DIRECT ACTION

In 1980, James Demeter was thumbing through a tube-electronics book when he stumbled upon the schematic for a cathode follower circuit. That circuit would
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In 1980, James Demeter was thumbing through a tube-electronics book when he stumbled upon the schematic for a cathode follower circuit. That circuit would become the basis for the Demeter VTDB-2 Tube Direct, the first commercially available tube direct-injection (DI) box. The VTDB-2 appeared on the scene at a time when the pro-audio industry was fixated on solid-state equipment and many studios were discarding tube units like poison apples. Although fully aware that his “tube direct box” was a new and wonderful-sounding invention, Demeter thought no one would give a hoot, so he failed to copyright the product category name.

Today, more than a dozen tube DI boxes are on the market, and they don't appear to be going away anytime soon. Recording engineers have fallen in love — or in some cases, back in love — with the sound of tubes, and tube direct boxes are now an integral part of the modern recording chain.

What makes tube DIs so useful and compelling that nearly all pro engineers feel they must own at least one? This article will explore the benefits of DI boxes in general and tube DIs in particular. I'll offer practical examples of how to get the most out of DIs, including proper interfacing with mic preamps, recording consoles, and guitar, bass, and keyboard amps. Because all tube DIs are not alike, I'll discuss the pros and cons of different designs and feature sets so you know what to look for when purchasing a new unit. Finally, I'll look closely at the features and audio quality of eight of the hottest tube DIs on the market. I'll begin by looking at how the tube DI box evolved and why it is such a useful recording tool.

STOP THAT RACKET

Unwanted noise can bring a recording session to a halt. One perennial source is unbalanced instrument cables used with electric guitars and basses. That's because unbalanced cables are prone to picking up radio frequency interference (RFI) from sources such as TV sets, computers, and digital-effects processors. They may also pick up hum or buzz from electromagnetic interference (EMF), the evil offspring of lighting fixtures, AC outlets, and power supplies.

Guitar cables act as involuntary antennae for those invisible demons. As you boost the output of a guitar or bass being played through an amplifier, you also boost any induced noise the cable picks up. Nothing is more distracting than hearing the ninth inning of a baseball game broadcast through your guitar amp while you're trying to lay down a hot solo.

A DI box solves such noise problems in two ways. First, it converts your instrument's unbalanced, high-impedance, instrument-level output signal into a balanced, low-impedance, mic-level signal, which is much more immune to induced noise. That allows you to run longer, balanced cables (from the output of the DI box) without creating a huge antenna for sonic garbage. Second, recording with a DI box lets you forego the use of microphones, which indiscriminately record environmental noise along with the musical performance flowing out of your amp.

The sound of a miked amplifier or acoustic guitar can be a wonderful thing, too, so you may not want to eliminate its contribution to a song. In that case, the DI track can serve as an adjunct to the mic signal(s), or vice versa, depending on how you choose to mix the tracks. In addition, blending the DI and mic tracks together can help increase the overall signal-to-noise ratio of the combined signals.

There are other worthwhile reasons to use a DI box besides simple noise prevention. A good DI box also preserves signal quality in other ways, and typically, it provides a sound that is quite different from that of a miked amp.

SOUND OF MUSIC

Most readers know that long guitar cables can act like capacitors, killing high frequencies and resulting in a dull, lifeless sound. A good DI box can condition the instrument's output signal; that makes it less susceptible to the negative effects of long cable lengths and thus preserves the high-end sparkle of the instrument. But that isn't the only way a quality DI box can preserve the integrity of the original signal.

Every element of the recording chain, from room acoustics to recording medium, exerts a persistent influence on your tracks. Your room, for example, may impose boomy or weak bass, comb filtering, flutter echoes, or other unwanted sounds on your recordings. Or perhaps your guitar amplifier is a little ragged-out after its umpteenth bar gig, and you can hear the tubes giving out and a speaker starting to break up. Moreover, that $400 mic doesn't sound as great on guitar as you had originally thought — not to mention that it's picking up the sound of the drums pounding away in the adjacent room.

A DI box is immune to room sounds and acoustic bleed from other instruments, so it eliminates those problems. In addition, it lets you record direct to tape (or hard drive), thus bypassing the mic and guitar-amp stage of the chain. The result is a cleaner, drier, more focused sound. What's more, the recording setup is greatly simplified: there's no need to hassle with mic choice and placement, mic stands don't clutter the performance area, and you can bring the performer into the control room to discuss arrangement tweaks without worrying about control-room monitor bleed.

