Homebuilt Hardware

PT239x Delay

Posted in Synth DIY by admin on the April 5th, 2007

This page discusses the PT239x series delay chips made by Princeton Technologies. Here I will post notes about my experiences with these ICs as well as complete designs for delay circuits. All of the PT chips used in these projects can be purchased at small bear electronics

PT2395 Echo: PT2395 data sheet. This is a voltage controlled echo module. It is based around the Princeton Technology PT2395 delay IC.


The main features are:

  • voltage controlled delay time
  • voltage controlled feedback amount
  • delay time tracking anti-alias filters
  • external feedback path
  • short/long mode (64Kb or 256Kb RAM)

This circuit can not reach delays short enough for chorus or flanging. The shortest delay possible is some tens of milliseconds. I cannot remember exactly how long it was but, I can say that at minimum delay time, with no feedback, the echo is still barely audible. The maximum delay is about 1.5 seconds in 256Kb mode. Since I first posted this design, I have received several emails asking about a printed circuit board for the project. There is enough room for improvement that I won’t be selling a PCB for this module. Maybe one day I’ll build an updated version.: Version 1 Documents:

  • VC-Echo Schematic (03-05-2005) - schematic diagram. If you have an old version make sure that R19 should be 270K-300K instead of 100K. My 78L05 regulator was running a little hot. Try a 7805 or put something on the 78L05 to sink the heat. It has been pointed out to me that the pin numbering on the LM317 is incorrect.
  • VC-Echo Documentation (03-05-2005) - Circuit description and calibration procedure.
  • vc-echo-panel.fpd - This is a front panel express design file that matches the Oakley Sound panel format.

I have recorded some sounds to give an idea of what the module is capable of. Every time I go to record something I get side tracked just playing with it, but here is what I have: Sounds:

  • Random Sequence - random arpeggiator sequence. the CV inputs were not connected yet when I recorded this. towards the end I’m slowly adjusting the delay time pot.
  • Modulation - just a slight amount of modulation from a slow traingle VCO. An oakley equinoxe phaser is in the feedback path which is also modulated by an LFO.
  • echo-verb2 - a slightly more complicated patch. 2 VCOs, 1 lowpass VCF, 1 VC-Echo, 1 ADSR, 1 LFO and a TLN-156 reverb. a slow moving LFO modulates the trauma setting on the the TLN-156, the filter cutoff and also triggers a very slow ADSR. the ADSR modulates the delay time. stereo left and right are recorded twice. both are the same patch but the LFO phase is different on each channel.

PT2396 Short Delay Effects (planning): PT2396 data sheet. The PT2395 is a good chip for echoes, but I am also interested in chorus and flanger effects using a few very short delay lines. In preparation for this project, I am going to be experimenting with the PT2396 IC. The PT2396, has an internal RAM but still requires the external clock. Some information in the data sheet leads me to believe that the RAM has multiple taps. Using a 4-bit digital input, the delay time can be set between 12msec and 200msec. In addition to the digital input, the external clock can be a VCO. I have ordered several PT2396 chips to experiment with. For this project to work, I will need to be able to clock the chip over a wide range. The data sheet specifies a 2MHz clock, but does not specify minimum or maximum clock rates. I am hoping that the PT2396 allows clock frequencies similar to that of the PT2395. At this point I have used a PT2399 clock output (with my VC-current sink) to control the PT2396 chip. The initial results look promising but I only did a quick test. This project is somewhat stalled right now since I’m working on other things but eventually, I plan to build a high frequency VCO (2-25MHz). At the least, I think a few PT2396 chips could make a decent chorus effect.

PT2399 Notes: PT2399 data sheet. The PT2399 is PTC’s latest delay IC. It is also the easiest to work with. This is a single chip delay with a built in VCO and 44Kb RAM. It can produce a deccent range of short echoes. I would have used this for my original VC-Echo but, unlike the PT2395, its RAM cannot be increased. The example circuit in the PT2399 data sheet will work ok except I wasn’t happy with the distortion at longer delay settings. Having most of the circuit on a single chip is nice, and Princeton Technologies told me in an email that this one is superior to the others. This seems to be their only chip that was not a replacement for some other delay IC. The delay time is set by a resistor connected to pin 6 (VCO pin). The voltage at pin 6 stays at 2.5V so the adjustable resistor is aparently setting the current for the VCO. I know of three ways to add voltage control to the PT2399. The method that I have used is an NPN current sink attached to pin6 of the PT2399. Scott Bernardi used a current source controlling a Vactrol to adjust the resistance. Scott Stites has also informed me that a FET voltage controlled resistor works well. The circuit I have built uses has current limits between 0mA and 3mA.

PT239x Current Sink

PT2399 VC-Current Sink

The current through R1 sets an initial current (2.9mA measured). Any CV added reduces the output current. I haven’t tested what minimum current is needed, but when I find that out, I will probably add a resistor from the PT2399’s pin6 to ground. The opamp I am using is actually a TL072. The maximum current does not occur exactly at 0V CV. To improve it, R4 could be replaced with a smaller resistor and a trimmer. Perhaps a 24K and a 10K trim pot. The diode D2 is meant as extra protection for the PT2399.The voltage at pin6 of the PT2399 (2.5V) is not so constant. An increased pin 6 current decreases its voltage voltage. The pin 6 voltage drops about 150mV from 0 to 3mA current. I also checked the reference voltage (pin 2). The reference voltage also drops a little with an increased pin 6 current. There is about 6mV change across the 3mA. Some CV feedthrough can be expected because the reference voltage is used on the internal opamps’ non-inverting inputs and probably most of the other analog circuitry. I suppose the chip was not actually meant to operate with a 22MHz clock. PTC reccomends a clock of 4 to 5MHz.At this time, I have only been testing the amount of control I have over the PT2399 clock frequency. The clock output is very nasty looking, I have been running the clock through a 74HC393 binary counter to clean it up and divide it down. The measurments I have taken were done using an oscilloscope to view the divided clock. This is not terribly precise, but I would esitmate about 21 to 22MHz when the current source is at 3mA. This is right about the maximum frequency that the data sheet shows. I think it will go even faster, but I’m afraid of destroying any chips. After I get a complete circuit built, I may try and see how fast it really can go. The divided clock could be used to control a pair of LTC1063 (or similar) switch capacitor filters for the input and output.

Some related websites:

  • Ken Stone - home of CGS synth modules. He has a great delay development board for the PT2395. I used this during the intial development of my PT2395 Echo
  • Scott Stites - There are many sound samples of a PT2399 delay used with synthesizers. Look under the ‘old site’ for PT2399 info.
  • Scott Bernardi - has a complete PT2399 delay module here with voltage control of delay and feedback
  • General Guitar Gadgets - another PT2399 delay. This one is designed for guitars and has a low bandwidth, and no voltage control.