[sdiy] Configurable Phase Shift Project

[scottnoanh at peoplepc.com]

Hey Michael,

I tried the FET distortion method from Mark Hammer's page - used an MPF-102.
I've had to diddle a bit with the values, scaling and input voltage to get a
fairly decent response, but it most certainly is an improvement in many
cases.

I figured using the expo response combined with the parabolic waveform
wouldn't work all that well, but it does.  There are some cases where I
prefer the normal triangle - not necessarily because of  fast/slow
modulation.  This sounds quite good even with some fast settings - the
response of the LDR itself takes care of the depth of modulation versus mod
frequency to a great extent (not nearly so much as, say, a VTL5C2, but the
5C3/2 does it adequately).  More rather, the normal triangle sweep allows
some unusual sweep patterns, kind of like the Compact A has a fairly unusual
sweep.

I figure I can put in the waveshaping circuitry with the implication that,
for external modulation,  a 10Vp-p triangle wave centered around 0V is
expected for the hypertriangular waveform, and make the waveform selectable
via switch.  10Vp-p is what most modulars will put out (at least mine does),
so that shouldn't be an issue.

Thanks for bringing that up!

Cheers,
Scott


----- Original Message -----
From: "Michael Bacich" <weareas1 at earthlink.net>
To: "synth diy" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, March 11, 2006 3:23 PM
Subject: Re: [sdiy] Configurable Phase Shift Project


>
> On Mar 11, 2006, at 11:19 AM, <scottnoanh at peoplepc.com> wrote:
> The higher value of capacitance always brought a certain amount of pitch
> bend on the lower end of the sweep.  I started wondering if that was
> because, as the VTL5C3/2 goes higher in resistance, the tolerance of
> resistance begins to vary more greatly from device to device.  Taking
> that
> into account, along with a suggestion from a list member, I lowered
> the LDR
> parallel resistance from 1.5M to 1M, and that seems to have improved
> things
> on the lower end quite a bit.
>
> With both phasers and flangers, there is always more apparent pitch
> bend at the low end of the modulation.  This becomes more apparent at
> faster modulation rates.  Adding lots of juicy regeneration/feedback
> seems to make this worse, unfortunately.  You can always minimize
> this unwanted pitch shifting by reducing the overall modulation
> depth, but where's the fun in that?  We all love deeeeeeep phasing
> and flanging modulation.
>
> There are two ways that I can think of to keep the modulation fat and
> deep, but reduce the unwanted pitch bending:
>
> 1.  Use an LFO that changes its frequency (and hence, its shape) as
> it goes through its cycle.  Craig Anderton famously did this on his
> PAIA Hyperflange design.  He used a CEM3340 VCO for his LFO, and he
> fed a little bit of the LFO's triangle wave output back into the
> 3340's modulation CV input, so that the LFO would speed up during the
> triangles peak, then slow down at the low part of its cycle.  He had
> a special name for this type of LFO -- a "hyper-triangle", or
> something like that.  The end result was that you could use much
> greater modulation depths on the flanger, because the relative LFO
> speeds were better matched for the high and low frequency ranges of
> the flanger's comb-filtering.  Another way of describing this is that
> the modulation intentionally moves more slowly while it is working on
> the frequency range that provides the most dramatic "action" -- which
> is very useful with a deep, resonant flange effect, especially at
> very sloooooooow LFO rates.  I'm sure that this method would work
> just as well with a phaser.  One possible down side to this method is
> that the up and down modulation sound will have a kind of lop-sided
> quality to it -- this may or may not sound good at certain LFO rates.
>
> 2.  Create an LFO scheme that would allow your modulation depth to
> change as the LFO goes up and down.  That could be done by using the
> raw triangle LFO itself to modulate a VCA/attenuator (such as a
> LM3080).  Run the LFO's output through the VCA, which would, in turn,
> output a triangle wave whose amplitude varies in direct proportion to
> its relative phase (that is, higher amplitude when the wave is
> peaking, and lower amplitude when the wave is low).  Use this self-
> amplitude-modulated LFO to modulate your phaser.  This should also
> help minimize the pitch-shifting, still allowing fairly deep apparent
> modulation depths.  The effect would be similar to Anderton's idea,
> but with a more triangular overall modulation shape, which might
> sound less lop-sided at certain LFO rates (particularly at faster
> rates).
>
> I have tried neither of these methods using actual LFO's, but I have
> simulated their effects by manually adjusting LFO speed and depth
> while the LFO cycles, and both methods seem to work pretty well.  My
> apologies if I have described the actual electronic methods used not
> so accurately -- please refer to the Craig Anderton Hyperflange
> schematic for better details.
>
> Michael B.
>
>
>