[sdiy] Idea for PT2399 chorus

[Ullrich Peter]

You can also make a circuit that changes the resistor on pin 6. 
Why not changing the resistor by a weighted network (like in R/2R DACs) that are switched by analog switches?
Maybe old DACs can be connected directly as some of these parts had the DAC resistor ladder accessable on pins.
This would be an option if you want digital control (via MIDI).

Ciao
Peter

________________________________________
Von: synth-diy-bounces at dropmix.xs4all.nl [synth-diy-bounces at dropmix.xs4all.nl] im Auftrag von Tom Wiltshire [tom at electricdruid.net]
Gesendet: Samstag, 07. März 2009 14:11
An: sdiy DIY
Betreff: [sdiy] Idea for PT2399 chorus

Hi all,

I was reading about Scott Swartz' PT80 delay pedal for guitars last
night:
        http://www.generalguitargadgets.com/index.php?
option=com_content&task=view&id=125&Itemid=26

I was also finding out about the various ways of adding voltage
control to the PT2399 delay chip on which it is based. The delay time
is set by a resistance on pin 6 which sinks current from the pin and
sets the clock VCO speed. There are three basic methods;

1) Current sink
        http://www.homebuilthardware.com/index.php/projects/pt239x-delay/
2) FET used as variable resistance.
3) Vactrol
        http://home.comcast.net/~sbernardi/elec/og2/og3_echo.html

The trouble with Scott Bernardi's vactrol solution is that it uses a
current source to control the brightness of the vactrols LED, so in
fact it finishes up being more complicated than the current sink on
its own. Also the output of an LED isn't that linear when fed from a
current source.

Then it occurred to me - why not use PWM to control the LED
brightness? The vactrols LDR response is slow enough that it would
provide PWM filtering, and PWM provides a much more linear output
from the LED. Seeing as I already have a PIC chip which acts as an
LFO and produces a PWM output, I thought I could easily knock
together a quick chorus.

I'll let you know how I get on, but it seems like a promising idea.
I'm also keen to try with a slightly larger chip. For instance, the
16F767 has three separate PWM outputs, so you could program up a
triple chorus, and either have independent LFOs for each delay, or
phase-shifted delays like in many string synths. Since all the clever
bit (the LFOs) is then done in software, the actual circuit would
amount to one PIC, three vactrols and three delay chips with
associated Rs and Cs.

T.


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