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.
