Vactrols and Optocoupliers
Vactrols are sometimes called Opto-isolators, photocouplers and optical isolators.
They all use a similar idea, though this post is mainly about vactrols.
One side has a a light source of some sort
This could be a LED (or an incandescent bulb).
The other side uses something which is photo sensitive.
This could be a LDR (Light dependent resistor) or a photosensitive transistor.
Applying a voltage to the LED causes it to light up. The light falls on the LDR.
The intensity of the LED directly controls the photo-transistor / LDR.
Thus this resistor becomes voltage controllable.
This is a Doepfer LPG... based on a Buchla 292 module.It uses vactrols.
Through the use of a vactrol, anything which is normally
controlled by a potentiometer or variable resistor can become a
voltage-controlled parameter. Applying a voltage to the vactrol's LED is just like turning the knob on the potentiometer.
In addition, since the LED and LDR can be controlled by different circuits we are thus electrically isolating the two different circuits.
The hollowed space between the LED and photo-transistor/resistor can be made using glass, air, or a transparent plastic.
The resistance of the LDR doesn't change instantly.
It takes time ... the change is not linear either.
"It may take a few
seconds until the LDR reaches it's dark resistance (i.e. the maximum resistance
without illumination)" Doepfer.
A brief History
In 1967 Vactec introduced a compact RO (resistive optocouplier) branded as Vactrol.
Unlike the tube-coupled ROs of Fender and Gibson, Vactrols were sealed
and sturdy devices. They originally used incandescent
bulbs. In the early 1970s, Vactec replaced the incandescent
bulbs with LEDs. This increased the switching speed and probably also their reliability.
In 1983, Vactec was purchased by Perkins-Elmer. Their vactrol division was spun out from the company and changed their name to Excelitas in 2010.
In the European Union, the production and distribution of Cd-based photoresistors was banned after January 1, 2010. This decision marked the beginning of the end of Excelitas vactrols. However, CoolAudio started manufacturing the VTL5C3 and Xvive is manufacturing the complete range.
Commercial vactrols usually contain LEDs that usually produce infrared light.
The detector is a semiconductor-based photoresistor made of cadmium selenide (CdSe) or cadmium sulfide (CdS).
Resistive opto-isolators are the slowest type of opto-isolator .
Switching times usually exceed 1 ms, and for the lamp-based models can reach hundreds of milliseconds.
Cadmium-based photoresistors exhibit a "memory effect": their
resistance depends on the illumination history; it also drifts during
the illumination and stabilizes within hours.
-----
Since it's getting more difficult to find vactrols, making your own may one day be the only option.
These are the light dependent resistors which I like to use.
GL5549
The type of LDR is impt.
It should have a dark resistance of at least 0.5M ohm.
I like a slow attack (or response time).
VTL5C3s have a slope of 20, a dark resistance of 10M Ohms and a very slow response time.
LDR type nr 5516.are good too.
The 5516 specs say it gives about 5 to 10 kOhms at full brightness
The colour of the LED will have a bearing on how the vactrol works.
So experimentation is in order.
This is an example of a DIY vactrol.
I read somewhere, that yellow LEDs make the most efficient circuit.
The best match
is I think between green and green/yellow. A blue LED is probably the worst possible
choice.
Also remember that LED colours are normally not standard or very precise from one manufacturer to another.
Here is another example of a DIY vactrol.
I used black shrink wrap.
To get the best possible performance, the LED and LDR should have equal wavelengths.
This will require checking datasheets to
find a good match. Model |
- |
Size (mm) |
Light resistance(ohm) |
Darkness resistance(ohm) |
Max voltage (DC): |
Max power consumption |
Operating temperature |
|
GL5506 |
- |
5X2 |
2K~5K |
0.2M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5516 |
- |
5X2 |
5K~10K |
0.5M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5528 |
- |
5X2 |
10K~20K |
1M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5537-1 |
- |
5X2 |
20K~30K |
2M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5537-2 |
- |
5X2 |
30K~50K |
2M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5539 |
- |
5X2 |
50K~100K |
5M |
150V |
100mw |
-30C° ~+ 70C° |
|
GL5549 |
- |
5X2 |
100K~200K |
10M |
150V |
100mw |
-30C° ~+ 70C° |
A bit about the Buchla 292 LPG
The 292 Quad Lopass Gate is very famous for its use of vactrols,
The LPG is kind of a VCA
You will find that the minimum release time associated with a vactrol is a lot longer than that of the typical (non-Vactrol) VCA,
It is thus impossible to produce snappy drum sounds using a 292
The drum sounds tend to ring for about 30 ms after triggering
This occurs even if the 281 envelope generator that is controlling the LPG is set for
its minimum value.
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There are a few commercial vactrols you may come upon.
This is a NSL type vactrol
The single vactrol NSL. the white dot marks the cathode.
Below are some Perkins-Elmer/Excelitas VTL 5C3/2
VTL 5c3/2..... I used them a lot in the buchla "clones"..
These are getting really hard to find.
You may come across these Xvive versions.
I understand they are made in China
The manufacturer says they are a modern clone of the old Perkins-Elmer VTL53C
These are I understand also Xvive vactrols
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