Temporary intro to vactrols

Vactrols, or each an LED and a light-weight relying resistor (LDR) in a light-tight housing, are present in analog music electronics circuits like audio compressors, voltage-controlled amplifiers (VCAs), voltage-controlled filters (VCFs), and different purposes.

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These days, analog ICs are used for this goal, so vactrols have develop into fairly uncommon. Considered one of their principal benefits was and stays the low giant sign distortion in comparison with transistor circuits.

Then again, they’re gradual and sluggish when pushed by small management currents and have a nonlinear attribute curve.

Fortuitously, the attribute curve of the conductance versus management present is extra linear than that of the resistance. That is advantageous, as an illustration, for VCFs with a frequency response proportional to 1/RC. For music electronics purposes, nevertheless, exponential management of the conductance is most popular since voltage-controlled circuits use the “volt/octave” attribute, whereby with every volt of further management voltage, the cutoff frequency of the VCF doubles.

One other benefit of exponential vactrol management is the truth that the LED present by no means turns into 0 [y= exp(x) > 0] and thus the LDR by no means reaches its full darkish resistance, which has a constructive impact on the response time of the LDR.

A vactrol circuit

Often, a pair of transistors is used to transform a linear management voltage into an exponential present. Within the case of a vactrol, nevertheless, the pair of transistors might be changed by the LED itself, which is like every diode a voltage-controlled exponential present supply.

For temperature compensation, two matched LEDs are required, just like the transistor circuit.
Determine 1 reveals the simulated circuit of the exponential vactrol management.

Determine 1 An exponential vactrol drive the place a reference LED is used to transform a linear management voltage into an exponential present, and two matched LEDs are used for temperature compensation.

The LED2 is operated with Iref = -V/R4. At CV=0, the present within the vactrol LED2 is similar, and the resistance of the LDR is about to the center of the specified resistance vary by way of Iref, right here about 30 µA.

As CV will increase, the voltage on the cathode of LED2 decreases, however the voltage between the anode and cathode will increase in order that the LED present will increase exponentially.

With a unfavourable CV, the voltage throughout LED2 decreases accordingly, in order that the LED present decreases exponentially. The vary of the LDR resistance is decided by summing amplifier U1’s achieve. In sensible purposes, a variety of ~ 1 MΩ (CV = -5 V) to 1 kΩ (CV = +5 V), is used, so {that a} VCF might be tuned from 20 Hz to twenty kHz.

Thermistor R3 improves the temperature drift of the LED present. Nonetheless, the LDR’s temperature dependence stays at roughly 0.2%/Okay, which makes the vactrol circuit much less appropriate for high-end VCOs.

For different purposes (VCF, VCA), the temperature drift is sweet sufficient, and usually, the thermistor might be omitted.

Determine 2 reveals the simulated resistance curve and LED2 present at 20°C and 40°C.

Determine 2 The simulated resistance curve and LED2 present at 20°C and 40°C.

Sensible notes

A small PCB was developed for the circuit. The SMD LEDs are normal white varieties in a 5730 case. Vactrol LED2 is on the PCB prime facet and illuminates two GL5537 LDRs, that are organized at an angle of roughly 45 levels above LED2.

By barely bending the LDRs, they are often mechanically trimmed for matching resistance. A small black 3D-printed field and a PCB with black solder masks forestall exterior gentle from affecting the circuit. Circuits with two and 4 LDRs illuminated by one LED have been efficiently examined to implement 2nd- and 4th-order VCFs.

Uwe Schüler is a retired electronics engineer. When he’s not busy along with his grandchildren, he enjoys experimenting with DIY music electronics.

Associated Content material

• Vactrol – A Lazy Stroll
• Computerized Road Gentle Circuit
• LDR = Gentle Dependent Resistor = Photoresistor
• Electroschematics LDR circuits

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