Michael Cooper Recording By Michael Cooper
Tube DI boxes are all the rage. Plug into this user's guide and comparison test for a full report. 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 gear as if it were poison. Though fully aware that his "tube direct box" was a new and wonderful-sounding invention, Demeter thought no one would give a big hoot, and so he failed to copyright the product category name. Today, there are over a dozen tube DI boxes on the market, and the category doesn'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? In this article, we'll explore the benefits of using 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 created alike, I'll discuss the pros and cons of different designs and feature sets so you know what to look for when considering the purchase of a new unit. And finally, we'll take a close look at eight of the hottest tube DIs on the market, comparing their features and audio quality. Let's 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 quickly bring a recording session to a halt. One of the perennial sources is unbalanced instrument cables used with electric guitars and basses. That's because unbalanced cables are prone to picking up radio frequency interference (RFI), as well as interference from 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 like involuntary antennae for these invisible demons. So as you boost the output of a guitar or bass being played through an amplifier, you also boost any induced noise that the cable picks up including the baseball game being broadcast through your guitar amp or the obnoxious electronic buzz coming from the bass. A DI box solves these 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. This 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 allows you to forego the use of microphones, which indiscriminately record environmental noise along with the musical performance flowing out of your amp. Of course, the sound of a miked amp or acoustic guitar can be a wonderful thing, too, and so you may not want to eliminate its contribution to a song. In that case, the DI track can serve as an auxiliary 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 for the combined signals. (I'll discuss how to combine mic and DI signals later.) There are other good reasons to use a DI box besides simple noise-prevention. A good DI box will also preserve signal quality in other ways, and typically it provides a sound that is quite different from 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, making it more immune to the negative effects of long cable lengths, and thus preserve the high-end sparkle of the instrument. (I'll explain how a DI does this in a bit). But this 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 amp is a little ragged-out after its umpteenth bar gig, and you can hear the tubes giving out and one of the speakers 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 sound and acoustic bleed from other instruments and so it eliminates these problems. In addition, it allows you to 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 on the fly without worrying about control-room monitor bleed.
New Input Appreciated
Not all DIs are created equal. In fact, the first DIs produced sounded dreadful. At the time 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, making the sound dull. The only way around this was to raise the transformer's impedance to the point that there would be an unacceptable loss in signal level (as much as 40 dB!). A few companies, Demeter's included, had already made DIs that used semiconductor devices (instead of transformers) on the inputs. This design preserved the instrument's high end with virtually no loss in signal level. However, these solid-state DIs had an edgy quality to them, not to mention harsh overload characteristics. Demeter realized that a tube could be manipulated to deliver the extremely high-input impedance that a DI needs in order to sound transparent. (We'll discuss the importance of a DI's input-impedance spec momentarily.) A tube also preserved the input-signal level, and made the sound richer, to boot. Demeter scrapped both the input transformer and semiconductors, stuck a dual-triode tube in their place, and the first tube DI box was born. Since that seminal launch, successive tube DIs have incorporated some significant design changes. Let's look at the pros and cons of different designs to help you decide what's best for your applications.
Most tube DIs available today employ a dual-triode tube. The first triode buffers the input signal; the second boosts the output level of the box. But some manufacturers prefer not to boost the output stages of their DIs at all, 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. The fact is, 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. Indeed, that's one reason we tested a bunch of units for this article to see, after all 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 to electronically balance their DI outputs (in lieu of transformers, which automatically reduce output levels by roughly 20 dB). Why don't all manufacturers use this hybrid design tube plus 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, and 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 on stage, for example a transformer-coupled DI output will do the best job of keeping induced noise to a minimum. The second reason for using an output transformer is that it allows the manufacturer to maintain an all-tube design. After all, solid-state devices sound edgy, right? Again, it's not that simple. What matters is quality both in components and design. A high-quality hybrid or solid-state DI, for example, can sound considerably better and warmer than a poorly designed all-tube box. Why is that? Well, all transformers distort signals to some degree; they tend to saturate the sound, especially when hit with excessive input of very low frequencies, and they can also cause a mild attenuation of highs. Indeed, the best transformers are sought after precisely because they saturate the sound (in a musically pleasing way, that is). Bear in mind, too, that, although an all-tube DI may sound smoother or warmer than a hybrid box, your signal, before reaching final mixdown, will almost certainly run through gobs of semiconductors located "downstream" in the signal chain.
