Similar to the famous "Univibe": All pass stages
from discrete transistors, LDRs as voltage controlled resistors.
Different from the Univibe: Six stages, feedback, wet/dry mix, two separate LFOs for modulation. A special "Colour" function that can be programmed with little "plug-in devices" ...
( JC Maillet
has done a lot of profound work around the original Univibe, with light
bulb and all.
Make sure to check out his page !)
At first I wanted to name my little box "JH-HV1 Hexa Vibe Optoelectronic 6 stage phaser" - but it felt kind of pompous. Then I proposed it as "Uranus Phaser" to the Synth-DIY mailing list, and learned about the special side effect that this planet's name has in the English language. Ok, I hope Neptune is a better name. If not, I'll go on numbering my future projects JH-xyz ...
The voltage controlled resistors:
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 the LED lies horizontally in the "corridor" made of the 6 LDR's. The spaces between these components are filled with hot melt glue, 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 ...
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 guitar preamp like the Univibe).
By accident, I found a very interesting "Colour" (or resonance) mode. At first, I had 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, as long as I used a high resistance value (50 ... 100k). But as I tried a 10k Pot, things became 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. Something you would not try to design in the first place, but such a pleasant sound that you want to keep if you have once heard it.
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 ...
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. I don't care too much about that, as long as it sounds good. But keep in mind that if you build this device, yours will most likely sound slightly different, or you will have to find your own set of capacitor values.
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. If I want a fast phaser (important for S&H modulation !), I use the ladder circuit from the ARP Quadra, or some OTA design.
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 ... (There is a current limiting resistor at the driving opamp output, but note that the feedback loop starts after this resistor. So full voltage control of the LEDs, and the resistor will only make the opamp go to its rails in "emergency" case.)
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. The LFO's LED will also be dark when the LFO is disabled. You can use different types of footswitches here. With a normal (mono) Footswitch, the ring contact of the jack is shorted to GND, so one LFO will always be active, and you can switch in an additional second modulation with the footswitch. Or use a dual footswitch with stereo plug and control both LFOs. Last, not least, you can build a special footswitch that will short either ring or tip to GND at one time, so you can switch between the two LFOs.
External "Colour" jack:
The 12k feedback resistor (see above) is used as normal connection of a stereo 1/4" jack. You can plug external passive networks in here. Solder a few resistors and capacitors into a 1/4" plug, and you have a little "Colour Program Module". I've tried 470 Ohms + 100nF from Ring to GND, and 12k from Tip to Ring, for example. This will boost the higher frequencies directly at the first stage (which is no all pass anymore, in that case), and some medium frequencies (depending on the light intensity) because of the feedback loop. I like the possibillity to experiment here, even after the box is finished.
All drawings copyright J. Haible (C)1996