuit uses the LM13700 operational transconductance amplifier ( OTA) chip. As the LM13700 gives two OTAs, we’ll build two VCAs in one 6HP module – you can never have too many right? 

Photo of the N8 Synth Eurorack VCA shows both the front panel and breadboard layout

Credit where credit is due. The LM13700 was first introduced in the 1970s, so it is no surprise that many VCA circuits have been designed around it in the intervening years. The VCA circuit presented here is based on a design published by Electric Druid. Their site has an excellent and detailed explanation of the design considerations of working with the LM13700 chip and lots of cool synth goodies in their shop, including the LM13700, if you need to get your hands on some.

The LM13700 VCA schematic

Circuit schematic for LM13700 synthesizer VCA
Eurorack Dual VCA schematic - click to expand
Eurorack breadboard and control deck layout for a dual LM13700 VCA
Eurorack Dual VCA layout - click to expand

This being a dual voltage controlled amplifier circuit, our schematic includes two VCAs, Ch1 & Ch2. The two VCAs are functionally identical, so we’ll focus on channel one and save me some typing and you some reading.

Our Eurorack VCA can accept two control voltages via J1 and J2. J1 assumes that we have a standard(ish) 0-5V range, while RV1 allows us to attenuate the control voltage arriving at J2 for any CV sources with a range above 5V.

RV2 lets us set an offset, or you could use it as an over-engineered attenuator. The three CV sources are summed by the op-amp U1B, which scales them to a level the LM13700 can handle, converting the combined control voltage to the current required by the Iabc input of the LM13700. This current sets the OTA’s gain controlling the attenuation of the audio signal arriving at J3. R5-R7, R17, and R21 set the gain of U1B, giving us the scaling.

Driven by the output of U1B, Q1 supplies the current to the OTA via R18. R18 ensures that the current to pin 16 of the LM13700 cannot exceed the maximum 2mA if one of the CV inputs is accidentally shorted to the 12V rail. D1 protects Q1 from negative control voltages.

C1 provides AC coupling while the voltage divider, R8 and R14, attenuate the incoming audio signal to a level the LM13700 can handle. U3B converts the current output of the OTA back to a voltage and returns it to a nominal 5V peak-to-peak signal.

As noted above, the VCA circuit is based on one published by Electric Druid. We found that with sufficient noise on the CV inputs, the VCA may not fully close. To remedy this and ensure we hear the sound of silence when there is a 0V CV input, RV5 lets us take the control voltage a little below 0V when the RV2 is fully counterclockwise. If you like a bit of noise, you could replace R1 and R3 with 120K ohm resistors, remove R2, R4, RV5, and RV6 and connect pin 1 of RV2 and RV4 to GND.

Bill of Materials

We’ll build our Eurorack dual VCA module using a 6HP 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 VCA, and the Eurorack Control Deck makes mounting our controls a breeze.

The 6HP Eurorack Prototype kit and the pots and jacks 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 synth modules, stock up on these components, as they are ubiquitous in the schematics you’ll find online.

CategoryRefValueNotes
6HP 2x6 Eurorack Prototype Kit--Includes Eurorack Breadboard, 6HP Control Deck, 6HP Panel, 40-pin headers (PH1 & PH2), and 2x8 power header - J9.

Add to basket

HardwareJ1-J8PJ 3001F3.5mm vertical mounting jack socket

Add to basket

PotentiometerRV1, RV3100K9mm vertical PCB mounting trimmer ( Alpha RV09AF-40 style )

Add to basket

PotentiometerRV2, RV4100K9mm vertical PCB mounting potentiometer ( Alpha RD901F-40 style )

Add to basket

Hardware--2x Davies 1900H style knobs, white

Add to basket

ICU2LM13700Dual Operational Transconductance Amplifier
ICU1, U3TL072TL072 Op amp
CapacitorC7-C14100nFCeramic 50V
CapacitorC1,C2,C15,C1610uFElectrolytic. 50V, 5mmx11mm
CapacitorC3-C41nFCeramic 50V
CapacitorC5-C620pFCeramic 50V
DiodeD1-D21N4148Small signal diode
Hardware--2 x 8 pin DIP IC socket for U1 and U3
Hardware--16 pin DIP IC socket for U2
PotentiometerRV5-RV650KRM-063 6mm Vertical Variable Trimmer
ResistorR1,R3,R8,R12100K1/8W or 1/4W 1% metal film
ResistorR18,R2012K1/8W or 1/4W 1% metal film
ResistorR2,R4180K1/8W or 1/4W 1% metal film
ResistorR23,2418K1/8W or 1/4W 1% metal film
ResistorR21,R22,R23,R261K1/8W or 1/4W 1% metal film
ResistorR5,R6,R7,R9,R10,R11330K1/8W or 1/4W 1% metal film
ResistorR17,R1933K1/8W or 1/4W 1% metal film
ResistorR13-R16620R1/8W or 1/4W 1% metal film
TransistorQ1-Q22N3906General purpose PNP silicon transistor

Constructing the Eurorack Dual LM13700 VCA

Building the breadboard Eurorack VCA circuit

Our VCA circuit is built on an N8 Eurorack breadboard. If you’ve used a sprung breadboard before, you should feel 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 an overview 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 end of the wire 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. For this module, it may be easier to leave the wires highlighted in orange, on the diagram, until last, as these go over or are routed around the components. 