NEW INPUT APPRECIATED

Not all DIs are created equal. In fact, the first DIs sounded dreadful. When the Demeter VTDB-2 Tube Direct arrived on the scene in 1980, most DI boxes were passive. Those early passive units employed an input transformer to buffer (alter the impedance of) the input signal. For reasons beyond the scope of this article, the input transformer would cause the instrument's signal to lose high frequencies and make the sound dull. The only way around that was to raise the transformer's impedance to a point at which there would be an unacceptable loss in signal level — as much as 40 dB. (For a more in-depth explanation of how a DI box works, see “Square One: Going Direct” on p. 96.)

A few companies, Demeter's included, had already made DIs that used semiconductor devices instead of transformers on the inputs. That design preserved the instrument's high end with virtually no loss in signal level. However, those solid-state DIs had an edgy quality to them, not to mention harsh overload characteristics.

Demeter realized that a tube can be manipulated to deliver the extremely high input impedance that a DI must have to sound transparent. A tube also preserves the input-signal level and makes the sound richer to boot. Demeter scrapped the input transformer and semiconductors, inserted a dual-triode tube in their place, and the first tube DI box was born.

Subsequent tube DIs have incorporated significant design changes. A look at the pros and cons of different designs should help you decide what's best for your applications.

INSIDE STORY

Most tube DIs available today employ a dual-triode tube. The first triode buffers the input signal; the second is often used to boost the box's output level. Some manufacturers prefer not to boost the outputs of their DIs at all, however, arguing that everyone already owns a mic preamp (and thus can boost the signal as needed) and that adding another stage of amplification inside the DI only degrades the signal quality.

As with most things electronic, there's no simple right and wrong. A high-quality output-level boost circuit can be made to sound great. Likewise, a DI-box design could conceivably forego the boost circuitry, only to degrade the signal through an oversight elsewhere in the signal path. That's one reason I tested a variety of units for this article — to learn, after the theory is laid to rest, how the units actually sound.

The outputs of most DI boxes typically run in the — 40 to — 15 dBm range (mic or instrument level). However, some manufacturers achieve hotter levels by using semiconductors in lieu of transformers, which automatically reduce output levels by roughly 20 dB, to electronically balance their DI outputs.

Why don't all manufacturers use this hybrid design, tube and semiconductors, to preserve 20 dB or so of gain? There are two reasons. First, transformers have far greater ground-lifting capability than electronically balanced circuits, so they reduce hum much more effectively. If you'll be using your DI only in a control room, where cable runs are typically short, an output transformer is not critical. However, for applications that require long cable runs — playing onstage, for example — a transformer-coupled DI output will do the best job of keeping induced noise to a minimum.

Using an output transformer also allows the manufacturer to maintain an all-tube design. After all, solid-state devices sound edgy, right? Again, it's not as simple as that. What matters is quality — in components and in design. A high-quality hybrid or solid-state DI, for example, can sound considerably better, and warmer, than a poorly designed all-tube box. That is partly because all transformers distort signals to some degree; they tend to saturate the sound when hit with excessive input, especially of very low frequencies, and they can also cause a mild attenuation of highs. Indeed, some people seek out particular transformers precisely because they saturate the sound (in a musically pleasing way, that is). In addition, although an all-tube DI may sound smoother or warmer than a hybrid box, your signal will almost certainly run through gobs of semiconductors located downstream in the signal chain before it reaches final mixdown.

PROGRESS IMPEDED

Just the same, I strongly consider one specification when deciding which DI box to purchase: the measure of the unit's input impedance. A DI box's input impedance has a profound impact on the sound of both passive electric instruments with magnetic pickups (such as electric guitars and basses) and acoustic instruments fitted with piezoelectric pickups (such as acoustic guitars and mandolins).

A magnetic pickup is basically a coil or inductor. An inherent property of inductive devices is that, as the impedance rises, the device is able to pass increasingly higher frequencies. For that reason, DIs that offer significant input impedance will generally produce, for example, more sparkly electric-guitar tracks because they don't load down the magnetic pickups.

