1.1.Light Dependent Resistors (LDRs)
An LDR is the marriage of a photoresistor with a light emitting diode (LED) in a small sealed package approximately the size of an M&M candy. Each LDR has 2 pairs of wires. One pair connects to the LED while the second pair connect to the photoresistor.
The resistance of an LDR varies in proportion to the brightness of the LED which is in turn dependent on the amount of electrical current running through the LED. As the LED becomes brighter the resistance of the photoresistor decreases.
Through precise control of input current, LDRs can smoothly regulate resistance over a wide enough range to provide effective high performance audio volume control.
LDRs optically isolate the control circuitry from the audio signal passing through the photoresistor. The audio signal only encounters a variable resistance that is regulated by photons (i.e. light) and not electrons.
All Tortuga Audio preamps utilize LDRs to control volume (i.e. to attenuate the audio signal).
Why Do We Use LDRs for Volume Control?
We use LDRs because the resulting sound quality is amazing. Adjectives like clear, open, unveiled, articulate, and uncolored all apply to the LDR. All while maintaining excellent bass and overall dynamics even in a passive preamp.
If LDRs are so great why then why don’t all preamps use LDRs? Reasonable question. Here’s why not.
The simple reason is LDRs are notoriously difficult to use for volume control. They are inherently nonlinear which means they act differently at different levels of resistance. They are also not consistent from one to the next even within the same make and model from the same production batch. Their performance curves may even drift slightly over time. LDRs also have higher distortion characteristics than most other alternatives although not enough that it really matters subjectively.
For all the above reasons most audio designers have avoided taking on the challenge of using LDRs.
Tortuga Audio took on this challenge several years ago and has not looked back. We overcame these technical challenges. We are currently on our 4th generation of our LDR preamp control technology.
How are LDRs Used for Volume Control?
Volume control with an LDR is conceptually similar to a potentiometer so lets look first at how the common potentiometer (“pot”) is used for volume control.
Potentiometers used for volume control typically have a fixed resistance between 10k and 100k ohms. The audio input signal connects at one end of this fixed resistance and the other end is connected to ground. The output signal from the pot is a third connection to what’s referred to as the “wiper” which slides along the pot’s fixed resistor. This is shown in the diagram below.
The variable resistance above the wiper (the output) is the series resistance (Rseries) and the variable resistance below the wiper is the shunt resistance (Rshunt). The sum of Rseries plus Rshunt equals the rated impedance (10k, 50k, 100k etc. ) of the pot.
The resistance Ratio is defined by the formula: Ratio = Rshunt/(Rshunt + Rseries). This Ratio also happens to be the voltage ratio of Vout divided by Vin such that Vout = Vin x Ratio. A pot’s attenuation level at any point along the wiper can be expressed in decibels which is defined as 20 x log(Ratio). Putting all this together your arrive at dB = 20 x log (Vout/Vin) where Ratio = Vout/Vin = Rshunt/(Rseries + Rshunt).
Thus, when Rseries is zero the volume is maximum (no attenuation) and when Rshunt is zero the volume is zero (maximum attenuation).
With LDRs, attenuation is achieved by varying the resistance levels of 2 separate series and shunt LDRs to achieve specific resistance ratios that correspond to specific dB attenuation levels. While a pot does this mechanically, the resistance levels of both the series and shunt LDRs must be done electronically. Doing so reliably and repeatedly can be a challenging design problem.