Dual Decay Eurorack Point-to-Point Circuit

The purpose of this instructable is to show how you can make a DUAL DECAY circuit for your modular synthesizer. This is a point-to-point circuit free of any pcb and demonstrates another way to build functional synthesizer circuits with minimum parts and low cost. It can also be a fun way to learn more about an electronic circuit and how it works.

I do not take credit for this circuit design. It is exactly 1/2 of the BMC043 4x Decay. You can learn more about the original design and what it does here. Barton Musical Circuits is a great resource for learning more about synthesizer module designs because all the projects have schematics available with explanations of what's happening in the circuits.

I took half of this circuit to make 2 decay units from a single TL074 in a point-to-point version. The parts cost is less than $5.00 for the final version I made with a laser-cut panel.

I write this tutorial as a document to my own learning journey and this project requires that you have some basic understanding of electronic components and how to use a soldering iron. There's only one other person that I know of at the moment doing this type of modular synth construction. You can find his tutorials here.

If you are game enough to put this together, let me know how it turned out. Run into any trouble, find a mistake, something I wrote doesn't make any sense at all? Hit me up so I can take a look and make edits to improve the instructions.

THE BOM

1x TL074 quad op amp

2x 1n4148 general purpose diodes

2x LED any color

2x 100k potentiometers (B104)

2x knobs

4x jack sockets

1x 10pin eurorack power header (optional depending on your format)

RESISTORS
2x 10r

2x 220r

3x 1k

2x 2.2k

2x 2.7k

3x 100k

CAPACITORS
4x 10uF electrolytic cap

4x 100nF ceramic cap

This is the eurorack panel design I put together for laser cutting.

You can download this file and modify it as you see fit.

DOWNLOAD PDF

This project can be modified to fit your needs and synth format.

Good for both circles and squares.

The pins on the potentiometers are numbered 1,2,3 from left to right. Bend pins 2 and 3 together and solder for both. (pic 1)

Now we are working with 2 of the 10uF electrolytic capacitors and the 220r resistors and the diodes. One leg of the 220r resistor will be twisted with the anode (long leg) of the 10uF capacitor and the cathode leg of the diode. Twist those legs all together and spread them out in a star like it's shown in (pic 2).

Each potentiometer will have the cathode of the 10uF capacitor soldered to the joined pins 2 and 3. The other side of the 220r resistor will go to pin 1 of the potentiometer. (pic 3)

Place the potentiometers, jacks and LEDs as shown in (pic 1)

For the jacks and LED placements, orient them so that all ground leads can meet in the center as shown in (pic 2). Solder these points together in the center.

INFO: The TL074 is a quad op amp with 14 pins. In this circuit, one op amp of the TL074 is being used as a comparator to "compare" two voltages and output a 1 at the positive side or a 0 at the negative side, in order to indicate which voltage is higher.

Orient the chip so the divet is point away from you, the leg at the top left is pin 1 and the leg on the right is pin 14. View the pinouts and numbering here for more detail on how the legs are numbered. This is important because the circuit will not work if you make the wrong connections. You should be able to follow along with the photos as a reference.

INSTRUCTIONS:

1. Bend up the power pins 4(+) and 11(-) (pic 1).

2. Bend pins 6 and 7 together and solder as well as pins 8 and 9 together and solder. (pic 1).

3. Bend pins 2 and 13 up and back towards each other then solder. See (pic 2).

Everything we've done on one side of the TL074 is the same as the other side so far.

NOTE: When building point-to-point circuits, the path of connection is really up to you. The main goal is to make all the correct connections so the circuit will work. There is a good amount of creative construction that can be learned through trial and error. Building the same circuit several times can reveal better ways to do things. Throughout this build, I am positioning the components and trimming legs while being mindful of placement to keep things clean, compact, and prevent the possibility for short circuits. As the project is built up, the structure will become reinforced and sturdy.

Now for the 100k/1k resistor voltage divider.

Twist together the ends of a 100k and 1k resistor, solder the twisted legs and then trim. (pic 1)

The trimmed twisted end will get soldered to the folded up pins 2 and 13. 1k goes to the ground connection and 100k goes to +power connection pin 4 of the TL074. (pic 2)

NOTE: The 1k resistor is placed so that the leg trails off the back end of the TL074. Just let that hang there and we will use this leg as our ground connection for the rest of the circuit.

INFO: Your input signals can be a gate trigger, lfo, audio source, anything you feed it and as long as it's giving off at least 1v, the decay will be engaged.

For this build, our input pins on the TL074 are pins 3 and 12.

- Cap those inputs by placing a .01uF (100nF / 104) ceramic capacitor on pin 3 and another .01uF cap on pin 12. View pics 1,2,3,4. The other ends can hang free for now. Those points will hook up to your input jacks later on.

