[sdiy] phaser cv law

[JH.]

I've created a web page for this, with diagrams and the original mutron and
schulte schemos:

http://jhaible.heim.at/phasers/jh_phasers.html

JH.

----- Original Message ----- 
From: "JH." <jhaible at debitel.net>
To: "diy" <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, February 14, 2007 11:04 AM
Subject: [sdiy] phaser cv law


As I'm currently working on a phaser, I started to do some research about
the pole frequency vs. cv law of certain phasers.

Of course there's the expo function - I've built several phasers with expo
law in the past, and somehow it's the natural thing to do in an OTA-based
phaser.
FET-based phasers have a similarly nonlinear CV law, I think: closer to
quadratic than expo, but all in all a similar shape of nonlinearity.
And then there's the Aries and Tau, which ave an 1/x law, which creates even
steeper slopes towards the high end of the cv range than an expo function.

Some phasers are different, however.
One of my favorites, the Schulte Compact A, uses an incandecent lamp / LDR
combination, and I was curious to learn about the CV law of _this_.

(A LED / LDR combination would be _approximately_ linear, despite the very
nonlinear courves that are usually plottet for Vactrols: it's the G_LDR vs.
I_LED that matters, not the R_LDR vs. I_LED that is always plottet. Of
course this basically linear behaviour can be bent to other CV laws, for
instance by creating the I_LED with an expo converter. But I disgress.)

So, I wanted to know about the incandescent lamp / LDR combination.
Last night I made some measurments on a random-picked old
incandescent-lamp-based Vactrol (VTL3A21) and plottet the LDR conductance
vs. lamp voltage in a spread sheet. (The Compact A drives the lamp with a
voltage, not with a current source.)

The result is a courve that is basically exponential in its lower range, but
linear in its upper range.

I did some listening tests and compared my "Compact Clone" (incandescent
lamp based) phaser with the exponential law, OTA-based phaser of my JH-3
modular. When I set up a wide sweep that goes down to low frequencies, and
up to cover the whole audio range, and a similar "overall sweeping sound" on
both devices, I noticed a subtle but important difference:

The expo sweep would pass thru the upper part of the audio range "too
quickly", compared with the incandescent lamp sweep.
It's an interesting effect of it's own, but a notable difference. (In that
setup, the way the sweep goes thru the _low_ range was made identical on
both devices - here's where you don't have as much freedom as in the high
range, if you want a phasing that is "deep", but not "detuned".)

My curiosity raised, I wanted to know the CV law of another famous
optoelectronic phaser, the Mutron Bi-Phase.
The Bi-Phase uses a LED / LDR combination, and a special nonlinear drive
circuit. I had always assumed this were a crude expo approximation. But
Spice analysis showed a (now) familiar courve: expo-like around the diode
knee in the low range, and linear in the high range. So, basically the guys
at Mutron have approximated the behaviour of an incandescent lamp, using a
LED and a bending the LED control current.

Now I think I'll implement something similar in the phaser of the Matrix FX.

Comments welcome - I know others on the list have done more research on
different phasers.

JH.