An inexpensive tube preamp with oodles of digital features.
Whether you are tracking vocals on digital tape, capturing an orchestral performance with a hard disk recorder, or cutting distorted guitar tracks on an analog deck, most signals require a preamp to boost the source signal to an operable level. The dbx 386 is designed for personal-studio owners who want a high-quality, 2-channel tube preamp coupled with loads of flexible analog and digital I/O. Considering the 386's generous feature set, the unit is surprisingly inexpensive and will likely find its way into many small recording studios and touring rigs.
IN CONTROLInput into each of the 386's two preamps is controlled by a knob labeled "Drive." Up to 60 dB of gain is available for microphone signals, and a 30 dB range (ñ15 dB) is offered for line- and instrument-level signals. The unit's vacuum-tube stage is positioned after the preamp section.
Unlike some tube preamps, the dbx 386 does not provide separate input-gain and tube-drive controls; rather, the amount of tube coloration is linked directly to the Drive control. However, thanks to the unit's separate analog and digital output controls, you can saturate the tube stage more at the input and compensate for the increased level at the outputs.
Other front-panel controls for each channel of the preamp section include a 48V switch for phantom power, a 20 dB pad for the microphone input, a phase-inversion switch, and a 12 dB/octave low-cut filter for keeping out unwanted low-frequency rumble (see Fig. 1).
The 386 provides two output-level controls - one analog and one digital - and both types of outputs are simultaneously active (more on this later). Each channel of the preamp also has a single, 12-segment LED meter for monitoring output levels. (No input metering is available.) The output meter can be switched between analog (dBu) and digital (dBFS) scaling at the touch of a switch; the switch glows red for analog and green for digital readings. When metering stereo signals, it's important to set the meters so that each channel reads the same type of output.
Between the 386's two preamp control panels is a section labeled "dbx Type IV Conversion System." This section comprises five lighted button switches: Dither (SNR superscript 2 or TPDF); Shape (noise-shaping curve); Sample Rate (44.1, 48, 88.2, or 96 kHz); Word Length (16, 20, or 24 bits); and Output Format (S/PDIF or AES/EBU). The Sample Rate switch is unlit for 44.1 kHz operation and uses yellow, red, and green lights to indicate other rates. The other four switches glow red or green to indicate their status.
I/O, SILVER!One of the 386's strongest features is its panoply of inputs and outputs (see Fig. 2). Balanced analog inputs are available for microphone and line-level signals on the rear panel, and separate unbalanced instrument jacks are provided on the front. A Line Select switch determines whether the line-level inputs or the mic input is active.
Balanced XLR and 11/44-inch analog outputs are simultaneously available on the rear panel. The 11/44-inch outputs can also be used in an unbalanced configuration simply by inserting tip/sleeve plugs. Each channel also furnishes a TRS-insert jack for setting up an effects loop after the tube section and before the output section - ideal for patching in a compressor or EQ.
Both AES/EBU and S/PDIF digital outputs are provided. Although data is sent out both connectors at the same time, the proper formatting for both is determined by the Output Format switch on the front panel. In other words, if AES/EBU is chosen at the switch, then AES/EBU-format data will come out both the XLR and RCA coaxial connectors. Care must therefore be taken to properly match digital formats when connecting digital gear.
As mentioned earlier, each channel has separate pots for the analog and digital outputs, and the outputs are simultaneously active. This very cool feature makes it possible to feed up to three devices at a time from the 386's outputs: two analog devices and one digital. In addition, word-clock BNC input and output connectors on the rear panel allow synchronization with other digital gear.
AT ANY RATEThe sample rates and bit resolutions for the input A/D conversion and the digital outputs must be identical; they cannot be set independently. This does not present a problem, though, because the analog insert is placed directly after the tube stage, and the 386 has no digital input. Because there is only one conversion process (and no digital processing of the signal), independent resolutions for A/D conversion and output are unnecessary.
The best news is that the sample rate for the A/D conversion can be set with a front-panel control to 44.1, 48, 88.2, or 96 kHz, and the word length can be 16, 20, or 24 bit.
Building on the previous dbx Type I, Type II, and Type III technologies, the Type IV Conversion System is a proprietary process that, according to dbx, captures a much wider dynamic range in the more linear area (that is, the upper bits) of the A/D converter. Type IV is a single-step encoding process and is basically a sophisticated compressor that squeezes the upper dynamic range of the signal (and its peaks) into the upper bits of the digital word. This system allows better overall performance from the A/D converters and is said to emulate the natural compression effect of analog tape saturation.
Along with the Type IV Conversion System, two types of dithering and two types of noise shaping are available. Combined, they do wonders for cleaning up the sound of dynamic sources that either would clip the converters or would have to be turned down to the point that much of the material would be in the lower range of bit resolution.
A major benefit of the 386 is that the Type IV peak compression (along with some additional processing voodoo, kept secret by dbx) happens before the A/D conversion rather than after, allowing hotter levels to be encoded. With many digital peak limiters, including the growing assortment of software plug-ins and all-in-one mastering boxes, the processing happens after the signal has been captured digitally. Any material originally encoded at a lower-bit resolution (such as low-volume material) will never be higher in quality, even if it is turned up via compression. But with Type IV, the quieter material can be encoded at a higher-bit resolution by taming the peaks on the front end of the conversion process.
MILD BUT WARMINGA wealth of features is welcome in most products, and the 386 is certainly rich in that department. But the bottom line for a preamp is its sound, and here the 386 comes up short.
The best part of the signal-processing circuitry is the tube section. In general, the 386's tube coloration is mild yet warm. Unlike the saturated tube sounds you get with a guitar amp, the 386 gives you a little blossoming, a modest tone coloration. It's pleasing and could be useful for a number of applications.
However, the preamp's overall sound is disappointing, even considering its reasonable price. I first tested the 386 on keyboards and vocals and noticed a thickened tone on keyboard sounds. On vocals, the sound was not well focused; it was sort of swirly, mushy, and phasey. The sound was similar from both the digital and analog outputs, so the converters apparently are not the source of the problem.
I attempted to record doubled acoustic guitar on several songs with the 386, with similar results. The guitarist - who is also a competent engineer - felt that the tone was "spongy" and lacked accurate transient response. In addition, the noise floor of the preamp was a bit higher than I like, though not as high as that of some low-cost tube preamps. We ended up recording through the Mackie console's internal solid-state preamps, which produced better results.
After our guitar date, we decided to test the 386's transient response by recording some percussion instruments that were lying around the studio. Sure enough, the claves, tambourines, and bongos lacked a real sense of punch and attack, regardless of the settings. The claves' sound was especially compromised. We captured an acceptable bongo tone, but the 386 didn't maintain any of the percussion instruments' crisp attacks.
REAL DEALThe most surprising thing about the dbx 386 is its low price. Considering that the unit is dual-channel, has tubes, includes decent converters, and sports a wide variety of high-resolution I/O, the features rating has to be high.
I can't recommend the dbx 386 as your main or only preamp. But you might make good use of it in situations in which a crisp attack and sharp focus are not crucial - especially if you want to add some nice, easygoing tube warmth.