Photos: Mark Ewing
The Presonus Blue Tube DP supersedes their existing Blue Tube and features what Presonus call Dual Path technology. Essentially this teams a solid-state preamplifier stage with a tube stage, where each of the channels functions as a microphone or instrument preamp. A 12AX7 dual triode is shared between the two channels so there is one stage of tube gain per channel. This tube is also run at well under the normal death-dealing voltages used in traditional tube products, so there's no safety risk. However, the authenticity of the added 'warmth' can vary in low-voltage designs, so you really need to judge them subjectively rather than assuming that they will sound exactly right simply because they have valves inside.
Presonus are by no means the first company to take this hybrid design approach, as solid-state input sections tend to be cheaper to build and less noisy than their all-tube counterparts, but they have managed to bring in the Blue Tube DP at a surprisingly attractive UK price and claim to offer the user the ability to create 'a fat warm tube tone or invisible solid-state transparency'. Predictably, the degree of warmth is determined by how hard you drive the tube, as warmth in this context is really just another way of describing subtle harmonic distortion. The visual design has been brought up to date with brushed aluminium, blue switch LEDs, and red LED backlighting to the tube, though the 1U half-rack shape and external transformer powering have been retained.
As the Blue Tube DP is simply a preamp without EQ or dynamics, the manufacturers have been able to provide all the essential features that you'd expect in a mic pre, specifically a -20dB pad, phase-reverse button, 80Hz low-cut filter, and individually switchable 48V phantom power. Each channel has two rotary gain controls, one of which works more or less conventionally to adjust gain while the other, labelled Tube Drive, sets the amount of valve gain. The Tube Drive control has an integral push switch that bypasses the tube stage completely — as you'd expect, this setting produces the least distortion and the best noise figures. The Tube Drive control increases the input to the tube stage, but reduces its output to compensate, so there's little overall level change while this is being adjusted.
For those who like to check the figures, the combined THD + Noise (unweighted) in solid-state mode is less than 0.005 percent, but can be anything from 0.01 to 30 percent when the tube stage is in circuit, depending on how hard you drive it. Metering is courtesy of two small, circular moving-coil meters, and there are also fast-acting overload LEDs located between the channel gain controls. These come on at +22dB, so there's a decent amount of headroom available.
All the connections are on the rear panel, including the PSU connection point. A hook on the rear panel provides somewhere to anchor the cable so that it won't fall out in the middle of the session. For the inputs, the designers have opted for Neutrik combi jack/XLR connectors, as these save on space and can accept both balanced XLR and quarter-inch, unbalanced jacks. The jack inputs have a high input impedance (1M(omega)) designed to accept instruments with magnetic pickups, but they can also accommodate line levels. By contrast, the line-level outputs are on separate balanced XLR and unbalanced jack connectors which are active simultaneously, providing a practical way of splitting the signal for zero-latency monitoring or other applications.
My first check was with a large-diaphragm capacitor mic using the Blue Tube DP with its tube stage bypassed. Although the maximum gain is limited to 57dB, that's plenty for capacitor mics, though it may be a hint on the low side for some dynamic models or for recording quiet sources. By contrast, most mixers offer 60dB of mic preamp gain with a further 10dB or so of gain available by pushing the faders up past their 0dB positions. To my ears, the sound is quiet and clean and, to be honest, fairly indistinguishable from any number of well-designed, mid-price mic amps I've tried in the past.
With the tube stage switched on, there seemed to be a fine line between not hearing much difference at all and hearing obvious distortion, so for normal work I'd use this below its midway setting and only venture into heavier distortion for special effects. There's quite a lot of distortion available at more extreme settings, no doubt to give flexibility to those using the unit as an instrument DI. The type of distortion it produces should be useful for taking the edge off hyper-clean rhythm or bass guitar and it could also suit many keyboard sounds, including tonewheel organ. It's less successful as an electric guitar treatment, because as soon as you get into the obvious distortion range, you need to follow up the preamp with a speaker simulator to attenuate the 'fizzy' high-end harmonics that would normally be removed by a typical guitar speaker. Having said that, used with care it can add a bit of tube character to an already 'produced' guitar sound from a Line 6 Pod or similar device, provided that you don't add so much drive that the sound becomes raggy.
In designing the Blue Tube DP, Presonus have managed to produce a cost-effective, sweet-sounding mic/instrument preamp that has the added benefit of tube coloration only when you need it — and even then, you're in full control over what you add. I don't think it sounds as subtle as a well-designed, high-voltage tube circuit, but used sparingly it adds to your creative palette and also allows you to add more warmth to DI'd electric instruments. As a desktop unit, the Blue Tube DP looks great, it's very easy to use, and it has no obvious vices. Even the gain control operates progressively without everything being bunched up at one end, which is a common failing with other low-cost devices. I would have liked just a little more available gain, but in the majority of studio situations there is more than enough available. Given the low UK price and very decent level of performance, you can't really go wrong.
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