[sdiy] Configurable Phase Shift Project

[scottnoanh at peoplepc.com]

Woah - just measured it, lowest res freq is much lower than I'd thought.
With no input and res cranked max, I got a 43 ms wave cycle, so around 23
Hz.  Top end was 0.15 ms, around 6666 Hz.  This was with 16 stages hooked
up.  This morning, hooking up those 4 extra stages pretty well hit the wall
with the control scheme - there was a noticeable drop in the high end
response (lowest resistance), which makes sense I suppose.  15V are feeding
the diodes, sucked through the Arp Style expo arrangement (ripped directly
off of Rene).  So, 20 stages (10 LED's) pretty well starts to limit things I
guess.  I had the same number of LED's in the 10 stage VTL5C2 phase shifter
I toyed with a couple of years back, but that one always seemed to have too
much range, if that makes any sense.  I double checked my cap value just to
make sure - sample cap I tried read 522 pF on my cap checker.

On my samples, I recall I wasn't using the full range.  I have enough play
with the initial phase control where I can make it linger longer on the top
or the bottom.  I've been concentrating on getting a symmetrical sweep.
Could be that low end is attainable, but perhaps not in a symmetrical sweep.
By symmetrical, I mean no apparent lingering on either end.  I should do a
sample with the full sweep, symmetry be damned.  I'll do a white noise
sweep, those are usually the most demonstrative.

Now I want to play with it some more, but my wife is trailing me, with
clucking tongue and hair cutting shears.

Cheers,
Scott


----- Original Message -----
From: <scottnoanh at peoplepc.com>
To: "JH." <jhaible at debitel.net>
Cc: "Synth-Diy at Dropmix. Xs4all. Nl" <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, February 22, 2006 7:52 PM
Subject: Re: [sdiy] Configurable Phase Shift Project


> 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
>