[sdiy] pt2399/rebote2.5 delay question

Derek Holzer derek at umatic.nl
Thu Sep 13 12:15:55 CEST 2007

Hi again,

I looked through one of my op-amp books (Jung : Op Amp Applications 
Handbook) and discovered that the type of low pass filter is a "multiple 
feedback" one. What followed was a huge pile of Greek letters and Arabic 
formulas that didn't do me a lot of good (I'm a bit mathematically 
dyslexic...). But at least I know what it's called now ;-)

Tom Bugs suggested:

> I think that it may be 1/(pi x R x C)
> & I reckon you need to look most at the RC networks around the Delay Out label at the base of the Rebote2.5 schem (so probably you mainly want to look around pins 13/14) --- 12k / 10n gives me 3kHz if I'm on the right track.. Halving 12k to 6k (just for calculation's sake) would raise that to 6kHz 

I'll try it, but it doesn't seem to fit with my understanding of the 
multiple feedback construction where pairs of resistor and cap values 
are changed together (this according to Lancaster's Active Filter 
Cookbook, which does a great job of explaining in simple terms how to 
adjust every kind of active filter *except* a multiple feedback low pass!).

I wonder, is there any kind of software for calculating component values 
in common circuits like this? Preferably something free or an online 
Java applet or something.

Thanks and best,

Derek Holzer wrote:
> Hi all,
> I'm working on a project with the Tonepad Rebote2.5 delay, based on the 
> PT2399 chip. The idea is to do a complex delay network where the signal 
> gets bounced around between different delay lines. But now that I've 
> listened to it a bit more, I realized that the low pass filter is too 
> aggressive. I'm losing too much signal on each pass of the delay.
> I'm guessing the main place to look is in the caps bridging pins 13/14 
> and 15/16. These are the bridges from the output of the LPF op amps back 
> to the negative inputs of those same op amps in the delay chip itself. I 
> might try throwing some different values in there, but of course this is 
> also tied to the resistor values.
> Is there a standard formula for computing the LPF cutoff frequency in 
> this situation? The configuration appears to be an inverting op amp, and 
> the spec sheet gives no info about setting cutoff frequencies.
> Here is the spec:
> http://www.princeton.com.tw/website/downloadprocess/downloadfile.asp?mydownload=pt2399_1.pdf 
> and here is the plan of the Rebote2.5:
> http://www.tonepad.com/getFile.asp?id=98
> Thanks much!
> derek

derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 81:
"Go to an extreme, move back to a more comfortable place"

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