Acoustic Guitar (November 1997) By Stephen Dick
One of the ironies of playing acoustic guitar is that just when you get good enough to play in front of people, you have to deal with the world of amplification and all the technical hassles that made you go acoustic the first place. One of these hassles is known as impedance. Technically, impedance is opposition to AC current flow. Most electric guitars (including acoustic guitars with built-in pickups) and amplifiers operate at high impedance. Mixing boards, most professional studio gear, and microphones operate at low impedance. If you’ve ever tried to run a microphone directly through a guitar amp, or plugged an electric guitar directly into a mixing board, you know the problems of unmatched impedance. Although you probably won’t blow anything up, you’ll have a hard time getting a decent sound. Many guitarists use a direct box to preserve their acoustic sound in the electric world of PA systems and studio mixing boards. A direct box is a transformer that converts a signal from one impedance level to another. It allows you to plug a high-impedance instrument, such as an acoustic guitar with a built-in pickup, directly into the mixing board in a recording studio or into the sound system at a gig. There are a number of advantages to using a direct box. One obvious advantage for acoustic guitarists is that you don’t have to try to get an acoustic sound out of an amplifier optimized for rock ‘n’ roll distortion. If your sound is essentially acoustic, a direct box allows you to bring that sound directly into the mixing board. Another advantage is the fact that low-impedance signals deteriorate less than high-impedance signals over long lengths of cable. Direct boxes can also be used as splitters, sending out a low-impedance signal to a mixing board and a high-impedance signal to a guitar amp, giving you the best of both worlds. The VTDB-2 Tube Direct Box from Demeter Amplification is a hand-assembled, professional-quality direct box. Approximately the size and shape of a hardback novel, the VTDB-2 has one quarter-inch input, a low-impedance XLR output, and a high-impedance quarter-inch output. It has a power switch, a ground lift switch, and a boost switch. At both the high-impedance and low-impedance outputs, the VTDB-2 delivers a beautifully warm, clear, strong signal. The most impressive aspect of the VTDB-2 is its transparency. Acoustic guitarists are used to hearing some kind of sound coloration whenever they plug in. With the VTDB-2, the sound that comes out is essentially the sound that goes in, only stronger. This strength comes from a Jensen transformer and a 12AX7 tube used to amplify the signal. It’s the same tube used in the preamp section of most tube guitar amps. It brings a noticeable warmth and fullness to the tone of the output signal. Paradoxically, what makes the VTDB-2 deliver such a strong low-impedance signal is that it operates at a higher-than-normal level of impedance. This extra headroom means that you can play with a wide dynamic range without overloading the system or creating distortion. The high-impedance output signal also gets an extra boost from the 12AX7. It acts like a pre-preamp, adding noticeable clarity to the sound. The boost switch increases the low-impedance output by 15 dB, which is useful when your input signal is a little weak or when you just want that much extra punch. That boost switch is the only control the VTDB-2 gives you over your sound. This may seem a bit Spartan for those contemplating using a direct box instead of an amp (and at $575 list, you may have to choose between the VTDB-2 and a good-quality acoustic guitar amp), but unlike an amp, which is designed to modify as well as amplify the signal, the only purpose of a direct box is to deliver a clean low-impedance signal. So, if all you want is to bring your acoustic sound into the mix with as little coloration as possible, the VTDB-2 will do the job beautifully with warmth and power.