AW: [sdiy] PT2399 site with sound files

[Scott Stites]

Hi Nils,

On Mon, 26 August 2002, "Nils Pipenbrinck" wrote:

>That just remindes me of a very cool feature of my
digital guitar chorus
>unit.

>It has a predelay feature. Combing is done with two
(or more.. who really
>knows) modulated delay lines. Both the feedback as
well as the source signal
>must pass the predelay each time. This gives a very
rich and deep
>chorus/flange where you can set the modulation and
feedback to extreme
>values without getting this ugly and cheap sounding
(my oppinion) up'n'down
>going effect you can hear from cheap chorus units.


>The more predelay you use the broader the sound gets.
Only drawback is, that
>you get some slapback style delay on your signal as
well.

>I think you can have the same effect by using two
delay chips. You won't get
>your BBD style flaging, but it gives extreme flanging
sound in a complete
>different manner.

That sounds like a very similar setup to my dual delay
contraption - I knew someone out there had had to have
already done something like that.  Thank you for that
description - the granularity I referred to is the
slap-back from the overly long delay time.

On my setup, the hard part of the flange sound is
setting a good balance for the intense flange effect
without actually affecting the pitch of the signal too
much.  There is a sweet spot, though.  And inverting
the loop signal for even/odd harmonics (a la Blacet TM
and my old ADA) does offer a wider range of timbres at
these short delay times.

The regeneration of both delays can be adjusted, the
'current' stage signal, or the amount of mix from the
opposite stage, or both (just a simple mixer setup for
each stage).  The comb filtering at the very low delay
times is very 'flanger-like' yet much different than
any flanger I have heard.  As for the sweep, I play
hell getting the simple linear 'up down' sweep using
the FET.  I think because of its nonlinear nature, the
FET (MPF-102) actually contributes very non-typical,
nonlinear sweeps.  I've tried linearizing resistors,
but with little luck.  Either due to inexperience
dealing with FETS (most likely), the combination of the
FET and the PT2399 resistance/delay slope, or just how
the PT2399 uses the current through the resistance.
  
The PT2399's delay time is controlled by the resistance
at Pin 6.  In my setup, I have a 50K pot in parallel
with an MPF-102 FET going to digital ground (Scott
Bernardi's design is using an opto-coupler, which is
advantageous for reasons explained later).  Not only
does the pot control the delay time, it also controls
the range of delay time affected by the FET (max delay
time possible with a given input level).  I also have a
CV offset control which allows the delay modulation, in
conjunction with a CV attenuator control, to work
within any region of the FET's response curve. 

Interesting things happen especially when both the
maximum and/or minimum delay times dictated by the
delay pot and the FET are 'exceeded' by the modulating
signal.

Feeding the the FET with a triangle wave that drives it
to the point of saturation near the top of the peak
causes the delay time to reach absolute maximum before
the modulating wave stops ascending, resulting in a
'break' or pause in the sweep.  

Adjusting the delay time/range pot to a very narrow
minimum to maximum delay time range, and then driving
the 'bottom' of the modulating signal (using the offset
control) into the region past the point of absolute
minimum delay time also generates a sort of pause,
since the modulating signal no longer affects delay
time.  This results results in a sort of second
'whoosh' per cycle when the modulating signal passes
back through the range where minimum delay time starts
to be affected and is climbing back to max delay time. 
Same with the bottom end, only in reverse.  This also
can be very interesting when controlling the delay time
with an EG that is also controlling a filter and VCA
before the delay stages.  Using a gated AR, the longer
you hold the note, the more chaotic the delay becomes
before release.  So you get a very 'feely' way of
introducing a little grating craziness into a note if
you want to.

However, there are times when I desire a more linear
response, that's just one aspect of Scott Bernardi's
design that I find appealing.  Also, the design I'm
using has enough controls to fill a very large chunk of
panel space and I'm not sure it's worth the overhead
(as interesting as it has been for me), plus there's
just too much circuitry inside (LODE - lack of design
experience).  Scott's delay CV setup and the ability to
control regen through CV are distinct advantages over
this control and connector happy thing. But I would
still go with two of them (Scott's design), I have
found the concept of dual delays so useful, especially
when you can cross-breed the regen signals.  I have no
afilliation with Small Bear or Princeton Technologies,
BTW :-), I just like the chips.....

Sorry to yak your ear off!

Take care,
Scott

________________________________________________
PeoplePC:  It's for people. And it's just smart. 
http://www.peoplepc.com