- Next twist 2 100k resistors binding their legs together. These twisted legs connect to ground. The other ends of the 100k resistors will connect to pins 3 and 12. Solder in place and trim off the excess leads. View pics 5,6 for reference.

NOTE: .01uF 100nF are two different ways to reference the same value capacitor and ceramic discs are usually marked with number codes for easy identification. The capacitor code for 100nF is 104.

Next we will add the 1k current limiting resistors that will protect the LEDs on each channel.

- Connect 1k resistor to pin 6 and 7 and another 1k resistor to pin 8 and 9 on the opposite side. The other end of those resistors can hang for a moment.

NOTE: The trailing resistor legs can dangle for now. They will be connected to the anodes of the LEDs soon.

We are getting very close, but before we go any further let's complete the ground connections for the potentiometers and jacks. With a scrap piece of wire, solder connections at the joined pins 2 and 3 for each potentiometer and run that wire to the center ground point for the jacks and LEDs.

Let's start securing the TL074 to the panel now. Orient the chip above the channel 2 potentiometer so the ground "tail" hovers above the jack sockets. The joined leads from the capacitor resistor pairs wired to each potentiometer can be connected next. Top pot is channel 1 and the bottom pot is channel 2.

The diode of channel 2 pot connects to pin 14. The twisted legs coming from channel 2 pot go to pin 10 as shown in (pic 1). Solder the twisted pair of leads from pot 2 to the diode connected at pin 14 since these are in close proximity already.

Since channel 1 pot is further away, I will use a wire to complete those connections as referenced in (pic 2). The twisted legs coming from channel 1 pot go to pin 5. The diode of channel 1 pot connects to pin 1.

Complete the ground connection from the tail of the dead bug to the central ground point of the jacks and LEDs as you can see in (pic 3).

Those dangling legs of the current limiting 1k resistors can now be connected to the LEDs.

The 1k resistor coming from pins 6 and 7 of the TL074 go to the anode of the LED for channel 1. See (pic 1).

The 1k resistor coming from pins 8 and 9 of the TL074 go to the anode of the LED for channel 2 and since it is further away I used a piece of blue wire. See "partially" in (pic 2).

Pair up a 2.7k with a 2.2k resistor and twist one side of their legs together. Do this for each pair.

Channel 1.
The leg of the 2.7k resistor will solder directly to pin 7 for channel 1 output and the leg of the 2.2k resistor goes to the ground rail "the tail". The twisted legs of both will connect to the tip of channel 1 output jack.

Channel 2.
The leg of the 2.7k resistor will solder directly to pin 8 for channel 2 output and the leg of the 2.2k resistor goes to the ground rail "the tail". The twisted legs of both will connect to the tip of channel 2 output jack. Sorry for the blurry pic, this section is shown in (pic 2) with a green wire going from the twisted legs to output jack.

NOTE: At this point the once wobbly floating TL074 should now feel very secure in place.
**I will test this instructable by building it and make sure to get good photos of these to include on this step.

So, this circuit is done once you power it. This part depends on your specific application. I build in the Eurorack format and use shrouded headers and ribbon cable to connect to a power distribution board that receives power from an AC switching power supply. Each person may do this a little differently but what remains the same for the circuit to work is powering the positive and negative rails with +/- 12v and connecting the ground.

You will also want to add a low-value resistor (10r) coming from the power source for positive and negative rails and some capacitors to filter noise from the power rails.

For the positive rail +12v, use a 10uF electrolytic capacitor. The anode of the cap goes to the positive rail and the cathode goes to ground. Also, place a .01 (100nf / 104) in series going from positive rail to ground.

For the negative rail -12v, use a 10uF electrolytic capacitor. The cathode of the cap goes to the negative rail and the anode goes to ground. Also, place a .01 (100nf / 104) in series going from negative rail to ground.

This is shown in the main schematic for power.

FINAL NOTES:

1. I've included images from 2 different builds of this same circuit to show the final work. Notice they each turned out a little different as I made adjustments to tighten up the build and found better ways to place components. So please remember, if you don't get it to work on the first try, just keep trying. It really doesn't cost that much to fail a few times. You'll get it eventually and you might find better ways each time.
   
   
2. This single module is pretty useless on its own. I plan on doing more of these Instructables for other types of modules so this one can have friends and they can all play together. In the meantime, there is only one other person I know of doing point-to-point Instructables for modular synths. Modular for the Masses (M4TM). Juanito is pretty crafty and is responsible for sparking my curiosity in P2P circuit building. So check out his page where there are more really cool Instructables to build P2P modular synth modules. Those projects will pair nicely with this one. Juanito's page is here.