Cause for delay

Here’s a little board:

It’s based around the Princeton Technology PT2399 chip, a CMOS echo/delay processor. The board adds the requisite support circuitry to take an audio input signal and add reverb to it.

I got this from an eBay seller who lists it somewhat hilariously as “DC 6-15V PT2399 Microphone Reverb Plate Reverberation Board No Preamplifier”. I don’t think there’s a very big plate inside that chip. Anyway, the price is right. Shipped from Shenzhen, China, I paid roughly $3.50 each… for five of them, because why not? Normally I’m not a fan of buying stuff from Chinese eBay sellers if it’s something I have a real emotional investment in actually receiving and being able to use, but this was a “let’s play with this” kind of a purchase. Worst case scenario, it wouldn’t show up or would be unusable and I would be unable to get a refund. Unlikely, and not that big a deal even if so. But in fact they did show up (took about three weeks) and worked.

I learned of this board via this instructable for building a standalone echo/reverb box. Which might be a thing I’d like to do at some point, but for now it’s turning into a Eurorack module.

What happens in the chip is it digitizes the input, stores it, and sends it via a DAC to the output after some delay. In the external circuit the output’s fed back to the chip, and is mixed with the dry input to become the board output.

The relative amount of dry and delayed signal in the output — the reverb level — is set by the potentiometer. So you can just put 6 to 15V on the VCC terminals, send in an audio signal on the input header, and take the signal off the output header into your system and, voila, reverb.

But there is a second parameter to this system, and that’s the delay time. It’s controlled by the resistance between pin 6 and ground, and on this board that’s a fixed 15kΩ SMT resistor labeled R27. However, near that resistor are three pads labeled S G G. If R27 is removed and a pot is connected to those pads, then you have a knob to vary the delay time.

You also have a way to lock up the board and render it useless, which is what happens if there’s less than 2kΩ from pin 6 to ground when the chip is powered up. If you want a very short delay time you can use a few components to make an anti latch up circuit (see the Electrosmash page linked above), or if not you can just add a series resistor.

I cut away R27, literally cut away with diagonal cutters. The delay time pot could be mounted on the board, but inconveniently enough it’s on a different edge than the level pot, which is less than ideal for a module panel! So I put the delay time pot on a perfboard and ran wires to the pads on the PT2399 board. I used a 50kΩ pot in series with a 2.2kΩ resistor. Here we are with the pot, resistor, and a power header mounted on the perfboard and connected to the PT2399 board.

I tested that and was satisfied with it. I also noticed the combination of those two boards was too long to fit between Eurorack rails. Oops. Time for the Dremel. I cut the perfboard a little shorter, leaving room to mount two jacks — then failed to find any jacks available that are perfboard friendly and would align to the same panel plane as those potentiometers, let alone to both the same panel plane and the same center line. So I cut the perfboard a lot shorter and went with panel mount jacks.

I drew up panel graphics and printed them on self adhesive label paper which I then sprayed with clear paint. I cut the graphic large, stuck it down to a blank panel, and trimmed it to size. Then I spread some CA glue around the edges. Once that was dry I sanded the edges to get them smooth and straight.

I drilled the holes using a step drill on a drill press. In the end the panel is… okay. The finish isn’t as even as I’d like it, and I think the label paper interfered with getting clean drill holes. The area around the bottom hole particularly, the first one I did, looks messy. Next panel I do, I’m going to try a few different ideas. This one’s not bad though.

Then it was just a matter of mounting the boards using the pots as support, and adding and wiring the panel jacks. On the bottom of the boards I epoxied a small piece of scrap perfboard across the two, to keep them from flexing while twisting the pots. And done.

Almost done. Needs knobs.

How’s it work? Pretty good. I mean, it’s not the world’s greatest reverb/echo module. It may be one of the world’s cheapest, though. I estimate the total parts cost was about $13.50.

I have four of these boards left. Could build a quad module! Or not. Hm, but maybe a dual module. With CV control. Use vactrols or photo FETs or something? Or maybe save the ambition for something more worthwhile?

6 thoughts on “Cause for delay

  1. Hey Mr Holmes,
    Nice tutorial you have written there. I have one question left (since i want to build such a beautifull delay too). Could you please post a Picture of the other side of the perfboard. I am very new to DiY Electronics and have no clue how to wire up the powerconnector.
    This got me thinking. where do i put the the red stripe of the eurorack power cable?

    all the best from cologne germany



  2. It’s kind of buried in the synth now but here’s a similar connector:

    You just connect one wire to the two pins at one end of the connector, another to the two pins at the other end, and a third to all six pins in the middle.


  3. Mr holmes,
    another quick question. does it mean that the wire “in the middle” is Ground? And does it run without Ground also? Cause i dont see a third cable in the picture of the module you posted in this blog post.

    All the best


  4. The power connection is for a standard Eurorack power cable, which has lines for +12V, -12V, and 0V (“ground”). Neither the module board nor my modifications make any use of the -12V rail, though, so only two wires, for 0V and +12V, connect from the power connector to the module board.When the key notch is pointing down as it is in the picture then the right two connector pins are +12V (connected to red wire) and the middle six pins are 0V (black wire). The left two pins are -12V and in this case are not connected to anything.


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