[sdiy] PT-80 delay for Synth Modular

[Dan Snazelle]

tom
can you explain a bit how you have made chorus with this chip? i havent seen any schematics for doing such a thing and have always wondered if its possible

thanks

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> From: tom at electricdruid.net
> Subject: Re: [sdiy] PT-80 delay for Synth Modular
> Date: Mon, 30 Jun 2008 13:23:51 +0100
> To: czvekan at gmail.com
> CC: Synth-diy at dropmix.xs4all.nl
>
> Csaba,
>
> Like you said, Scott Swartz's PT-80 delay is delibrately setting out
> to mimic the sound of older BBD-based delays, and that means quite a
> bit of lowpass filtering.
>
> Either BBDs or the PT2399 are both sampling the signal and then
> reconstructing it, so you need a lowpass filter after the delay line
> to smooth the sample steps out of the signal. The highest you should
> really make the filter frequency is half the sampling frequency (the
> usual nyquist limit). How high this is depends on how long the delay
> is. Shorter delays means a higher clock frequency and wider bandwidth
> (eg higher filter cutoff and better quality). At longer delays, the
> clock is running slower and you need a lower filter cutoff to help
> smooth out some of the digital steps in the waveform. At really low
> clock frequencies, you're getting something close to Tom Bugs
> "Bugcrusher" sample-rate reducer effect (http://www.bugbrand.co.uk/).
>
> The PT2399 datasheet has a really useful table which shows the clock
> frequencies and bandwidths. If you set the filter frequency too high
> for a given bandwidth, you'll start to hear more sampling distortion,
> but nothing terrible will occur in the world, so experiment with it.
>
> The PT2399 IC has op-amps on it, and these are used as filters, but
> the PT-80 circuit has an additional Lowpass stage (the mess of Rs and
> Cs running up to the 2N5088 tranny). If you wanted to lose some of
> the "muffled" sound you could remove this stage entirely, I'd have
> thought. Doesn't Scott make some comment on his page about copying
> this filter from some venerable analogue pedal on his page? It's
> there for historical accuracy rather than technical necessity.
>
> Hope this helps. I've used this chip a bit, but mainly for chorus
> (short delay equals pretty good bandwidth and quality) so I don't
> know exactly how far you can push it.
>
> Regards,
> Tom
>
>
>
> On 30 Jun 2008, at 11:05, Csaba Zvekan wrote:
>
>> Hi Group,
>>
>> I have built two PT-80 delays from General Guitar Gadgets .com
>> http://www.generalguitargadgets.com/index.php?
>> option=com_content&task=view&id=125&Itemid=26 .
>> Now this is originally a stomp box for guitars and has probably a
>> different input gain. I have tested it with my DIY Modular Synth it
>> works . But I am not quiet satisfied with two things.
>> 1. The Wet/Dry ratio is a little too dry. Meaning the original
>> signal is too loud ( all though the effect level is all the way up
>> to the max).
>> 2. The Sound of the delay is rather dull and has not quiet a
>> brilliant sound.
>>
>> My questions are : Where do I modify the circuit to have less
>> original signal and more effect signal? How can I have a more
>> cleaner delay sound ?(If this is even possible with the PT2399).
>> I also have to mention that I didn't have the 2N5088 high gain NPN
>> Transistor and was helping myself with the BC550C and BC547B. They
>> sound almost the same.I have purchased some 2n5088 thinking that
>> might be my problem . Can somebody verify that?
>>
>> here is the schematic of the delay. http://
>> www.generalguitargadgets.com/diagrams/pt80schem.pdf
>>
>> Also I have some other analog Guitar effects like
>> Chorus,Frobnicator,Tremolo and distortion built and was wondering
>> if I should modify the input stages differently for use with high
>> level synthesizers signals. And how would that look like ?
>>
>> Any input is of course greatly appreciated.
>>
>> Thanks
>>
>>
>> Csaba
>>
>>
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>
>
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