Uranus phaser

[Haible Juergen]

Hi,

just a few words about my latest project.
Six cheap surplus LDR's soldered to 12 pins
of a 14-pin DIL socket, sensitive side of each
pointing inwards. A red LED soldered to the
remaining 2 pins, the LED's wires bent so that
LED lies horizontally in the "corridor" made
of the 6 LDR's.
The spaces between these components are
filled with heat glue (sp ?), transparent enough
to distibute a diffused light.
The whole thing wrapped in black heat shrink tube,
remaining gaps closed with black paint.
Very compact hex "Vactrol" device ...

The circuit: 
Discrete all pass stages similar to the 
Univibe. NPN darlingtons as phase splitters, with
positive feedback to increase input impedance,
and capacitive coupling between each stage.
For details, see the univibe schemos. Just six
stages instead of four, and the 1st stage is like
the other ones (not a preamp like the Univibe).

Found a very interesting "Colour" (or resonance)
mode. First, I have variable mix of original
and phase shifted signal by wiring a potentiometer
from the emitter of the first stage to the emitter
of the last stage. Nothing special, if I use a high
resistance value (50 ... 100k). But with a 10k Pot,
things become more complicated. First, there is
some amount of feedback (10k / 4.7k) from the last
to the first stage, and second, the first stage will
not act as a unity gain phase splitter anymore,
because it sees a lower impedance at the emitter
than on the collector !
This is very unlike the Resonance on other phasers.
On other phasers, feedback would affect all peaks 
and notches in a similar way. With my method,
the 3rd (highest frequency) peak is much more
enhanced than the others, so the whole thing sounds
like a mix of phaser and band pass filter.
It's hard to describe, but the old overused word "warm"
hits the point, IMO.
As soon as I found this, I decided to use a 50k pot,
and a "color" switch that connects a fixed 12k resistor
across this pot. A single resistor, but very impressive ...

Capacitors:
As I could not expect the LDRs to match very well,
I included sockets for the all pass capacitors. I selected
them by ear, and ended up with 10nF x4 and 100nF x2.
No idea if the poles are similar or wide apart. Don't
care too much about that, as long as it sounds good.

Modulation:
The LDRs are slooow. Especially because they don't get
much light thru the glue. But that's just right for me.
It adds to the smooth and warm character.
Expo converter: Two LEDs in series. One is in the "Vactrol",
and one is on the front panel. (So you see directly what's
going on.) These LEDs are directly driven by an opamp
output, so the LEDs will convert a linear voltage to an
exponential current all by themselves. Why do it complicated
when things can be so easy (;->).
LFOs: Two of them. Triangle LFOs, 0.012Hz ... 10Hz each.
I like deep slow sweeps with a little bit of fast vibration on
top of it. Each LFO can be switched off by a footswitch jack.
A FET is shorting the integration cap then.

External "Colour" jack:
The 12k resistor (see above) is used as normal connection of
a stereo 1/4" jack. You can plug external passive networks
in here ... 

Name:
First I wanted to name it "JH-HV1 Hexa Vibe Optoelectronic 6 
stage phaser" - obviously not a good name. (;->)
Working title is "Uranus" now. But I want to make sure that this
name wasn't already used for electronic music devices, before I start
engraving a front panel. So, is there an "Uranus" already (other than
in the solar system) ??

Schemos comming soon on my site at Synthfool. A few weeks I
think.

JH.