The piezoelectric pickup, on the other hand, is essentially a capacitor. As impedance rises, capacitive devices are able to pass increasingly lower frequencies. Furthermore, piezos need to see a much higher impedance than magnetic pickups to be totally rid of loading effects and to pass full-bandwidth signals. So a DI must offer high input impedance to produce acoustic-guitar tracks with full bass content.

Modern tube DIs typically offer from 1 megaohm to 27 megaohms input impedance. Generally, units at the extreme low end of the range produce a softer, more muted sound, and those with a high input impedance tend to capture an extrasparkly sound (including more of the pick-strike sound, for example).

However, you can't specify the lowest acceptable input impedance — which is another reason why sonic comparison is necessary. But now you can see why the earliest DIs, with their transformer-coupled inputs that provided only 10 to 100 kilo-ohm impedance, stifled the highs on electric guitars and cut off the lows on acoustic guitars that have piezo pickups.

Using a DI box for a synthesizer will not improve the instrument's frequency response. Synths are not sensitive to the impedances you're likely to encounter with a mic pre or with the line inputs on your console, simply because they have no capacitive or inductive pickups to load down.

Ditto for an active bass guitar (that is, a guitar that has battery-powered electronics): the instrument's active circuitry preconditions the pickup's signal before it goes to the DI box (or wherever), and so it is not sensitive to impedance. You can therefore plug a synth or active bass guitar directly into an outboard preamp or console input, and it should sound pristine. You may still opt to warm up the sound first by patching the instrument through a tube DI box. However, if the DI features tube-gain boost circuitry (rather than a unity-gain audio path), it's possible that you are actually degrading the signal slightly by inserting that extra stage of amplification in the signal path — it all depends on the quality of the gain circuitry. In such cases, let your ears be the judge.

INS AND OUTS

Consider the external features when choosing a tube DI box. Any DI box worthy of consideration should offer a high-impedance, unbalanced ¼-inch input jack (for plugging in instrument cables); a low-impedance, balanced XLR output connector (for patching the DI's output to an outboard mic preamp or console mic input); and a high-impedance, unbalanced ¼-inch output jack (for sending the DI's signal to an instrument amplifier).

In some designs, a tube DI's unbalanced output is muted directly off its input jack — that is, it's wired in parallel with the input so that what goes into the box is exactly what comes out at the unbalanced output. When you patch that type of DI output to your amp, you'll get exactly the same sound as you would by plugging the instrument directly into the amp (as long as the cable lengths aren't unreasonably long).

In another design, the DI's unbalanced output may follow its tube input buffer. That offers the benefit of warming up an instrument's signal before it goes to an amp. Another benefit is that the tube input buffer gives the DI's unbalanced output a constant impedance that won't vary with frequency. Because an electric guitar's high-frequency response does vary with impedance, long cable runs tend to dull the instrument's highs. The constant output impedance of a tube DI's unbalanced output greatly mitigates high-frequency rolloff, letting you run cables as long as 40 or 50 feet to your amp without dulling the sound. That's why savvy live performers use a DI with a tube-buffered unbalanced output to feed an amp located across a large stage.

Other tube-DI manufacturers incorporate yet another design that places the unbalanced output after a tube-gain boost stage (in nonunity boxes). Although you certainly don't need that extra amplification if you're patching in to a guitar amp, the extra tube stage typically warms things up even more.

Having two DI outputs — an unbalanced out for your amp and a balanced out for your mic pre — allows you to play with two strikingly different sounds. You can choose which one you like best or use both patches simultaneously and combine their signals at your mixer for a more complex, layered sound.

GAINING CONTROL

A couple of other DI-box features are worth noting. Most DIs, whether tube or solid state, provide a ground-lift switch to reduce hum. Gain-boost circuitry may also be included; it is accessed with a switch that engages a fixed amount of boost or with a continuously variable knob.

Relatively few DIs provide a “speaker” or “amplifier” input jack. Those that do let you plug the output of your amp in to the DI. A switch on the DI must be set to Speaker mode so that the DI offers the correct input impedance for the setup; otherwise, distortion and equipment damage may result. Although having extra features never hurts, I've always found that is the worst way to use a DI box. The sound is always so bad that I can't see why anyone would want to run the electronic output of an amplifier through a DI box. If you want a “live” sound and you don't want to mic a speaker cabinet, several speaker emulators on the market produce results far superior to a DI box used in that manner.

All tube DIs come equipped with a power cord, and most also come with an on/off switch and power-status LED or lamp. That means that tube DI boxes require a nearby power outlet — an important consideration for live performers.

