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PreSonus Blue Tube

Stereo Tube Preamp By Paul White
Published October 2001

BABY BLUE

The Blue Tube preamp accepts both mic and instrument inputs, and lets you add as much tube warmth as you like using real valve circuitry.

Conceptually, the PreSonus BlueTube is a very simple device, providing two channels of mic/instrument preamplification in a half‑width, 1U rack case. A full‑width rack adaptor, able to hold two units is available. The 'Tube' part of the title relates to the 12AX7 valve used to 'flavour' the audio and, given that the unit runs from an 18V AC power adaptor, I suspect the tube runs at a reduced plate voltage compared with traditional tube circuitry.

On the front panel are two Neutrik combi jack/XLR connectors that are used to connect both balanced microphones and unbalanced, high‑impedance instruments such as guitars and basses — the Instrument input impedance is specified at 1MΩ. The dual‑servo gain stage dispenses with capacitors to deliver a good signal‑to‑noise performance at all settings of the gain control, and a pair of eight‑section meters monitor the output level. Gain is set conventionally using rotary controls with 40dB of gain‑range adjustment. A further two buttons per channel engage phase reverse and a 20dB pad, while a further, centrally mounted button delivers 48V phantom powering to both mic inputs. The outputs are on both balanced XLRs and unbalanced jacks.

The spec sheet is rather sketchy, as it quotes a very respectable noise floor of ‑94dBu, but doesn't say whether that is weighted or not, or for that matter what the maximum output level is. Similarly, the output impedance is quoted, but not the frequency response. Even so, the subjective performance gave no reason for concern.

So far, I've described a pretty conventional preamp, but the addition of a Drive control (up to 30dB gain) in each channel adds to the flexibility by providing the opportunity to drive the valve stage harder, when necessary, for adding controlled distortion to the sound. At lower settings, the expected tube 'warmth' is in evidence, while at higher settings, the added distortion is quite noticeable. Gain and Drive work in conjunction, so, to prevent overloading the unit, it is sometimes necessary to reduce the gain settings when using high drive levels, and vice versa. The meters soon let you know if you stray into forbidden gain territory.

PreSonus Blue Tube rear connections.Blue Tube rear connections.

Performance

In tests with typical project studio capacitor mics, the Blue Tube proved itself to be a very capable and sweet‑sounding mic preamp. At lowish Drive settings, it is able to capture something of the essence of a tube microphone, although if you advance the Drive control too far then the distortion becomes obvious and, to my ears, quite unpleasant. The reason so much Drive range has been included is because some people will want to use the Blue Tube to record guitars or basses and, at maximum drive with a typical guitar, the amount of overdrive seems to be somewhere in 'crunch' territory. This sti ll sounds pretty awful if you're simply DI'ing the result, because there's no speaker simulator to take the rough edge off the sound, so it's really too gritty to be useful. On the other hand, it sounds fine if plugged into a guitar amp or any of those recording gadgets that include a speaker‑emulation section.

We seem to be inundated with voice channel‑type products, but there's relatively little choice when it comes to simple mic preamps. Blue Tube fits the bill nicely, combining the role of a mic preamp with that of an instrument‑level DI box, and the Drive function adds greatly to its flexibility. Providing the Drive control is used somewhere in its first quadrant, the results are quite flattering, with vocals thickening slightly and guitars/basses gaining added definition, but higher settings are best avoided unless you have a speaker emulator to follow up with. Computer users in particular should find the Blue Tube makes a good companion to a soundcard with line inputs, as it will enable the user to record virtually any analogue audio source without recourse to additional hardware.

Pros

  • Simple and compact.
  • Good mic preamp with phantom power.
  • Useful tube Drive control.

Cons

  • Extreme Drive settings can sound unpleasant unless used in conjunction with a speaker emulator.

Summary

A practical and affordable solution for recording mics and instruments into a soundcard‑based studio system.

Published October 2001