Vactrols

Why I started with VACTROLS

I was preparing for a live gig for Silent Noise Revolution. Simply put: SNR is about piano on a bed of abstract noise. When you are playing piano, you don’t have extra hands left that you can use to control a modular. 

I was using a lot of sound sources, and these sound sources needed to switch on and off while I was playing piano. I soon realised that switching sounds on and off using VCAs which were in turn controlled by gate signals was not ideal: there were hard clicks during the switching! Using envelopes to control the VCAs ws a good solution, however: I soon had a case full of envelope and VCA modules to enable audio switching without unpleasant clicks. 

Time for simplicity and improvement!

The short description of the solution is: using vactrols, it is possible to replace many VCAs and enveloppes by simple vactrol and circuits!

What is a VACTROL?

A VACTROL is the combination of an LED (light emitting diode) and an LDR (light dependent resistor) in a single package.

When the LED shines light on the LDR, the resistance of the LDR goes down. That’s it!

If you expose an LDR to daylight and measure the resistance, you will find a value in the range of 1kOhm (the actual value depends on the type of LDR, and may be higher or lower). If you put the same LDR in a dark environment, the resistance goes up to 1MOhm or much higher. 

By putting an LDR and an LED in a package that does not let in daylight, we can control the resistance of the LDR by the light of the LED.

When the LED is off, the resistance of the LDR is very high, when the LED lights up, the resistance of the LDR goes low.

Applications

The great thing about the LED shining light on the LDR is that there is no electric connection between both components: the only connection between both components is light! This opens a lot of possibilities, like modding existing circuits without changing the original circuit. This way, you can add a potmeter or voltage control input to existing circuits. 

Vactrols are great to switch audio on and off without annoying clicks. This is because it takes some time for a vactrol to react to a transient from on to off or from off to on: switching off the LED does not mean an instant high resistance of the LDR, while switching on the LED does not result in low resistance of the LDR instantly. 

Aside from switching, controlling a resistor by a control voltage is very useful in VCO's, VCA's, LFO's, tremolo, vibrato, etc

Vactrols are not linear…

Aside the fact that a vactrol does not react instantly on a change of light intensity, there is no linear relationship between the LED light output and the LDR resistance. 

This means that when you double the current through the LED, the LDR resistance does not decrease by the same factor. In addition, an LDR has a memory effect: the resistance also depends on the light/dark situation just before the current situation. And that’s not the end of the story: every vactrol will behave slightly different. 

…but they sound very musical

However, the few milliseconds to switch them on or off tend to be very musical. Just as the human hearing system is not linear (to perceive double loudness of a sound, we need to make it 10 times ous loud from an electric point of view!) some of the vactrol imperfections match well with human hearing.

LPG

The Art  is of course in using the disadvantages as features! A good example is the so-called LPG or Low Pass Gate, very popular in west coast synthesis. An LPG is basically a vactrol switch, and its sound is determined by the transient between on and off, the low frequencies are passed earlier colpared to the high frequencies (fastest transients) get through later in time will be less compared to the lower frequencies: no unpleasant switch clicks! After this initial transition, high and low frequencies get through in the same way, unless a lowpass RC filter is added. 

Compressor

The first commercial use of vactrols was by Teletronix, who used a vactrol circuit in their audiocompressor LA-2. When I was mixing vocals in music productions, I was searching for the best compressor during several months, and finally found a software version of the LA-2 (€30k for a vintage hardware version was out of reach): it is incredibly soft and musically, even when you push it hard. 

Noise and distortion of an LDR 

The noise of an LDR is greatly dependent on thermic noise (like normal resistors), practically above 10kHz in case of 50 volts or more: negligible in most audio applications. For signals below 300 mV and a resistance below 100 kΩ, distortion levels are extremely low. The dark resistance of LDRs degrades gradually over the years.

Are vactrols expensive?

If you want a Xvive VTLC1 from Thonk, you will pay at least €5. If you buy a Chinese VTLC1 you will pay about 1€. I’ve tested both: Xvive vactrols are sealed from outside light because the components are poured into a mold, whereas a €1 vactrol is put in a plastic housing that has light leaks via the holes for the leads: one day I thought that a lot of 10 vactrols was out of spec because the off resistance was below 1MOhm, till I covered a vactrol with my hand: suddenly the resistance went up by a factor 10! Of course, in the darkness of your modular case this becomes less of a problem. In addition, 1 out of 10 cheap vactrols is different in terms of specs.

So far the conclusion is: if you only need a few vactrols and you want to be sure that the circuit begaves like intended: don’t hesitate to spend €5 per piece.

If you plan to use several vactrols: buy a bulk quantity and spend a few minutes to check the values in both off conditions (remember the impact of ambient light!) and on conditions (5V with 1k in series with the LED is perfect. Because the forward voltage of a red LED is about 1.8V, the remaining voltage over the 1k resistor is 5V - 1.8V = 3.2V, and the current through the LED is the same as the current through the resistor because they are in series: 3.2V / 1kOhm = 3.2 mA): mark the vactrols that show a different behaviour to be used for special projects.

So this is it, €1 for a vactrol? No! If you make your own vactrols, they will cost you just around €0.1!!!

When you go for DIY, you need to experiment with several types of LDR and LED.

The wrong type of LDR, LED or combination can make your vactrol useless.

I’ve made several dozens of vactrols, and for most of my projects, an 5539 type LDR combined with a “normal” 3mm red LED (8.7mcd/10mA) gives the best result.

If I would use a 20mcd/10mA LED, I would need 20/8.7 or 2.3 times less current to have the same brightness. But maximum brightness is not our goal here, it's about finding your own unique vactrol behaviour, to make your own unique sound!

Once you find a good receipe, you can make 25 vactrols for the price of a good coffee!

To be continued soon!