CREATIVE ROUTING

You can patch a DI in to your system in various ways. As with other types of tube gear, I always let my tube DI warm up for at least 20 minutes before using it to record. An hour is even better. A cold tube unit is a recipe for noisy and inferior-sounding tracks.

The simplest way to use any direct box is to patch the instrument into the DI's unbalanced input and patch the DI's balanced output into an outboard mic pre or console mic input. Whenever I record an acoustic guitar that has a pickup, I route the pickup's output through a DI box in that manner in addition to miking the guitar with a stereo pair of condensers (see Fig. 1). Recording each signal to a separate track allows me to choose any combination of the two miked tracks and one DI track at mixdown. I typically pan the two miked tracks apart (for example, at 10 and 2 o'clock) and add in a little bit of the DI track between them (at 12 o'clock) to anchor the sound.

You can usually fatten the sound further by delaying the DI track so that it's in phase with the two miked tracks. It takes about 1 ms for the guitar's sound to reach a mic positioned 1 foot away. Delaying the DI signal by the same amount (1 ms for every foot the mic is from the guitar) puts it in phase with the mic signals. That alignment technique reduces phase cancellations and comb filtering that would otherwise thin out the sound.

Electric bass guitar tracks can be fattened up using a similar technique (see Fig. 2). Simply patch the bass's output in to the tube DI's unbalanced input, the balanced output of the DI in to a mic preamp's or mixer's mic input, and the DI's unbalanced output to the amp. Mic the amp and route the miked signal to a separate channel on the board. Now you can record the DI and mic signals to separate tracks and combine them at mixdown, delaying the DI signal to align it with the mic signal for a fatter sound. If you're short on tracks, you can always align the two signals during studio sound check and submix them to one track while you record. (That setup works great for recording six-string electric guitars too.)

RULES OF ENGAGEMENT

More than a dozen tube DIs are on the market. Those evaluated for this article meet several criteria. First, all are dedicated DIs; no dedicated line preamps or mic preamps with DI inputs were included. All units feature tube-buffered inputs and a balanced XLR, mic-level output, and all fall within a specific price range — $200 to $800 per channel — so extremely high-end units are not pitted against budget models.

The eight boxes I will describe are the Aguilar DB 900 Tube Direct Box ($529), the AMB Tube-Buffered Direct-Injection Box ($595), the Anthony DeMaria Labs ADL 100-G ($599), the Demeter VTDB-2b Tube Direct ($599), the D. W. Fearn VT-I/F Vacuum Tube Instrument Interface D.I. ($1,500; 2-channel), the Manley Tube Direct Interface ($575), the Tube Works 4001 Real Tube Direct Input ($219), and the Uncle Albert's VTD-2A Vacuum Tube Direct ($450). Note that in cases in which a manufacturer offered mono and stereo versions of the same basic model, I reviewed only the mono unit. If you're interested in a mono unit I tested but are looking for a stereo DI, contact the manufacturer — a stereo version may be available.

All units tested feature high-impedance, unbalanced ¼-inch I/O, but they differ as to where the output is derived in the circuit. Some units have the output before the tube buffer, others immediately after, and still others after a gain-boost stage. In addition, all of the units provide an all-tube audio path and transformer-balanced XLR output except for the Tube Works 4001 Real Tube Direct Input, which features a hybrid design with electronically balanced outputs. Finally, each DI box tested is a portable desktop or floor unit (that is, sans rackmounts), and each has a ground lift. (For a tidy comparison of the units' features and specs, see the table, “Tube DI Box Features.”)

To test the units, I recorded electric guitar (a 1962 Fender Stratocaster), bass (Kramer Pioneer), and synthesizer (Roland Juno 106 set to a raspy pad) through all eight boxes and then compared the tracks. Each instrument/preamp combination was recorded to separate ADAT tracks against backing tracks of drum set and acoustic guitar. The guitar, bass, and synth tracks were recorded through each DI in turn, with the DI's output routed through a Millennia Media HV-3 mic preamp. I chose the HV-3 for its neutrality and solid-state design. I wanted as little coloration as possible from the preamp so as to better hear the tube characteristics of the DI box. I decided against recording any instruments through an amp because an amp's severe sound coloration precludes an accurate assessment of the DI box.

