[sdiy] pt2399/rebote2.5 delay question

Tom Wiltshire tom at electricdruid.net
Thu Sep 13 13:41:48 CEST 2007


Perhaps you could try this:


I've also used an Analog Devices tool which was good.


Hope this helps,

On 13 Sep 2007, at 11:15, Derek Holzer wrote:

> 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
> 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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