[sdiy] pt2399/rebote2.5 delay question

Derek Holzer derek at umatic.nl
Thu Sep 13 14:08:34 CEST 2007

Hi Tom,

thanks for your thoughts!

The ones on the bottom of the schemo *are* the internal filters at the 
input/outputs of the chip. The author just pulled them down to show the 
circuit more clearly. Actually makes more sense to show them that way, 
since the feedback/filtering part of the circuit is easier to follow 
than, say, the one in the PT2399 spec sheet! Have a look at the 
"advanced" spec sheet for the block diagram of the PT2399:



Tom Wiltshire wrote:
> Derek,
> one other thing...
> Are you using the two additional input/output filters at the bottom of 
> the Rebote Delay 2.5 schematic, or just the ones based on the PT chip's 
> internal op-amps?
> Am I right in thinking that it's the internal filters you're having 
> trouble with?
> Obviously for the external filters you have a lot more options, 
> including removing them entirely and replacing them with something else!
> T.
> 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"
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 63:
"Don't break the silence"

More information about the Synth-diy mailing list