All tracks were recorded at 20 bits and 48 kHz through the A/D converters of a Yamaha 02R digital mixer. The 02R was slaved to an Apogee Rosetta converter providing master clock (an amazingly great combination, by the way). Great care was taken to record all tracks at consistent levels, just below 0 dBFS.

I listened to all the tracks through high-end, ultraflat (50 Hz to 22 kHz, ±0.5 dB) KS ADM 2 monitors powered by a Hafler P3000 Trans-nova power amplifier. The monitors were coupled to an Acoustic Sciences Corporation Attack Wall acoustical environment. I evaluated DI tracks soloed and in the mix. All evaluations are entirely subjective — what you're getting are my opinions.

Aguilar DB 900 Tube Direct Box. I was so impressed with the single-channel, all-tube DI from Aguilar that I bought one when it first hit the streets last winter. It has an ultrapristine, minimalist audio path with no gain boost; hence, it offers the weakest output level of all the units I tested. But the trade-off is well worth it.

On bass, the DB 900 offers the best of all worlds: perfectly balanced tone, low noise, a clear top end with truckloads of nuance and air, and an extended, tight bottom. You can clearly hear the 12AX7 tube working its magic; electric bass sounds very rich and warm through this awesome DI.

Interestingly, my Stratocaster sounded a tad thin through the DB 900. But what the tone lacked in fullness it more than made up for with nuance, sparkle, and detail. On synth pad, the Aguilar dominated the field once again, offering by far the richest, most resonant, and clearest tone of the units tested.

AMB Tube-Buffered Direct-Injection Box. AMB's German-made, all-tube DI offers 15 dB of gain boost through a front-panel switch. A bombproof chassis and detachable AC cord add to the unit's portability. The single-channel, unventilated AMB became hotter than the other DIs, but according to AMB, that is not cause for concern because the unit's components are designed to withstand high operating temperatures.

The Kramer Pioneer bass had a slightly dull top end when played through the AMB DI, but the bottom end sounded tight and deep. The Strat sounded mellow and warm, if slightly lacking in clarity and openness. On the synth pad, AMB's DI offered the thickest low mids and overall darkest timbre of the units tested, if only by a hair.

Anthony DeMaria Labs ADL 100-G. The single-channel, all-tube 100-G offers a continuously variable rotary gain control, but it attenuates (by as much as 21 dB) rather than boosts the output level. A two-position switch places the unbalanced output immediately after the tube input buffer or after the gain attenuator. Even with no attenuation, the 100-G offered the second-weakest output of the eight units. That won't be a problem if you have access to a high-quality mic preamp.

The 100-G offers a vintage-style bass-guitar sound with a soft top end and overall fat tone. Middle-bass frequencies sounded slightly hyped to my ears.

The 100-G imparted a fatter tone to my Strat than the Aguilar, but it was not as fat or unrestrained as the Tube Works, AMB, and D. W. Fearn DIs. Compared with those units, the 100-G sounded a little stiff or compressed. Also, the 100-G was a bit shy on high-frequency detail on guitar. But this is a quality unit, and some of those distinctions, though audible, were subtle. On synth, the 100-G was one of the richest, most resonant-sounding DIs of the bunch.

Demeter VTDB-2b Tube Direct. The Demeter VTDB-2b is a modified version of the seminal VTDB-2. The single-channel unit has undergone no design changes since 1987 — but then, why should it? This all-tube DI is my overall favorite for pop, rock, and country electric-guitar tracks. It gave my Strat an extremely detailed and transparent tone yet was fuller sounding than the Aguilar DB 900.

Patched through the VTDB-2b, electric bass easily cut through the mix, and its sound was present and rich in harmonics yet not at all thin or harsh. The DI's deep bottom end balanced out the overall tone beautifully. On the Juno 106, the VTDB-2b sounded a bit prominent in the upper midrange but, again, not at all harsh. If you're looking for a fat, lush tube DI with a lot of presence and a torrent of rich overtones, the VTDB-2b is your ticket to paradise. This one goes on my must-buy list.

D. W. Fearn VT-I/F Vacuum Tube Instrument Interface D.I. In dollars per channel, the dual-channel VT-I/F is the most expensive DI that I tested. Also the prettiest and most impressive-looking DI of the bunch, the 15-pound cherry red unit features a chassis machined from solid ¼-inch-thick aluminum plate and finished with a tough polyurethane aircraft finish. Heavy-duty toggle switches (for ground lift and power on and off functions), custom Jensen transformers, and a detachable AC cord enhance the handcrafted unit's appeal.

