YOU USED to have to go to an audiophile’s basement to hear the “warmth” and “punch” of analog preamps and compressors. Now those formerly boutique items are nearly ubiquitous. Try these tips for optimizing outboard preamps.
Preamp Primer Tube preamps, as well as other oldschool designs involving transistors and transformers, leave their own sonic fingerprint. Drive a tube gently, and it adds subtle, pleasant harmonics to the sound, as well as rolling off sonic information we perceive as harsh, which can make even a very good digital keyboard sound less . . . digital. Transformer-coupled preamp designs give more thickness or “meat” to the sound, especially in the bass and low-midrange.
Pres in Practice Can you plug a keyboard into a mic preamp safely? Yes, provided you’re careful with the volume control on each. Technically, a preamp is anything that amplifies a signal. If a unit has enough gain to boost the relatively low signal a mic generates, it’s a mic preamp. That’s way more gain than you need for a keyboard, but you don’t have to max it out to benefit from its sonic characteristics.
The harder a preamp works to bring up a signal’s level, the more of its own tonal characteristics it imparts to the sound. Since a dedicated preamp, compressor, or direct box has higher-quality analog circuitry than the output stage of the average synth workstation, John Songdahl of Boston-based Professional Audio Design suggests breaking the “keep the keyboard volume at maximum” rule for analog output stages. “Just as with guitar amps, each keyboard’s output stage has its own sound and character, and to keep prices affordable, manufacturers favor chip-based output circuits,” he explains. “The trade-off is in the character of the sound. Older, non-chip-based output stage designs helped give analog synths like the Minimoog and Prophet-5 their sound. One way to get some of that mojo back is to rely less on your keyboard output, and make up the gain with a tube mic preamp” But Craig Anderton reminds us that with some digital synths, lowering the output level lowers resolution, which may or may not be desirable. As always, your ears will tell you more than a knob position.
Try A DI The more you want to work the preamp, the more you’ll need to reduce the keyboard’s volume to avoid distortion. Go too low, and the preamp will be magnifying a wimpy, noisy signal. A direct box (DI) hits the Golden Mean here, making the preamp think it hears a microphone even with the keyboard’s volume up. The payoff : You can now be more adventurous with the preamp’s gain. A passive DI is generally best here, because the transformer inside it is better at soaking up hotter signals. Active DIs give truer sound when you need to put a weak signal, such as from a guitar pickup or vintage Rhodes Stage piano, on a level playing field with mics and keyboards.