Synthrotek FOLD - DIY Build

As 2020 comes to a close, we wanted to finish up one more build. The guys over at Synthrotek sent over their new FOLD module, which is a dual function module in eurorack format. The FOLD is one part ring modulator and other part wave folder that can take both audio or CV sources. The FOLD is available as a fully assembled module, a PCB/Panel combo or the DIY Kit that we will be reviewing and building here. 

Synthrotek Logo

On with the build ... The Logic Board

First up are the resistors and there are a lot of them. While they are non-polar (direction doesn’t matter), its important to make sure you are identifying the resistors correctly. Synthrotek did a great job with their visual BOM as it shows exactly what component goes to what area on the PCB. They also have a cool resistor band identification tool that you can download here. 

There are 47 resistors and that means take your time when identifying and placing them. When we were unsure, we used our component tester that we have in the shop. Once identified, we placed them into the PCB and lined everything up to solder. We used a top-down soldering technique which kept us from us having to flip the board over and over. When we were done with the top section, we flipped the board and reflowed solder so that our solder points were perfectly placed. 

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Logic Board - Diodes and Capacitors

There are a total of 18 diodes in the kit and its VERY important to make sure they are lined up with the line on the silkscreen. Once placed, tack them into place and move  on to the capacitors (5 in total).

The electrolytic caps are polarized and its important to place them in the correct orientation. The long lead of the cap goes through the side of the silkscreen that has a ‘+’ on it. After they have been placed, pop in the 3 other ceramic capacitors and tack those down as well. Make sure they are standing a little above the PCB surface. If the ceramic coating is in the holes on the PCB, you could get varied results when using the module.

Time to move onto the IC’s and transistors. 

Synthrotek FOLD - Time Lapse Build

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Logic Board - ICs and Transistors

Next up are the IC sockets. Take care when populating these to not bend any of the legs, and to line up the notch in the end with the same notch that is displayed on the silkscreen. This notch indicates where pin 1 of the IC is. When you have them populated, carefully flip your project over and solder everything in place.

** A trick to getting these nice and flat is to only solder one or two pins of each socket, and then reflow the solder joint while gently pushing on the top of the socket to seat it flat against the PCB. Then continue on and solder the rest of the pins. **

The transistor and the 5v regulator are next up. These are both polarized, so make sure when populating them that you align the flat edge of the component with the corresponding flat line on the silkscreen.

Also, pay close attention to the number on the side of the component, as it is very easy to mix up the voltage regulator with the transistors (they look alike). Follow this step up with the 10pin shrouded header pins that need to face according to the silk screen. Lastly, tack down the remaining trim pot and call the logic board complete. Time to start the control board.

Control Board - the “Kenny Rogers” board

Before you start placing jacks and sliders, you have to place the 90 degree header pins. There is 1 set of these and they have to be mounted to the control board and not the logic board first. If this is done the other way around, you will not be able to mount the sliders the correct way and can end up causing way more work than what it originally called for.

As you are placing these, make sure the long straight pins are going through the “Kenny Rogers” board  and they are flush with the PCB. Solder one pin on the other side of the board and test fit the control board and the logic board. If everything lines up, tack down the rest of the pins on the control board. 

After soldering all of the header pins to the back control board, flush cut the excess pin headers protruding from the other side of the board (the non-Kenny Rogers side). Then, take your trusty iron and reflow the solder on the cut pins so that the sliders can fit over the top of the pins with no issues. 

Control Board - Jacks and Sliders

First and foremost, screw down the metal stand off, leaving the black hex screw for later. Place all of the jacks one by one making sure to line them up with the ground plane on the PCB. It’s ok if they touch, as these are really close quarters. Flip the board over and tack down the ground pins of each jack to hold them in place as you begin to place the sliders. Do not solder anything else at this moment. 

The sliders mount only one way and its pretty easy to see what way they go. Place these as well as the switch as we begin lining everything up to the panel. After making sure everything is in its correct position, place the panel over the sliders and jacks and hand tighten the nuts to the jacks so that the panel will stay in place. At this point, you can screw in the black hex screw to help keep the upper portion of the panel in place. 

