PT2399 based delay guys...a question.

Started by skiraly017, June 17, 2008, 03:51:45 PM

Previous topic - Next topic


I think I know the answer and this is probably more thinking out loud than anything.

If looking at the PT2399 datasheet (found here), pin 6 controls the delay time. In the Rebote and other PT2399 based delays what you see is Ra in series with a pot (let's say a 25K) and Ra affects the short and long delay time, so if we use datasheet math and place a 2K resistor in for Ra then the minimum delay time would be 52.3ms and the max delay resistance would be 27K and max delay time would be right around 340ms. Let's say I wanted to put a mark on the face of my delay pedal where the delay time will be 151ms which requires a resistance of 10.5K. Do I take into consideration Ra/2K when measuring the pot i.e. subtract 2K from 10.5K and measure/mark 8.5K on the pot or do I look for 10.5K on the pot and call it a day? If I've confused anyone I apologize. Let me know and I'll clarify as best I can. Thanks.
"Why do things that happen to stupid people keep happening to me?" - Homer Simpson


You need 10.5k in total so yeah you would need to measure 8.5k on the pot.

Mark Hammer

You will note from the datasheet that the time-vs-resistance aspect is not linear.  Hell, I don't know WHAT to call it.

This may be one of those deals where, if you want to know the actual delay time, and replicate it, that some sort of rotary switch with designated fixed resistance values is called for.  Alternatively, a rotary switch with designated ranges in series with a smaller-value (e.g., 5k) pot.  The switch would indicate the absolute minimum delay time for that range, and the pot would let you hike it up a bit from there.


Thanks for the responses. Your right Mark, it's a wacky sort of math but as long as I can figure out about where I need to be I'll be fine.
"Why do things that happen to stupid people keep happening to me?" - Homer Simpson

Mark Hammer

Well that's sort of the reason behind the suggestion of the switch+pot idea.  If you use the pedal in a spontaneous way and are content to fiddle with the delay-time pot and leave it at whatever position sounds good to your ears, cool.  At the same time, I understand that some folks need to be able to replicate specific repeat rates as if they were tempos, and being able to zero in on a specific range with dependability can be an asset.  The ranges need not be THAT specific.  For instance, a 3-position toggle could easily select 3 different delay ranges by adding a pot to a fixed resistance that you either leave as is, or shunt with one of two different parallel resistances.

My old blue MXR digital delay has a delay time not that different from a PT2399 (320ms), with 10 radio-style push-buttons to select the range, and a divide-by-2/multiply-by-2 pot for zeroing in with more precision.  Given the rather limited (by today's standards) delay time, 10 switch settings may seem a bit overkill.  On the other hand, it is set to do flanging and chorusing too so there are also delay times shorter than what a PT2399 would aim for.  In any event, the switch+pot strategy works really nicely for being able to "get back to" a setting you liked....assuming you need to be able to do that.


The delay time is more linear than you think!

Some time ago I decided to tackle this question.  First, I tried to think how to setup an internal clock with an external resistor (in a generic way). Probably a resistor sets a control current that charges a cap with a ramp, I thought, and this method is pretty linear.  Let's see: at almost 0 ohms you have 31.3ms delay; let's find which resistor value doubles said figure.  Ah, it's 2.8k which produces 61.6ms.

The PT2399 has an internal 2.8k resistor that adds to the external resistor, and delay time is set linearly according to the sum of the internal+external resistors.

Let's see which resistor value triples the minimum delay.  Triple delay is 93.9ms.  From the published table we see Rext is a value slightly above 5.4k.
On the other hand, theoretical resistor value would be 3*2.8k = 8.4k, minus the 2.8k assumed internal resistor, one gets: Rext'=5,6k.  Pretty close so far, but could be diverging, so let's see one more case.

Let's consider 11 times the base delay, or 11*31.3ms=344.3ms, pretty close to 342ms that has an associated Rext=27.6k.  Now, the theoretical Rext, or Rext'=11*2.8k-2.8k = 28k, which again is very close to 27.6k.

In summary, for the minority who might still be awake after the above explanation, you can consider the delay time directly proportional, or linear to the external resistor plus an internal resistor of 2.8k (AKA stopper resistor).

P.D.  It is possible to graph Rext v/s the delay time in Excel or Calc.  It is easy to notice a straight line is a very good fit for the points, but it intersects the Y-axis near -2.8k, the internal fixed resistor.


I think I get what you're saying but now my head hurts.  :icon_biggrin:
"Why do things that happen to stupid people keep happening to me?" - Homer Simpson

Cardboard Tube Samurai

MAybe you guys can answer a question that people have been asking me then. I've built the Rebote 2 and everyone I tell about it wants to know the delay time. So what range does it actually have?


Now that's an easy one:

Let's call 'Rext' the external resistor in kohms.  Then, the delay in msec can be calculated as:

Delay = 31.3 * ( Rext + 2.8 ) / 2.8

which can be simplified as:

Delay = 11.2 * ( Rext + 2.8 )


Example:  What goes on with a 50k delay pot at max?

Answer: 11.2 * ( 50 + 2.8 ) = 591 msec


You should expect some variation from chip to chip, perhaps +/-5% at least due to the manufacturing tolerances of the internal components.  Pots have large tolerances also, usually +/-10 to as much as +/- 20%.  Recently I bought some 1 Meg audio pots that actually measure 750k !!!



Quote from: ubersam on June 18, 2008, 09:20:11 PM
what is the PT2399's max delay time?
IMO useable max time is right around 400ms. After that you start getting noise and digital hash. People have pushed them to 600ms but there's way too much unwanted noise for me at that point.
"Why do things that happen to stupid people keep happening to me?" - Homer Simpson