Modding Rabote Delay 2.5

[Rethfing]

Hi! This time im not here because of broken pedal Im thinking what to do about this Rabote Delay 2.5. Built it today from tonepad, sounds very nice, but I couldnt get longer delay time than 0.5s, so I replaced delay pot with 100k. Its now capable of 1-2s delay, but there is a lot of noise after each repetition. I was wondering how to solve it. Few weeks back I built delay from a kit (here's link to documentation: http://efekty-diy.pl/pdf/Deep%20Blue%20Delay.pdf) and I must say that it sounds a lot better. There is noise, but its far from what Rabote does. Im not too good at reading schematics, so maybe one of you guys will think about that?

Also some guy on YT told me that he replaced 2x82nF caps with 220nF and the sound is brighter so Im going to try that soon.

here's link to tonepad's pdf: http://www.tonepad.com/getFile.asp?id=98

[Rethfing]

Just tried 3 other PT2399 ICs, and they all sound the same (maybe one of them is a little bit noisy, but the rest sounds the same and noise is still there). Also just after messing around with my delay I plugged in the delay I mentioned in my previous post. Result? Very little noise compared to Rebote. I should mentioned that before, my delay has no enclosure yet, can it affect noise? Or maybe I should change some capacitor value (forgot which one, read it in build reports on tonepad) for high frequency filter?

[Seljer]

The PT2399 is a digital delay chip. The memory in the PT2399 has a fixed number of stages, to adjust the delay time you adjust the sampling frequency. If you want a longer delay time you run it at a lower frequency, a lower sampling frequency also means a lower nyquist frequency (upper limit of what the DA can sample without artifacts). Read up on the theory of digital audio.

There are two low pass filters (the opamps in the PT2399 at the bottom of the schematic, before 'delay in' and after 'delay out') that cut away all the content thats above the nyquist frequenciy to minimise aliasing errors. This is what you're hearing when you extend the delay time beyond the reccomended values of the original circuit. This also why the Rebote delay has a 'warm' sound compared to larger digital delay units.

If you want clean long delays you either need to cut away all high frequency content to the point where you're only left with the stuff under 600hz or abandon the PT2399 chip. And if you search this forum you can see that people have tried to get all they can out of it, the usable limit of the chip is about 800ms.

[Rethfing]

So I either have bassy sound without highs or full sound with noise? There's nothing more?

[senko]

I, too, have run into a similar roadblock.  The PT2399 is simply just a limited IC, but it still has its place.  I'd suggest abandoning the >800ms delay because of the noise issue.

Now, if you really want to have some fun, I'd suggest playing around with the Vref, Pin 6, and the lp filter amps.  Just set up a voltage divider with a potentiometer and see how varying this level affects the delay length and audio output.  Putting an LED/resistor combo off of Pin 5 (Digital Clock) will give you a visual indicator of the internal sampling speed.  You'll hear it, too.

LFOs on Pin 6 will result in some sick delay and pitch modulation.  At faster LFO frequencies, it's a very nice vibrato (with a sine, square or triangle); at slower speeds, you really get very cool delay frequency modulation.  Cool stuff and only lightly explored with in such circuits as the Echo Base.  A CV (Control Voltage) input on this part of the circuit would allow you to use your analog synth gear with your guitar pedals. 

I recently put up a webpage (it needs some major work!) with some of the notes I have been collecting while experimenting with the PT2399.  Check it out here:
http://www.diyaudiocircuits.com/pt239-digital-echo-delay-integrated-circuit-breadboard-experiment/
http://www.diyaudiocircuits.com/tutorials/pt2399-digital-delay-analog-echo/

[Mark Hammer]

So I either have bassy sound without highs or full sound with noise? There's nothing more?

Pretty muchl

[Rethfing]

Im just wondering... What happens if I stack another PT2399 on the top of the one that is already on the board?

[Seljer]

Nothing really other than strage noise issues.

You need to wire them in series, which requires the entire circuit to be appropriated to the topology. If you do a search here, someone designed stereo PT2399 delay where you could also run the two chips in series and a variety of feedback routing options allowing for longer delay times/multi tap delays/ping pong delays/etc...

[Rethfing]

Ok, screw longer delay times, they're a bit useless anyway I hear some clicking sound on repeats, dont know if it just sound that way, or I am having some kind of problem. Its like every repeated sound has click on it.