Last modification: 01/19/08 19:26
fig. 1 front
ART's TubeMP mic preamps took the world by storm with their affordable series of project studio tube mic pre's. For the cost of direct box, project studios no longer have to rely on multi-purpose audio interfaces of dubious quality to serve as preamps for their guitars and vocal mic's. ART's single-channel compact mic preamps have been weaning themselves away from the "direct box" appearance for some time now, with the top-of-the-line but modestly priced ($129 list, $99 street) Tube MP w/USB looking very much like a computer recording interface.
Much more than a direct box, the MPUSB provides a Swiss Army knife of analog signal processing in a sturdy, compact unit. It offers an impedance-selectable balanced mic input in addition to a hi-Z unbalanced instrument input, with balanced and unbalanced line-level outputs for plugging into a mixer. However, taking the original MP version a step farther is its USB capability. By providing a 16-bit A/D converter w/low latency (<2ms) USB connection, the unit is ideal for DAW recording with or without simultaneous zero-latency analog monitoring. Considering that the unit is identical in all other respects to its non-USB equipped brother but only costs $30 more, project studio enthusiasts would be better off buying the slightly more expensive version as the latter is not upgradeable.
As a mic preamp, the MPUSB offers sophisticated channel-strip like functionality in a small box, minus the EQ. Besides the tube saturation, its feature set includes a selectable linear-phase (Bessel) 40Hz highpass filter, analog input limiter (!), phantom power and phase switch. This solves so many problems at once; you can clean the subsonics, warm up the signal (if the limiter's off - note where the tube sits in the signal chain) and lessen the possibility of generating a digital clip all at the same time. The 12-page manual is clear, but lacking in details; there's no signal flow diagram, for example. From empirical testing and observation, the audio path apparently goes like this:
Input Gain -> Rumble Filter -> Limiter -> Metering -> Tube Saturation -> Output Gain -> Analog Output & 16-Bit A/D
Now, I could be wrong here - it's entirely possible that the metering comes before the limiter, as the 0dBu LED performs double-duty as the light signalling that the limiter is compressing (if enabled) - something I confirmed on the test bench. What definitely is clear is that the limiting happens prior to the tube saturation as there's very little of it to be had once the limiter is on. If you thought it might be nice to fuzz up the signal a bit and then limit it, you're out of luck. Well, not completely - at very high signal levels (ones that would slam the +15db Clip LED if the limiter weren't on) a bit of warmth is audible, but not a lot.
Speaking of tube saturation - would you even be reading this far if the unit didn't have a 12AX7 in it? If distortion is what you're trying to avoid, it's very easy to do so; just treat the 0dB LED as if it was -5dBFS. Using a 1KHz test signal, I measured no additional harmonic distortion until about +4.8dB after the 0dB LED lit up, with increasing (and very musical) levels of saturation in the "sweet spot" above that until the CLIP LED lights up at +15dbU. Beyond that the clipping gets rather nasty, so you're on your own.
In the examples that follow, I recorded a brief clean bit of rhythm guitar into the TubeMP prior to running it through the TubeMP again in a second pass using a REAMP. While this is going on, I'm also varying the unit's input gain from min to max. This gives an excellent demonstration of the wide range of saturation possible from the unit, from pristine clean to warm and fat up on up to a Rolling Stones-level of overdrive before finally breaking up and becoming unusable. In the first two examples, the 0dB light was lit for about half the demo. The third example lit up the (+15dB) "Clip" LED, and the fourth example - being slammed with a limiter, was pretty much solid yellow.
TubeMP USB Saturation Demonstration
|1: Reamped with Input Gain Swell|
|2: w/Input Gain Swell, Limiter On|
|3: w/Input Gain Swell, +20dB On|
|4: w/Input Gain Swell, +20dB & Limiter On|
The ART TubeMP with USB is such a practical device that it makes you wonder about some of the things that ART left out. First: Why is the USB port unidirectional, when with a pair of audio connectors and a D/A converter the device could have doubled as a PC recording interface? There are a number of items like this, which makes me feel that there must certainly be an ART TubeMP USB II being worked on as I write.
Rugged, attractive & sounds great - clean or warm for guitar, bass, keyboard
Subsonic filter, limiter, phantom power and phase-flip are indispensible tools
Supports USB 1.1 & 2.0, according to manual
The only game in town at this price
A/D converters are only 16-bit
Analog and digital output levels aren't independent
ASIO drivers not bundled with unit; recommended commercial drivers cost extra at around $70 USD
No clip indicator or digital metering for the A/D. Input level meters are useless for this purpose
Pre-saturation Limiter almost defeats the purpose of having a tube. Could be more transparent.
USB audio is unidirectional from the unit to your PC, so don't throw away your sound card yet
Lack of playback functionality (see above) discourages use in ASIO mode (At least in SONAR, where its single-client ASIO exclusivity means you'll have no way to hear your mix!)
Wall wart power supply
No off switch
Built-in (pre A/D) analog noise gate
Bidirectional audio w/stereo inputs - throw away your sound card!
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