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PRESONUS BLUE TUBE

February 1, 2001
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An affordable mic preamp with tube warmth and features galore.PreSonus Audio Electronics seems it's out to prove good things do indeed come in small packages. First came the Blue Max Smart Compressor, a great-sounding half-rack unit unique in its class for providing compression presets for a variety of applications. Now PreSonus is filling up the other half of the rack tray with the Blue Tube, a stereo microphone and instrument preamp with a hybrid tube and solid-state circuit design. The unit utilizes a single 12AX7 tube and independent drive knobs for dialing in the desired amount of tube "warmth." At less than $100 per channel, the Blue Tube also lays claim to being the least expensive tube mic pre on the market.

REALM OF THE SENSESDespite its diminutive size and low price, the Blue Tube is well built and well featured. It uses Neutrik Combo connectors to provide two XLR mic inputs and two 1/4-inch high-impedance instrument inputs on either side of its blue, brushed-aluminum front panel. Controls are laid out in mirror image (from left and right of center), with each side providing a Gain knob and Drive knob, switches for polarity reverse, and a 20 dB pad. In the center is a global 48V phantom-power switch. All five switches are push-button style, and the four knobs are continuously variable. Each channel also provides an eight-segment output meter, with green LEDs at -28, -14, -9, -3, 0, and +3 dBu, a yellow "caution" LED at +9 dBu, and a red overload indicator at +18 dBu.

The Blue Tube's enclosure is very sturdy, and the knobs and switches have a nice feel. Although there isn't a dedicated power switch, a red front-panel LED indicates power on when the unit is plugged in through the included wall-wart power supply. (Using an external transformer is one way PreSonus could keep internal noise and costs down.)

The Blue Tube's rear panel provides XLR balanced and 1/4-inch unbalanced outputs. Each output has its own amp, allowing for simultaneous output from the two jacks on each channel. That is useful for sending a signal to two places at once, such as to an amplifier and a mixing board.

The Blue Tube's well-written manual explains that each channel contains a dual-servo gain stage - a design that eliminates the need for capacitors and lets the Gain knob "boost the desirable signal without increasing unwanted background noise." The manual also discusses optimum settings for various applications, and even encourages the user to experiment with tubes other than theprovided 12AX7.

APP HAPPYThe Blue Tube has dozens of uses. It fit into my bag so easily that I became accustomed to bringing it along to sessions and concerts, even when I didn't have a clear idea as to how I might employ it. But once I arrived at the studio or venue, I always found a use for it - and sometimes I could not have done as good a job without it.

The Blue Tube is very handy as a DI box for stereo instruments such as keyboards, either live or in the studio. Not only does it provide ample gain and distortion, but also, as suggested earlier, its simultaneously available outputs let you use it as a splitter. For example, you could send signal directly to a console or tape machine through the XLR outputs and to an amplifier or personal monitor through the 1/4-inch outs.

The Blue Tube also works well with electric guitar, whether as a preamp before a guitar amp or simply as a DI. I tested the unit as a DI using a Strat plugged into one of the instrument inputs and found that it yielded clean gain, effective and likable distortion, and any combination of the two. The Gain and Drive knobs interact in an additive fashion, with the solid-state amplification controlled by the Gain knob and the tube acting on the signal only as the Drive knob is turned up - a design that allows for a range of variations between clean and dirty tones. (Just remember, as with any tube unit, you should let the tube warm up before engaging the Drive knob.)

The Blue Tube is also helpful when you need extra juice to send a signal down a long cable. When recording a guitar or keyboard cabinet at home, for example, it's often necessary to position the cab far from the record deck - in a bedroom closet, for instance - to achieve adequate sound isolation. The Blue Tube provides plenty of gain for getting the signal to its destination.

Another feature I found very helpful was the Blue Tube's polarity-reverse switches. I typically use two mics to record guitar amp cabinets (usually a Shure SM57 and a Sennheiser 421). Because the sound arrives at the two capsules at different times, that technique can lead to phase distortion. A quick and easy way to hear if the two capsules are out of phase is to reverse the polarity on one of the two mics. However, not many mixers - especially among those commonly found in personal studios - provide polarity reversal. The Blue Tube's independent polarity-reverse switches really come in handy, making it easy to quickly hear phase problems.

The Blue Tube is a great tool for location recordists who specialize in stereo recording, whether to DAT, hard disk, or whatever else. Many DAT recorders provide mic preamps, but only a few offer phantom power. Similarly, if you record direct to hard disk using, say, a PowerBook, you'll need preamps and phantom power.

For live recordings of bands, I used the Blue Tube to power various mics, including pairs of Shure SM57s, AKG C 414s, Countryman Iso-Omnis, and Radio Shack PZMs. The unit worked well with all of these. I even used the Blue Tube solely as a phantom-power box during a live recording using a transformer-isolated snake splitter (a device that cannot pass phantom power because of the transformers).

CRITICAL CONSIDERATIONSOne slight misgiving I have about the Blue Tube is its audible noise floor, which is high enough to be problematic during pianissimo passages or while recording particularly quiet instruments. For that reason, the Blue Tube would not be a first choice for critical recordings of some acoustic instruments or ensembles. (According to PreSonus, the noise problem resulted from the steep gain curve of the potentiometer used in the original Blue Tube's Drive circuit. PreSonus has since replaced the offending pot, and the Blue Tube now ships with a potentiometer that provides more subtle gain control in the first half of the pot's rotation, resulting in a lower noise floor. At zero tube drive gain, the noise floor specification is better than -97 dBu.)

Also, whereas the preamps in the Blue Tube sound quite good, they do not sound as pristine as some higher-end preamps I compared them to. This, too, was more noticeable on recordings of quiet acoustic instruments. Again, however, price and features must be taken into account - and considering the price of this box and the numerous amenities it offers, the preamps' quality is quite impressive.

One surprise was how effective the Blue Tube was at saving live stereo DAT recordings from sounding cold and sterile. I dialed in a small amount of tube warmth while recording an electric band, for example, and ended up with a DAT recording that sounded punchy and real as opposed to digital. Yes, I actually added distortion to the recording. But after listening to sections of the same performance recorded without the tube circuitry engaged, I preferred the sound with the bit of analog distortion the Blue Tube added.

BLUE WITHOUT THE BLUESConsidering its low price, the PreSonus Blue Tube far exceeded my expectations. It's a solid, good-sounding, easy-to-use unit that covers a wide range of applications. The Blue Tube serves as a microphone preamp, instrument DI, phantom-power supply, polarity re-verser, distortion box, signal splitter, and signal amplifier for long cable runs - indeed, one could easily shell out as much for any one of the functions it provides. That's what I call a great value.

In addition to the versatility afforded by its healthy feature set, the Blue Tube's small size and rugged construction make it ideal for transporting from gig to gig. I highly recommend it as a multipurpose tool for any engineer. And if your digital stereo recordings sound a bit cold, this box lets you dial in the right amount of tube warmth.

The Blue Tube is also a great companion to the PreSonus Blue Max Smart Compressor. Keyboard and bass players, for example, may want to consider pairing the two in their equipment racks. The units fit neatly together in a rack tray (available from PreSonus for $39.95), providing a versatile preamp/DI and affordable dynamic control in a single rack space. They look nice together, too!

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