Waveform Magazine Project 1 - PT2399 Delay Module - DIY Build

When it was announced that there would be a magazine dedicated to modular synthesis, you can imagine the feedback from the modular community. Waveform Magazine was born in the summer of 2019 and upon its release it created a following.

To much of everyones surprise, not only was this a magazine that reviewed modules, manufacturers, artists, etc.. it also contained its very own DIY projects.

Project #1 was a upcycled karaoke reverb PCB that would be transformed into a eurorack delay. Otherwise known as the PT2399 Delay or Waveform Magazine Project #1, this little 5HP module inspired a whole new group of DIYers due to its new found glory of being published.

Waveform Magazine PT2399 Delay Module

PT23.. what?

At the heart of Waveform Magazine Project #1 is the PT2399 chip. The PT2399 is a CMOS echo/delay processor developed by Princeton Technology Corp.

This digital chip includes an ADC (Analog to Digital converter), 44Kb of RAM to store the samples and a DAC (Digital to Analog converter). This chip was created as a simple solution to add delay/reverb/echo to karaokes and set-up entertainment systems. Pretty rad eh?

The PT2399 is readily available and has a minimum delay of 30ms and a maximum of 340ms (that could be extended up to 1 second at the expense of sound quality) which makes it perfect for delay, echo and reverb effects.

Waveform logo yellow

Ok, so what does all that mean?

What it means is, the Waveform Magazine Project #1 uses a PCB from a karaoke machine, modified it and made it into a delay module for your eurorack system. 

This module was originally mentioned here, as well as on a Synth DIY Facebook page.  The gang at Waveform Magazine brought the PT2399 Delay module to life by making it available as a module/kit for the eurorack community. (Thanks guys!). There is a difference between the two modules though.

The original did not have a way to modulate delay time or feedback, Waveform Project #1 was modified to do so, giving it more bang for the buck. Let’s get onto the build shall we?

Project #1 - The Build

While there are several options of ordering available, we opted for the full DIY kit. This included the new Waveform panel, the “karaoke 2399 reverb” board and all the necessary components to build the completed Project #1.

Upon receiving the kit, we cracked it open and began laying out our components. The Waveform Magazine Project #1 can be daunting at first glance. This is primarily due to the all of the jack wiring and how it all starts to look like a spaghetti monster prior to panel mounting. We will try to break it down in the simplest of ways and hopefully it will make sense.

PT2399 Delay Module - Time Lapse Build

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First up, you have to desolder the 1/4 input and output jacks on the 2399 board. Take precautions not to lift the traces on the board or you could have some odd issues. Along the bottom of the board you will also see the power input.

Desolder this piece as well as it will be replaced with a 10 pin power shrouded header. You will also be removing all but 3 pins of the header so that it will sit flush with the 2399 board and allow power to be displaced appropriately.

Right next door is the R/L IN header. We pulled that off and cleaned up the solder points because we would be coming back to that later in the build.

2399 Board 1

Bill of Materials (BOM)

  • 1 x pt2399 based karaoke pcb
  • 1 x Waveform delay faceplate
  • 4 x 3.5mm mono audio jacks
  • 2 x 5mm LEDs (we used red LEDs)
  • 2 x LDR
  • Shrink tube (for vactrols)
  • 1 x 10 pin shrouded header
  • 2 x 1/8w 1k resistors
  • Extra wire
  • Eurorack power cable (10/16)

Purchasing Options:

  • Faceplate only = 10$
  • PT2399 Reverb Board = 8-10$
  • Full DIY Kit = 45$
  • Assembled = 109$

Trim the pots and make some vactrols (DIY style)!

After removing all of these components, you want to head on over to the pre-existing pots and cut the tab off the front facing portion of the pot. This ensures that the panel will mount flush when you get to that point in the build.

Vactrols are next. We used the supplied 5mm LED and the LDR (light dependent resistor) and lined them up so the top of each component was touching each other. We rolled this up in electrical tape and then covered it with the supplied shrink tubing. (Pat yourself on the back, you just made a DIY Vactrol). Set these aside for now and now, it’s time to make spaghetti.

Spaghetti Time...

At the time of our build, we reached out to the editor of Waveform for a little clarification on the wiring and within a few hours, they updated their build doc and emailed us back to let us know that they were on their game. (Talk about customer service!!). We have supplied their photos in this write up for reference.