Like the Aguilar DB 900, the all-tube VT-I/F shuns gain-boost circuitry in favor of a minimalist audio path. Nevertheless, the VT-I/F offers considerably higher output level than the DB 900. The VT-I/F is one of the few tube DIs that places its unbalanced output before the tube input buffer.

The VT-I/F lent a rich tone to electric bass. The top end was a tad muted, but the low end was extremely tight and deep. My Strat oozed warm, liquidy, round tones through D. W. Fearn's cream machine. Synth pad tracks recorded through the VT-I/F confirmed that this was the mellowest-sounding DI tested.

Manley Laboratories Tube Direct Interface. The Manley Tube Direct Interface features five presets that are switched using a stepped rotary knob on the faceplate. The presets vary the corner frequency of a 6 dB — per — octave highpass filter, optimized for recording different instruments. The Bass Full setting is 3 dB down at 12 Hz; Bass Medium rolls off at 42 Hz, Guitar/Synth at 100 Hz, Guitar Medium at 250 Hz, and Guitar Bright at 550 Hz. All five curves begin to gently roll off above 8 kHz.

The Tube Direct Interface can be operated in Unity or Console Boost mode. The modes are selectable using a front-panel switch. In Console Boost mode, the signal gets a 17 dB gain boost.

The Manley unit I tested had one disconcerting quirk: the detachable AC cord did not fit into its IEC connector securely. As a result, the power temporarily failed whenever I gently lifted the unit to view its rear-panel connections. (According to Manley Labs, that problem has not been reported on other Manley units.)

On electric bass, this all-tube DI offered a well-balanced tone with the exception of a slightly understated top end. The overall timbre on electric guitar was warmer than that produced by the Aguilar DB 900 and Demeter VTDB-2b, but not as detailed. The Manley DI lent a nice overall balance to the sound of the synth pad. The timbre was a tad clearer than the AMB's, but not quite as clear in the upper mids as the sparkly Demeter.

Tube Works 4001 Real Tube Direct Input. Live performers will appreciate the hands-free control offered by the inexpensive Tube Works 4001, which offers large rocker switches for gain boost, ground lift, power on/off, and normal (instrument)/speaker impedance settings, all conveniently located on the unit's top chassis panel.

The speaker impedance setting works with the unit's speaker/loop out 14-inch jack, which is wired in parallel with the DI's input jack. To use that setup, set the DI's impedance rocker switch to speaker, patch your amplifier's output in to the 4001's input jack, and patch the speaker/loop out jack to your amp's speaker. That arrangement lets you send your amplifier's output through the 4001 without killing your speaker's output, so you can simultaneously mic your amp. (As mentioned earlier, I've never found that sort of patch to sound good, but having the extra capability doesn't hurt anything.)

The hybrid (tube and solid-state) 4001 provides 12 dB of switchable gain boost, which, in combination with its other circuits, gives it a higher output level than all the other test subjects. The 4001 gets fairly warm, though not as hot as the AMB Tube-Buffered DI Box. The unit uses a lump-in-the-line power supply with a detachable but flimsy power cord, which detracts somewhat from its roadworthiness. (According to Tube Works, the external power supply was chosen to further reduce noise in the unit.)

The Tube Works 4001 lent a soft, cottony top end to bass guitar, but the bottom end was tight and deep. In fact, this DI offered the deepest bass of the bunch. On electric guitar, the 4001 could have used more high-frequency detail, but the overall tone was otherwise wonderfully balanced. The 4001 also lent a wonderful tone to my Juno 106; the timbre's richness and resonance were second only to the Aguilar DB 900's. Considering its modest price, the 4001 is a surprisingly good performer.

Uncle Albert's VTD-2A Vacuum Tube Direct. The VTD-2A features an all-tube audio path, custom-built output transformer, and continuously variable output-level control. The output-level control boosts the DI's gain, but not at a tube stage. Rather, it changes the resistor network just before the output transformer so that the transformer receives more input level. (The amount of gain boost the circuit provides was undocumented.)

On electric bass guitar, the VTD-2A did not provide as deep a sound as the other review units. But the sound was the most live, taking on a slightly amplified character. I could really hear the 12AX7 tube's magic in this unit. The tone was richly textured and present, though less so than the Demeter VTDB-2b's.

