[sdiy] Idea for PT2399 chorus

[Ullrich Peter]

Hi Tom!

OK, this makes sense... Smoothing the DAC is not possible the easy way.
The cleanest solution would be taking a higher resolution DAC and interpolating 
between the steps - but thats too much effort.

Ciao
Peter

________________________________________
Von: Tom Wiltshire [tom at electricdruid.net]
Gesendet: Samstag, 07. März 2009 15:45
An: Ullrich Peter
Cc: sdiy DIY
Betreff: Re: [sdiy] Idea for PT2399 chorus

Peter,

Yes, it would be possible to do it the way you suggest, and you're
right that some resistor string DACs would do the job. Alternatively,
a digital potentiometer (almost the same thing) would do it.

The advantage for me of the vactrol solution is that the clock
frequency will slew from one to another, rather than abruptly
jumping. This should give a more "analog" effect. I'm worried about
digital stepping creeping in otherwise.

The PIC LFO includes a heavy filter to ensure that the output is
truly analogue and doesn't show any digital stepping. The vactrol has
the same effect here. Without it, I think you'd hear the 10-bit
resolution of the PICs output.

Regards,
Tom


On 7 Mar 2009, at 14:01, Ullrich Peter wrote:

> 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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