The jacks consist of 4 jacks. IN, OUT, Feedback Modulation Input and Time Delay Input.


  1. wire the ground to the ground of the OUT.
  2.  wire the tip to the MIC2 (where you removed the 1/4 input jack) and the LIN (next to the red LED) on the backside of the 2399 board.


  1. the ground wire is connected to the INPUT jacks ground. 
  2. the tip of the jack will be connected LOUT at the top of the board. This will also be used to connect the ground wires from both jacks. You can use about 4 inches of the supplied grey wire to make these connections. Strip the grey sheath, cut the red wire, strip the white wire and connect it to the tip of the output. The remaining wire strands will be rolled up and attached to the grounds. 


  1. Take the Vactrol and connect one lead of the LDR side to the ground lead [shorter lead] of the LED and use wire to attach that to the ground on a new jack. This new jack is our feedback modulation jack. Connect this jacks ground to either the input or output jack so that all of the grounds on the three jacks are connected. the ground wire is connected to the INPUT jacks ground. 
  2. Next, solder a 1k resistor to the positive lead [the longer one] of the LED, and solder the other side of the resistor to the tip of the feedback modulation jack. The other LDR lead is to be attached via wire to the topmost pads [bridge the two topmost pads together] of the feedback pot on the underside of the PCB. 


  1. Connect one leg of the LDR to the ground lead [shorter lead] of the LED and attach that to the ground on a new jack. This new jack is our delay time modulation jack. Connect this jacks ground to the grounds of the other jacks so that all 4 jacks grounds are connected.
  2. Solder a 1k resistor to the positive lead [the longer one] of the LED, and solder the other side of the resistor to the tip of the delay time modulation jack. The other LDR lead is to be attached via wire to the topmost pads [bridge these pads together too] of the delay [2nd from top] pot on the underside of the PCB.

Putting it all together...

It’s time to take everything and mount to the panel. Start with the input and output jacks and attach them to the panel. Tighten the jacks down with the provided nuts. Next, mount the feedback modulation and the time delay jacks to the panel, careful not to cross any metal to metal anywhere.

Pro-tip: Carefully cut a corner out of the lower portion of the PCB out. This will allow the jack to mount easily and not put pressure on the panel. This also keeps the panel from being bent when placing all of the components together.

Prior to attaching the ribbon cable (ensuring that the red stripe is opposite the red LED), you will place your knobs on the pots. We didn’t really care for the knobs that came with the kit, so we opted for clear Davies 1900H clones.

That’s it, you have completed the module!

Before mounting the Waveform PT2399 Delay in our case, we tested it with our Synthrotek TST module and it was ready to go. We popped it into our rack and fired it up. The red LED switched on and it was ready to be patched.To our surprise, this thing was a little beast. 

We patched an oscillator through it, kick drums and even samples from our TG ONE. It handed everything we threw at it. (Make sure and check out the quick links below).

Note: the output is only mono, so you would need to patch it into something else prior to taking it straight into your DAW.

PT2399 Delay Testing
PT2399 Delay Testing

Available Resources

The Final Words…

In the end, the Waveform Magazine PT2399 Delay module was a fun and semi-daunting build. You really need to pay special attention to the jack wiring and panel mounting. You can easily cross things up. We have to admit that we did this once, and had to order a new 2399 board when ours went up in smoke. (hey, no one is perfect and we had one too many Rockstars when we were building the first time around…LOL!).

Waveform Magazine did an excellent job of packaging and panel design. Though the assembly instructions were not 100% clear at the time time of our first build, they were quickly updated with concise instructions on jack wiring within hours upon our request. After seeing a quick wiring photo, we knew exactly what was to go where. (Another big shout out to WF).

This project along with its brethren, the Gateway Oscillator and The Switchenator not only helps build upon the Waveform family, but it gives builders another chance to try their hands at DIY.  There is no reason that anyone with a modular system should miss out on this sweet little delay. 

Update: At the time of this article, we ordered up 2 more kits and are in the process of completing one of them. Since we have multiple racks (3 total), we wanted to have one in each space. One can’t have enough delay in your rack and this kit is inexpensive and meaty all at the same time.

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