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

3. If you are using DIL sockets for the ICs add these next. If not, solder in the two TL072 op-amps and LM13700.

4. Capacitors. Solder in the small ones and then the big ones, along with the two trimmer pots RV5 & RV6. We’ve shown vertical-mounted RM-063 trimmers. You could substitute these for horizontal RM-065s by running the wires connecting pins one and two under the body of each trimmer.

4. Solder in the 2×8 pin header (J9) that is our power connector. This can be mounted on either side of the breadboard.

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

If you skipped the wires coloured orange on the diagram in step one, don’t forget to add them now.

Photo of a Eurorack breadboard LM13700 Voltage Controlled Amplifier circuit

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-amps. 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 LM13700 Dual VCA Control Deck

Our LM13700 VCA module has eight PJ-3001F 3.5mm jack sockets for CV and audio signals and four potentiometers to set the level of those signals. We’ve used 9mm Alpha RD901F-40 style pots with Davies 1900h style knobs for the level controls and 9mm trimmers to attenuate any hot CV sources over 5V.

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. The side of the Control Deck a pot is mounted on determines the order its pin connections arrive at the edge connector, on the front 1-2-3 on the rear 3-2-1. 

In this VCA design, we mount our jack sockets and pots on the front of the board, the side with screen-printed boxes. If in doubt, reference the photos and layout diagram before soldering.

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

6HP Eurorack Prototype Kit ( click to expand )

1. On the front of the Control Deck, add the short wires connecting the “d-bus” to the spare horizontal C pad below each position where the 3.5mm jack sockets (J1-J8) will be mounted. Solder these in place from the rear.

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. On the rear of the Control Deck, add wires between the d-bus and the vertical A Pad in positions 5 & 6, where RV1 & RV3 will be mounted. Solder these in place on the front of the Control Deck. Again we are using the d-bus to connect pin 1 of the pots to ground.

3. Position the 40-pin female header on the rear of the Control Deck and solder it into place. 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.

4. Dry-fit the four potentiometers RV1 – RV4  on the front side of the board, as illustrated. As explained here, if the pots have support tabs on their top and bottom edges, these should be bent under the pot’s body.

5. Dry fit the eight jacks on the front side of the board, as illustrated. If in doubt, check out this guide for correctly positioning jacks on the Control Deck.

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

7. Leaving the panel attached, flip the control deck, solder the pots, and jacks into position. Jacks J1-J3 and J5-J7 need their switch pin connected to ground. We achieve this by soldering the two pins together on the rear of the Control Deck as illustrated.

8. Connect the finished Control Deck to the breadboard using the 40-pin headers.

Important: the side of the Control Deck pots are mounted on changes the order in which their connections arrive at the edge connector. On the front, 1-2-3. On the rear, 3-2-1. Make sure you mount them on the correct side of your Control Deck.

Testing, Testing, check one two.

Before you power up your 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.

Trimming the VCA

You may find that your new DIY VCA doesn’t fully attenuate the audio signal when the control voltage falls to 0V. The two trimmers, RV5 and RV6, allow the cut-off point to be tuned by applying a small negative voltage to the CV input, ensuring complete attenuation with a 0V CV.

To adjust the cut-off for Channel 1, connect an oscillator ( or another source of audio ) to the VCA’s input (J3) and the VCA’s output (J4) to a mixer, your synth output module or an oscilloscope if you want to be scientific. Leave both CV inputs unconnected and turn RV2 fully anti-clockwise. If you can hear the oscillator adjust RV5 until the audio signal is fully attenuated. Now slowly turn RV2 until you can hear the oscillator again. If your adjustment of RV5 results in a dead spot ( you need to turn RV2 a long way before you hear the oscillator ), adjust RV5 in the opposite direction to improve the response of RV2. Repeat for Channel 2.

Making your Eurorack VCA look beautiful

The final step is to make your DIY synth module 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 Ford Capri Orange is your thing, you can download our finished label, ready for printing.

Download the LM13700 VCA label template.

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

Photo of the front panel of the DIY Eurorack VCA module

10%

off, especially for you 🎁

Join the N8 Synth mailing list to receive an exclusive discount and keep up to date on our latest products, tutorials & offers!!

We don’t spam! Read our privacy policy for more info.T&Cs apply.

Shopping cart
Sign in

No account yet?

Start typing to see products you are looking for.
Shop
0 items Cart
My account