Uncle Albert's tube DI is an excellent choice for electric six-string guitar tracks needing a little extra verve. Although lacking somewhat in low-bass tone and sounding a tad bright — not generally a problem because low bass tones on electric-guitar tracks don't usually help out in a full-band mix anyway and may even mask the drums and bass — this DI made my Stratocaster sound really lush and alive. I received similarly good results recording synthesizer through the VTD-2A.

DOWN THE TUBES

If you're on a tight budget and can't afford a clean, quiet mic preamp with at least 60 dB of gain, consider buying the Tube Works 4001 Real Tube Direct Input for your DI duties. It is easily the best-sounding tube DI in its price range, and it offers generous amounts of clean gain boost. Just be aware that the 4001's flimsy power cord makes it more vulnerable to rough handling and therefore less suited to itinerant use or to placement near drunken musicians and bar patrons.

If, on the other hand, you have a high-quality mic preamp at your disposal that can crank out at least 60 dB of gain, the Aguilar DB 900 is a must-have. The DB 900 exhibits all of the audiophile qualities one could hope for in a piece of pro-audio gear, delivering a clear, detailed, tightly focused, and well-balanced sound from deep lows to airy highs. The DB 900 is my favorite DI for recording electric bass guitar and synth.

The Demeter VTDB-2b is my first choice for recording electric guitars, at once offering sparkling detail, warmth, and presence. It also sounds outstanding on electric bass and synthesizer. It delivers a present sound balanced with tight, deep bass.

Those searching for a mellow guitar tone will want to investigate the D. W. Fearn VT-I/F Vacuum Tube Instrument Interface D.I., the AMB Tube-Buffered Direct-Injection Box, and the Manley Tube Direct Interface. Fearn's DI wins top honors here.

Tube DIs can give your instrument that focused, warm sound you've been lusting after, without costing you an arm and a leg. This is one time when having your music go down the tubes is a good idea.

Michael Cooperis the owner of Michael Cooper Recording, located outside the beautiful resort town of Sisters at the base of the Oregon Cascades.

TUBE DI BOX FEATURES Manufacturer/Model Channels All-Tube Unbalanced Output Topology Input Impedance* Gain Boost Price

Aguilar DB 900 Tube Direct Box1yesTB12.8 Munity$529AMB Tube-Buffered Direct-Injection Box1yesTB20.0 M15 dB$595Anthony DeMaria Labs ADL 100-G Direct Box1yesTG10.0 M0 dB**$599Demeter VTDB-2b Tube Direct1yesTB27.0 M16 dB$599D. W. Fearn VT-I/F Vacuum Tube
Instrument Interface D.I.2yesP1.0 Munity$1,500Manley Tube Direct Interface1yesTB10.0 M17 dB$575Tube Works 4001 Real Tube Direct Input1noTB1.0 M12 dB$219Uncle Albert's VTD-2A Vacuum Tube Direct1yesTB>1.0 MN/A$450KeyTB: unbalanced output follows the tube input buffer but occurs before any gain stage.TG: unbalanced output is placed after the tube input buffer and gain stage.P: unbalanced output is wired in parallel with the DI's input, placing it before the tube input buffer.* Input impedance is noted only for high-impedance, unbalanced instrument input.** The ADL 100-G's gain control attenuates up to 21 dB, instead of boosting gain.

MANUFACTURERS

Aguilar Amplification LLC
tel. (800) 304-1875
e-mail info@aguilaramp.com
Web www.aguilaramp.com

AMB (dist. by the John Hardy Co.)
tel. (847) 864-8060
e-mail johnwellshardy@home.com

Anthony DeMaria Labs
tel. (845) 256-0032
e-mail anthonyadl@aol.com
Web www.adl-tube.com

Demeter Amplification
tel. (818) 994-7658
e-mail info@demeteramps.com
Web www.demeteramps.com

D. W. Fearn
tel. (610) 793-2526
e-mail dwfearn@dwfearn.com
Web www.dwfearn.com

Manley Laboratories
tel. (909) 627-4256
e-mail emanley@netcom.com
Web www.manleylabs.com

Tube Works (dist. by Genz-Benz Enclosures)
tel. (480) 941-0705
e-mail info@genzbenz.com
Web www.genzbenz.com

Uncle Albert's Amplifier
tel. (800) 416-2444
e-mail uncleal@iei.net