Tech Tip:Debunking Preamp Myths
Don't believe everything you read . . .
By Craig Anderton
Preamps—just like mics, speakers, and record company lawyers—are shrouded in mystery. They provide that all-importrant link between a mechanical transducer, such as a mic or guitar pickup, and the circuitry that follows it, such as a mixer or computer audio interface.
There are a lot of preamps out there, from cheapo tube preamps to audiophile devices that costs thousands of dollars and sound like electronic silk. People get very passionate about their preamps, but that's because there are distinct, subtle differences among the various types that even those without golden ears can hear. However, as with most "golden ear" topics, there's a lot of mythology surrounding preamps—some of it true, some of it half-true, and some of it flat out wrong. Let's sort out fact from the fiction.
- A preamp has to use a Class A amp design to sound any good. There's no denying that a Class A amplifier, where one or more transistors are conducting current all the time, is theoretically capable of the lowest amount of distortion. Class B amplifier configurations, which are far more common, use separate transistors to conduct a waveform's positive and negative portions. They got a bad rep because in their formative years, there would be delays as the signal switched back and forth between the two transistors, thus leaving a "notch" in the waveform. (Class A/B amps tried to fix this by overlapping the conduction, but this wasn't always an ideal solution.) These days, though, Class B circuits can be designed that have no audible, and often no significantly measurable, distortion; and they save energy because they don't dissipate the significant amounts of power that Class A circuits do. Still, when only the best will do, it's hard to go wrong with a good Class A design.
- The higher the impedance with guitar, the better. This is true only with passive pickups, and then only if you add "up to a certain point." An excessively high impedance is more likely to pick up hum, noise, and other interference. Usually, an impedance of 250kâ¦ to 2Mâ¦ is more than sufficient to avoid loading down a guitar with passive pickups. (Note that mics with built-in preamps and guitars with active electronics are almost always relatively unaffected by the input impedance of the stages they feed.)
- High impedance is the only way to go with guitar. Not always. It's certainly true that lower impedances reduce a passive pickup's high-frequency response and overall level, but this may be desirable if you're feeding a subsequent distortion stage. Rolling off the highs by loading down the pickup can produce a creamier distortion effect. In fact, one company (Radial Engineering) makes some direct boxes and preamps with "drag" controls that emulate the effect of having a low input impedance.
- Tubes sound better than solid state. The reason why many people prefer the sound of tubes is not because they sound more accurate, but because they can color the sound in a subjectively pleasing way. Tubes add a natural compression, possible slight high-frequency reduction due to the "Miller effect," hiss, and harmonic accentuation. Regarding solid state, a well-designed preamp -- whether using bipolar transistors, FETs, or integrated circuits—can certainly sound transparent and accurate to my ears; although conversely, there are some highly skilled tube designers who make extremely accurate tube-based products. In any case, even though computer emulations of tubes get better all the time, there's still nothing quite like a tube sound.
- The preamp has a huge influence on the way a dynamic mic sounds. Yes, but I think a more accurate phrasing is that the mic has a huge influence on the how the preamp sounds—a preamp may have a certain "color" with one mic, but not with another. This is because a dynamic mic presents an inductive load that interacts with the preamp's input stage, which can alter the overall frequency response. Some preamps have adjustable input impedances, so you can dial up the best sound for your particular set of mics.
- You want to match the output impedance and input impedance of the mic and preamp. That's true if you're installing telephones, where the object is a maximum transfer of power. What we want is a maximum transfer of voltage, which means that for the most accurate signal transfer, a stage's output impedance should be 10-20% the impedance of the stage it feeds. However, note that the impedance changes with frequency; for example, a guitar pickup's high frequencies exhibit a higher output impedance than low frequencies (this is why low impedance inputs reduce highs). Add other variables to the equation, and it's no wonder different combinations of gear can sound very different.
- An expensive mic preamp will always sound better than a cheap one. All things being equal, most people can hear the difference between a high-end boutique preamp, and the "let's-hit-a-price-point" preamps found in, say, budget mixers. However, it's important to remember that certain combinations of preamp, mic, and cable may lead to an inexpensive preamp sounding better for that particular combination of gear, or with a particular signal source. So, even if you have a really great preamp that cost as much as the GNP of some third-world nation, try out some of your other preamps as well. They may do the job better in some cases.