Q: “Why do some mic preamps have stepped attenuators, and why do many of those cause a pop in the audio signal when moved? It makes it impossible to change levels during a performance.”
A: Stepped attenuators are used in many different types of preamps in most cases for reasons of quality. Compared to standard potentiometers a stepped attenuator can be more precise, is “recallable,” and can provide a higher integrity path for audio. Understanding the way a potentiometer works will provide some insight. A potentiometer uses some type of strip of resistive material, and then a wiper slides across it to provide variable levels. A stepped attenuator, on the other hand, is basically a series of switches, with each position having a specific value of resistor tied across it. Building an extremely high quality, or “audiophile” potentiometer can be considerably difficult and expensive due to the nature of the construction. A stepped attenuator is generally as good as the quality of switching and the resistors that are used. It’s not difficult to incorporate extremely high quality resistors into a stepped system, which obviously yields a higher quality result. Additionally the contact that is made between a wiper and the long surface of a potentiometer can become degraded rather easily compared to what happens in a switched system. So, while the two may start off sounding equivalent, a stepped system will generally retain its quality long after the pot’s quality starts to degrade. This degradation that occurs in pots is what we eventually hear (once it gets bad enough) as a scratchy sound when moved. It’s a result of the contact between the wiper and resistive material becoming degraded and inconsistent, which clearly has ramifications on the integrity of your audio.
The popping is caused by the design of certain (not all) gain stages. There are a few different ways to come up with it, and the details of each aren’t really important to get in to here. Suffice to say that any type of switching of audio signals potentially can cause pops (anyone who’s ever made a simple A/B switch for guitars has discovered this). The pops can be prevented, but it requires extra components in or around the signal path to do so. A simple capacitor can work wonders for this type of problem, but they can also introduce phase shift problems and other high frequency anomalies to the signal. Since one of the guiding principles of many high end preamps it minimal componentry in the signal path it is sometimes decided to live with the popping to obtain higher overall sound quality. For the most part this makes perfect sense because it generally isn’t necessary to be able to change input levels during the actual performance. Even if there were no pop most engineers find the abrupt level change makes it impractical. It’s also worth noting that some designs using potentiometers have the same problem, and the pot just helps to disguise it. If you’ve ever quickly turned a gain pot on a preamp and heard a whooshing or subtle scratching sound you’ve heard this same problem in disguise. Were that a stepped attenuator it would be a pop.
The best remedy for all this is a good sound check, which hopefully will preclude you from needing to change levels later. If your circumstances dictate the need for a preamp that doesn’t pop we’d recommend you get one without a stepped attenuator. There are some designs that use stepped attenuators, but offer a trim control for fine adjustments. In some cases the trim control can be switched in our out of the circuit as needed.