The Valvotronics Tube Amplified Direct Box ($400) offers some interesting features that should appeal to electric-bass guitarists. A “bass tilt” switch provides a 6 dB-per-octave lowpass filter with a corner frequency at 500 Hz — excellent for producing pillowy electric-bass tracks devoid of high frequencies (think reggae music). Also provided is a continuously variable rotary control that provides up to 20 dB of bell curve cut roughly from 1 to 1.25 octaves in bandwidth and centered at 7.5 kHz. This passive filter does a great job of diminishing pick noise; the filter can be switched out of the audio path to accommodate bassists who play their instruments with bare fingers.
Other notable controls include a continuously variable attenuator knob that provides up to 6 dB of level cut and a switch that lifts signal ground (pin 1) on the unit's balanced, low-impedance XLR output.
In addition to the XLR output (which you patch to your mic preamp for further amplification), three unbalanced tip-sleeve I/O jacks are provided: a high-impedance instrument input, an unbuffered Thru jack (wired in parallel with the input jack; this provides a mult that you can route to your guitar amp), and a buffered Tune Out jack (for connection to a tuner). No power switch is provided; plug in the captive AC cord and the unit is on. The box is built like an armored truck and is well suited for both studio and live work. A convenient top-mounted handle makes it easy to pick up and carry the unit.
The box has no owner's manual (or spec sheet), an oversight that should be corrected to facilitate understanding and optimal use of the unit's more unusual controls. However, I dug up some interesting facts by contacting the manufacturer. For instance, the instrument-input's impedance is specified to be greater than 1 Mž. The primary audio path is all-tube, with a custom-wound transformer output (the Tune Out circuit is buffered by an op amp). A single 6AK5WA RF pentode is used in a triode configuration to buffer and amplify the input signal. With the attenuator knob turned fully clockwise (no attenuation), the unit's frequency response is purported to be “flat to beyond 100 kHz.”
The Valvotronics Tube Amplified Direct Box is easily the biggest-sounding DI I've heard on electric-bass guitar, dishing out insane amounts of rich tube saturation and corpulent bass frequencies. Designer Robert Derby attributes this to his special tube circuit, which he says produces “a high amount of second-order distortion without thirds or fifths.” The signal actually sounds slightly compressed, but in an immensely pleasing way. Another plus is that the box's output level is very hot.
With my Roland Juno 106 keyboard plugged in to the Valvotronics tube DI, I got wonderfully lush synth-brass sounds and deep, punchy synth-bass tracks. Guitarists will also get good use out of the Valvotronics DI. Although it isn't the most sparkling DI I've heard for recording electric guitar, my 1962 Fender Strat nevertheless produced pleasingly clear and well-balanced tones through this box.
The only problem I had with the unit was a persistent AC hum and buzz, which didn't go away even when I engaged the unit's ground-lift switch or lifted the AC plug's ground lug. Valvotronics has since determined the cause of the problem — a redundant wire originally included as an extra precaution against electric shock in the case of someone opening the box without first unplugging it from the AC — and removed it.
Aside from the noise produced by the review unit, the Valvotronics Tube Amplified Direct Box sounds phenomenal, and is quite distinctive when compared with other units on the market. Add the unit's generous and innovative feature set and rock-solid construction to the mix and you have an incredible bargain.