The light touch

Before we move on to learning about the upcoming Stellar Gain Cell DAC's unique volume control I've been asked to touch upon a few outlier technologies to control level. We've been learning the importance of level controls. They are the single most difficult design challenge in a preamplifier when sonics matter. In most volume controls (but not all as we shall see in Stellar's Gain Cell DAC) we use resistors to divide the signal. The quality of that resistor, as well as the mechanism that controls its value (a pot's wiper or an attenuator's switch) determines how good or bad it sounds. In nearly all cases resistors have fixed values. Even a pot, known as a variable resistor, is actually fixed at one resistance value. For example, a 10K pot is a fixed value resistor. It becomes variable by rubbing a metal contact up and down its surface, picking off points of different resistance. Its overall value remains constant. A rather unique resistor that we have yet to discuss is the opposite—a truly variable resistor. Known as an LDR (Light Dependent Resistor), this device has no fixed value. As its name implies, the value of its resistance is dependent on the amount of light applied to its surface. LDRs (or photoresistors) may sound exotic, but chances are excellent you have several in your home. These devices are used in all manner of consumer products: automatic lights, camera light meters, clock radios, alarm devices (as the detector for a light beam), night lights, outdoor clocks, solar street lamps and solar road studs, etc. Crafted from an increasingly banned substance, cadmium sulfide (it is the cadmium that is being banned), these photoresistors can also be used in a volume control. It's easy to imagine how. Place an LDR in series with your musical signal. In the darkness you would hear no music because LDRs exhibit very high resistance without light. Shine a light on the cadmium cell and resistance decreases, from several megaOhm to a few hundred, depending on the light's intensity. As resistance decreases, music flows through. This is a rather clever solution to our problem of building good sounding volume controls. Instead of suffering the imperfections of the pot, or the complexity of the stepped attenuator, a simple LDR and a lightbulb solve the problem. Unfortunately, like all things in engineering, it's never quite so simple. There are two main problems with LDR attenuators: the audio performance of the cell itself and variability. With respect to performance, all photoresistors have a sound to them regardless of materials: cadmium sulphide, Lead sulphide, indium antimonide, Ge:Cu photoconductors. This "sound" as we have seen in past posts isn't beneficial. What we hear is molested audio. No volume element makes music better. They can only damage what's there. The goal of the engineer is to do as little harm as possible. For my money, LDRs are not as good as the best resistors. Variability is perhaps their biggest challenge. No two cells provide the same resistance for a given light output. Worse, even the same cells vary with temperature and other environmental impacts. Imagine the difficulty of building a stereo volume control that uses two cells. Tracking the channel-to-channel volume within tight standards, like 0.1dB (which is our own), is a tricky feat that gets even more difficult in a balanced volume attenuator where 4 cells are needed to track each other to the same standards. There are a few companies successfully implementing LDR volume controls. It's a unique path, but one advantageous for its uniqueness more than its contribution to the betterment of high end audio performance.