Hang in there… We are almost done….

Once everything is lined up on the control board, flip the board over and tack down the rest of the pins on the jacks, switch and the sliders. We used our Synth DIY Tool Set to tighten down the nuts on the jacks, as we got ready to complete the build.

The final part of the build is mating the two boards. Bring the two boards together and line up the right angled pins. Tack down middle pin on the logic board and once everything is flush, tack down the rest of the pins. Congrats!! You have just completed building the Synthrotek FOLD module!

Prior to placing into the rack, you will want to test the module and calibrate it. Synthrotek supplied easy to follow instructions on how to calibrate the module and it takes all of about 5 min.Quick and easy. Once this step is complete, pop the module in your rack and start patching!

The Completed FOLD Module

Calibration and Testing...

Prior to patching, you need to go through the calibration process which is listed on the assembly guide on Synthroteks’ website or below.

Tools you will need:

  • A Eurorack system with a sine or triangle wave oscillator, a second audio signal and a CV source.
  • An oscilloscope and buffered mult (optional, but helpful)

With your Eurorack system powered off, plug the FOLD into your eurorack power via the included 10 to 16 pin power cable. All three LEDs on the pots should light up.

Ring Mod Calibration

If you are building FOLD v2.2 you can calibrate your module using the trimmer pot on the PCB marked R48. To do this, patch an audio signal into Ring Mod input 1 and do not patch anything into Ring Mod input 2. Next, trim R48 until there is as little audio coming out of the Ring Mod output as possible. You can check this by ear, or better yet with a scope.

The ring mod circuit only uses the three bottom left jacks; none of the other jacks or controls apply to it.

  • Patch two separate audio signals into IN 1 and IN 2.
  • Patch the output into a mixer; the two signals should combine together. Some frequency combinations will sound better than others, so mess around with it a bit!

Wave Folder

General functionality:

  • Slide all the pots down to 0%.
  • Plug a sine or triangle wave into WAVE FOLD IN jack.
  • If using an oscilloscope: patch the right-side (wave folding side) OUT into a buffered mult input, and patch the outputs of the mult into your o-scope and output/line level mixer.
  • With the MIX pot at 0%, you should have a dry signal. On the o-scope, you should see a normal triangle or sine wave.
  • Slide the MIX pot to 100%. Your amplitude should flatline or come close to this. There should be little or no audible signal.
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Features and Specifications


  • Ring modulator
  • Taps switch selects 2, 3, 4, 5 or 6 stages of folding
  • Bipolar CV input over Level
  • Offset for adjusting wave symmetry
  • Wet/dry Mix

Current Draw:

  • +12V: 50 mA
  • -12V: 40 mA
  • +5V: 0 mA


  • Module Height: 3U
  • Module Width: 4HP
  • Depth: 60mm
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Synthrotek FOLD - Purchasing Options

Synthrotek has a couple different options when it comes to purchasing the VERB module. We wanted to keep it simple by just placing images of the options and linking out to the direct purchase. The prices are excellent and the quality of this build is way worth it! Head on over to SR and tell them we sent ya!

Assembled Module
Panel/PCB Combo
White Synthrotek Logo

Synthrotek FOLD - Documentation

Our Final Thoughts . . .

As we are just getting to know the FOLD module, we have to say that the slider series from Synthrotek really stands out in our rack. The ambient lighting and the small footprint allow it to be a strong stakeholder in the case. With its dual functions and intermediate build, there is no reason not to have this module in your rack. 

Wavefolders are one of our favorite things to play with and when patched with CV, it can really add some flavor to your game. Do yourself a favor and head on over to the links provided above and pick one up. Tell the gang at Synthrotek that we sent ya. 

Comments or Suggestions?

If you have questions about the build process or suggestions/feedback about this article, feel free to let us know by contacting us  We look forward to hearing from you!

Until the next build … 

~ f i N

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