[sdiy] Configurable Phase Shift Project

[scottnoanh at peoplepc.com]

Hi again,

> > I'm using the VTL5C3/2 dual element phase shifter and TL074's.  The
timing
> > cap is 560 pF.
>
> That comes as a big surprise! What's the lowest resonant frequency
(feedback
> up)
> you can get with this? I think in the Compact A is adjusted to 50Hz for
the
> lowest possible
> peak. Your samples definitely have something of the Compact A tone, but I
> thought
> the A goes lower. And it "dives" lower, with a linear ramp controlling
> incandescent
> lamps, so the modulation stays longer in the higher range than in the low
> range on a
> log Hz scale, but it dives down to 50Hz quickly and then rises again.

I haven't measured the lowest freq yet, but I'd say you're right - nowhere
near 50 Hz, I'd imagine.  I'll take a look and see.  I arrived at the 560 pF
value based on a balance between sweep linearity and range (based in turn on
my ear).  I probably should investigate it more before buying a pile of
styrenes.

Thanks for the info on the Compact A.  I'm thinking that's the one used in
TD's Phaedra(?).

> > I really want to try the Quadra compandor method that you used in the,
> well,
> > Quadra phase shifter as well as the Storm Tide flanger.
>
> May be worth a try, but it will also change resonant behaviour completely.
> Would be interesting to switch it in and out.

That would be interesting.  I'd sure like to give it a try, anyway.

> > I tried it just a bit with an NE570 yesterday, but my results were
> abysmal.
> > It was sort of a cursory check, and I think I'll try the 4007 based
> > circuitry of the Quadra and Storm Tide.  I'm not sure if this is
correct,
> > but it looks like controlling the expander with the rectified control
> > voltage of the compressor will certainly help.
>
> I think so, too. But you could also do this with the NE570, connecting the
> VCAs of both halves to the rectifier of one half.

Oh - should have tried that!

> > As an aside, I notice Dr.
> > Moog used the 4007 at least for compression in the Moog Stage Phaser.
>
> The old 12-stage phaser has a highly unusual, very interesting compander.
> It's built around two dual Vactrol stages, and it has an envelope detector
> with fast attack, fast release, and a sort of "hold" time. Ween the input
> level fades away, the CV does a staircase kind of thing, which apparently
> causes
> less harmonic distortion than just using a release time.
>
> Don't know about 4007 in Moog - is this in the Moogerfooger pedal ?

The same one you're talking about, only you looked at it a bit more closely
than I did.  I just downloaded it yesterday, saw the compression trim pot
next to a 4007 and hadn't looked to see where it *really* went.  Looking at
page 1, I noticer there's a dual element Vactrol.  Doh!

I'll try to get a measurement on the lowest res freq.

Cheers,
Scott