Impeding Your Progress
Just the same, there is one specification I strongly consider when deciding which DI box to buy, and that is the measure of the unit's input impedance. A DI's input impedance has a profound impact on the sound of both passive electric instruments with magnetic pickups (electric guitars and basses) and acoustic instruments fitted with piezo-electric pickups (such as acoustic guitars, mandolins, and so on). Let's examine why this is so. 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 this reason, DIs that offer a really high input impedance will generally produce, for example, more sparkly electric guitar tracks because they don't load down the magnetic pickups. The piezo-electric pickup, on the other hand, is essentially a capacitor. An inherent property of capacitive devices is that, as impedance rises, they are able to pass increasingly lower frequencies. Furthermore, piezos need to see a much higher impedance than magnetic pickups in order to be totally rid of loading effects and to pass full-bandwidth signals. So a DI must offer very high-input impedance in order to produce acoustic-guitar tracks with full bass content. How high is enough? Modern tube DIs typically offer anywhere from 1 megohm to 27 megohms of input impedance. Generally, I find that units at the extreme low end of the range produce a softer, more muted sound, whereas those with a high input impedance tend to capture an extra-sparkly sound (including the "twing" from pick strikes, for example). That said, there is no way to specify an exact minimum input impedance yet another reason that sonic comparison is necessary. But now you can see why the earliest DIs, with their transformer-coupled inputs that provided only 10 to 100 kilohm impedance, stifled the highs on electric guitars and cut off the lows on acoustic guitars with piezo pickups. Note that 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, one that has battery-powered electronics): the instrument's active circuitry pre-conditions the pickup's signal before it goes to the DI box (or wherever), and so it is not sensitive to impedance. Therefore, you can plug a synth or active bass guitar directly into an outboard preamp or console input, if you like, and it should sound pristine. Of course, 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), you may actually be 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 Now let's look at the various external features you should consider when choosing a tube DI box. Any DI box worthy of consideration should offer a high-impedance, unbalanced, 1/4-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, 1/4-inch output jack (for sending the DI's signal to an instrument amplifier). In some designs, a tube DI's unbalanced output is multed 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 this 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 (that is, as long as the cable lengths aren't unreasonably long). In another design, the DI's unbalanced output may follow its tube input buffer. This offers the benefit of warming up an instrument's signal before sending it 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 roll-off, allowing you to run cables up to 40 or 50 feet long 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 that's located across a large stage. Other manufacturers of tube DIs incorporate yet another design: placing the unbalanced output after a tube-gain boost stage (in non-unity boxes). Although you certainly don't need this extra amplification if you're patching into a guitar amp, the extra tube stage typically warms things up even more. Having two separate DI outputs an unbalanced out for your amplifier 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. (I'll detail the setup for this shortly.)
A couple of other DI-box features are worth noting. Most DIs, whether tube or solid state, offer a ground-lift switch to reduce hum. Gain-boost circuitry may also be included, accessed either by a switch that engages a fixed amount of boost or by a continuously variable knob. Relatively few DIs provide a "speaker" or "amplifier" input jack. Those that do allow you to plug the output of your amp into the DI. A switch on the DI must be set to speaker mode so that the DI offers the correct input impedance for this setup, or distortion and possible equipment damage may result. Though having extra features never hurts, I've always found that this 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 yet don't want to mic a speaker cabinet, there are several speaker emulators on the market that produce far superior results than using a DI box in this 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. (The low-cost Bellari Stereo Tube Direct Box one of the few tube DIs not tested here ships without a power switch; the unit comes on as soon as it's plugged in.) Creative Routing Now that you know what to look for when buying a tube direct box, let's look at various ways to patch a DI into your system. As with other types of tube gear, I always let my tube DI warm up for at least twenty minutes before using it to record. An hour is even better. A cold tube unit is a recipe for noisy, inferior-sounding tracks. The most basic 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 (see Fig. 1). Whenever I record an acoustic guitar that has a pickup, I route the pickup's output through a DI box in this manner, in addition to miking the guitar with a stereo pair of condensers (see Fig. 2). 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 and sneak in a bit of the DI track between them to anchor the sound. You can usually fatten up 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 one 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. This 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. 3). Simply patch the output of the bass into the unbalanced input of the tube DI, the balanced output of the DI into a mic preamp's or mixer's mic input, and the DI's unbalanced output to the amp. Mic up 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 soundcheck and submix them to one track while you record. (This setup works great for recording six-string electric guitars, too.)
