Digital Delay Effects: PT2399 IC

I am going to make a digital delay effects using two commercial delay effect modules with PT2399 Echo IC processor for my DIY euro rack module project, which will be coupled with Doepfer BBD delay (analog) module. Of course we can use more than three delays effects to derived various regular or irregular rhythmic (time)-textural based effects. However, the objective of building this effect is to have affordable and less building time digital delay effects with multiple delay parameters that can be adjusted with CV (0-5 +v and 0-10 +v) through vactrol circuit based, resulting unique experimental processed found sound such as using sensor based control voltage.

So let’s get started by referring to the delay effect principle process diagram below to identify the adjustable delay processing parameters which is similarly applied into the PT23999 IC. Click here to understand more on what is delay effect and how it works.

From the diagram above and further referencing on this (click me) in-depth PT2399 IC application analysis we can ‘hack’ the commercial PT2399 delay effect module for adjustable delay processing parameters as follows:

  1. Input (In)
    • Installing Buffered Attenuator with 100K ohm linear potentiometer at Audio IN on the module for adjusting the amount of incoming audio signal. The attenuator can be voltage controlled via Vactrol CV (0-5 OR 10 +v) as switch for ‘tap’ delay effects.
  2. Delay Time (DT)
    • Installing 100K ohm linear potentiometer at S-G-G pins on the module with 2K pull-up resistor whereby S to 2K resistor to ground pot (1), G mid to out pot (2), G right to Vin pot (3). Note that to enable the installed potentiometer with 2K pull-up resistor, remove the resistor R22 block CAREFULLY little by little at the center of the resistor on the module with small sized grinding or carving bits mini drill until you left with light shiny solder points (for less primitive approach, you can use hot solder if you know how). The potentiometer can be controlled via Vactrol CV (0-5 OR 10 +v) in parallel with potentiometer Ground and Vin. In addition, pitch shifting effects is observed during adjusting the pot (potentiometer) values through ramping gain (smooth knob turning) = ? + LFO cv out = cool!). From here, you can achieve echo effect (reflected sound) through 100% wet mix below.
  3. Mix (Dry/Wet)
    • As it is. However, this can be controlled via Vactrol CV similar to DT. With roughly between 30-40% DT, adjusting the Mix pot LESS than 100% will produce reverb effect (difference time of arrival between reflected sound and original sound source produce some sense of space). You can experiment this parameter by replacing the default 50K pot with 100K pot to hear how much texture changes occurred from the dry sound source (try the experimentation via breadboard).
  4. Feedback Multiplier (Feed)
    • This is a distortion feedback effect through looped feedback process and the amount of this effect can be re-adjusted/ manipulated by removing the resistor R21 block which has 20K ohm resistor value (click here to learn how to remove it through hot soldering) and replace it with 20K or 22K ohm linear potentiometer, which can be further controlled with Vactrol CV like above. The R21 micro-resistor resulting 0 feedback multiplication at 20K ohm. By varying the resistor value below 20K ohm we can have feedback multiplication between 0.00 to 1.00 (example 10/20K = 0.5 multiplication). Note that if we want to us 20K or 22K ohm pot for this parameter, the default Mix 50k Ohm pot may need to be replaced with 100K ohm pot for non-linear output. (click here if you need to know why). But you can experiment it with just a 100K ohm pot for both with additional 10K ohm resistor at Mix pot (click here for more info) and listen to the changes sound and create your own magic recipe!.
  5. Output (Out)
    • Buffered with op-amp.
  6. Power (Reset)
    • Installing on(off) 1NO/ 1NC momentary switch –_|_– to power reset the IC (resetting/ clearing the delay effects if things getting more ‘unwanted’ chaos or noise). Again, you can add Vactrol CV similar to input for such as if you want to apply random CV. Note that the module is specified with built-in power regulator (to 5 +v which the IC working voltage is between 4.5-5 +v) and capable of handling 15 +V (max).

The PT2399 Echo IC is specified as able to provide 340ms (0.34 sec.) max DT and 30ms (0.03 sec.) min DT at specific clock speed (pin 5). Therefore, you can feed the primary PT2399 output into secondary PT2399 input to double the maximum DT (2 x 340 ms) = chaining more PT2399 out-in!

Photos will be uploaded soon.

Conclusion, there is a lot of time-based effects that can be derived and produced with PT2399 delay (ECHO) IC processor and you can achieve ‘grain’ delay effects with much shorter DT (hint: shortest DT, 100% wet and more chaining for pitch shifting and longer delay). However, I will move on with building the DIY vactrol circuit using affordable LDR and white LED with current limiting resistor (to avoid the LED from burning out!) and the buffer attenuator/amplifier circuit using dual op-amp TL072 or quad op-amp TL074. Note that I am curious about other alternative way of single current limiting circuit to be used for both 5 and 10 +v CV option (is it with transistor or diode?). We will discuss more on this for another page.

Cheers! and happy tweaking.


Gluing together two pt2399 modules.
Basic assembling with two DIY Vactrols (black tubes) for input attenuation. The PT2399 module audio input line audio signal level is at 0.7 – 1.4 V, more than that we will get distortion. Therefore, we need to attenuate the standard eurorack 5V audio signal within maximum range of 1.4 (or 2.5 V to allow audio distortion effect) using voltage divider potentiometer of 100K ohm with 51K ohm pull-up resistor at V in pot. The audio output of the PT2399 modules is pot-gained up to 5V audio signal output via TL072 op-amp.
The vactrol (LDR-LED) is used to control the input attenuation which is added at the ground pot as above. To activate the vactrol and to controls its range, you need to increase the pot to 100% (low resistance) or less and input the CV 0-5V (DC) to activate the 5mm LED (with 510 ohm). I will add additional two vactrols to control the reverb time and feedback amount. The C1-R3 off-set high pass filter above is included at PT2399 audio output which then input into TL072 op-amp.

P/s. +9V and -9V power supply with two 9V batteries for TL072/074 op-amp circuit testing.