Rules of Engagement
Now that you know some cool ways to use a DI, let's proceed to the evaluation and comparison test. As mentioned, there are more than a dozen tube DIs currently on the market, so our first task was to establish criteria that would ensure we compared apples to apples. To that end, we picked only dedicated DIs; that is, no dedicated line preamps or mic preamps with DI inputs were included. In addition, we selected only units featuring tube-buffered inputs and a balanced XLR, mic-level output. We also determined a price range $200 to $800 per channel so as to not to pit extremely high-end units against budget models. We settled on eight boxes: the Aguilar DB 900 Tube Direct Box ($529), AMB Tube-Buffered Direct-Injection Box ($595), Anthony DeMaria Labs ADL 100-G ($599), Demeter VTDB-2b Tube Direct ($599), D.W. Fearn VT-I/F Vacuum Tube Instrument Interface D.I. ($1,500; 2-channel), Manley Tube Direct Interface ($575); Tube Works 4001 Real Tube Direct Input ($219), and Uncle Albert's VTD-2A Vacuum Tube Direct ($450). Note that, in cases where a manufacturer offered both mono and stereo versions of the same basic model, we reviewed the mono unit only. So if you're interested in a mono unit tested here but are looking for a stereo DI, contact the manufacturer there may be a stereo version available. All of the units we tested feature high-impedance, unbalanced, 1/4-inch I/O, but they differ as to where the output is derived in the circuit. Some place the output before the tube buffer, others immediately after, and still others after a gain-boost stage. Additionally, 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 of the DIs tested here is a portable (desktop or floor) unit (that is, sans rack mounts), and each provides a ground lift. (For a tidy comparison of the units' features and specs, see the table "The Heat Is On.") 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 nine 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 output of the DI 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 of the instruments through an amp because every amp colors the sound so severely that no viable conclusions could be drawn. All of the tracks were recorded at 20 bits/48 kHz through the A/D converters on 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, ultra-flat (50 Hz to 22 kHz, ± 0.5 dB) KS ADM 2 monitors powered by a Hafler P3000 transnova power amp. The monitors were coupled to an ASC ATTACK Wall acoustical environment. I made my evaluations both by soloing the DI tracks and by playing them back in the mix. Note that all evaluations are entirely subjective what you're getting here are my opinions only. Aguilar DB 900 Tube Direct Box I was so impressed with this single-channel, all-tube DI that I bought one when it first hit the streets last winter. It features an ultra-pristine, minimalist audio path with no gain boost; hence, it provides the weakest output level of all the units tested here. But the trade-off is well worth it. On bass, the DB 900 offers the best of all worlds: perfectly-balanced tone, very low noise, a clear top end with truckloads of nuance and air, and a very extended, tight bottom. You can clearly hear the 12AX7 tube doing its magic here; electric bass sounds extremely rich and warm through this awesome DI. Interestingly, my Strat 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
This German-made, all-tube DI offers 15 dB of gain boost via a front-panel switch. A bombproof chassis and detachable AC cord add to the unit's portability. The single-channel AMB got hotter than the other units, and its unventilated chassis raises some concern about tube life. 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 at the expense of clarity and openness. On 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
This single-channel, all-tube DI offers a continuously variable rotary gain control, but it attenuates (as much as 21 dB) rather than boosts the output level. A 2-position switch places the unbalanced output either 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. But this is not cause for concern as long as 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 not as fat or unrestrained as the Tube Works, AMB, and D.W. Fearn DIs. That is, compared to those units, the 100-G sounded a little stiff or compressed. I also felt that the ADL was a bit shy on high-frequency detail on guitar. But I must stress that this is a quality unit, and some of these 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, vintage VTDB-2. The single-channel unit has undergone no design changes since 1987 but then, why should it? This single-channel, all-tube DI is my overall favorite unit 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 had a sound that was very 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 synth pad, 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 we tested. Also the prettiest and most impressive-looking DI of the bunch, this 15-pound, cherry-red unit features a chassis that is machined from solid, 1/4-inch thick aluminum plate and finished with a tough, polyurethane aircraft finish. Heavy-duty toggle switches (for ground lift and power on/off functions), custom Jensen transformers, and a detachable AC cord enhance this hand-crafted unit's appeal. Like the Aguilar DB 900, the all-tube D.W. Fearn 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 very 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. And synth pad tracks recorded through the VT-I/F confirmed that this was the mellowest-sounding of the DIs tested here.
Manley Tube Direct Interface
The Manley Tube Direct Interface features five presets that are switched via a stepped, rotary knob on the face plate. The presets vary the corner frequency of a 6 dB-per-octave bass rolloff 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. This mono, all-tube DI has one disconcerting quirk: the detachable AC cord does not fit its IEC connector securely. As a result, the power would temporarily fail whenever I gently lifted the unit to view its rear-panel connections. On electric bass, the Manley offered a tone that was well-balanced, 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. On synth pad, Manley's DI lent a nice overall balance to the sound. The timbre was a tad clearer than that offered by the AMB tube DI, 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. This mono DI 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 in conjunction with the unit's "speaker/loop out" phone jack, which is wired in parallel with the DI's input jack. To use this setup, you set the DI's impedance rocker switch to "speaker," patch your amplifier's output into the 4001's input jack, and patch the speaker/loop out jack to your amp's speaker. This arrangement allows you to 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 this sort of patch to sound good, but having this 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, although not as hot as the AMB Tube-Buffered Direct-Injection Box. The unit uses a lump-in-the-line power supply with a detachable but flimsy power cord, detracting somewhat from its road-worthiness. 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 was second only to what the Aguilar DB 900 delivered in richness and resonance. Considering its modest price, the 4001 is a surprisingly good performer.
Uncle Albert's VTD-2A Vacuum Tube Direct
This mono DI 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 via 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 this 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, although not as present as what the Demeter VTDB-2b provided. 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 Strat sound really lush and alive. I got similarly good results recording synth 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 placement near drunken musicians and bar patrons. If, on the other hand, you have a high-quality mic pre 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, delivering 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 yet warm sound you've been lusting after, without costing you an arm and a leg. This is one situation in which having your music go down the tubes is a good idea.