Mic Preamp & Optical Compressor

Published in SOS April 2013
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Reviews : Preamp

ART Tube MP/C.

ART's Tube MP/C combines a hybrid 12AX7 (ECC83) valve preamp section with the company's 'Vactrol' optical compressor, and the whole thing is all about tonal character.

Paul White

Optical compressors are renowned for their tonal flattery, but an output transformer is also employed specifically to add coloration (although this is rather subtle: the amount of distortion added before you pile on the tube drive is actually only around 0.1%!).

The mic/instrument preamp section offers up to 80dB of gain in total, with 40dB gain via the input gain control, a gain button to add a further 20dB, and yet another 20dB available courtesy of an output level control. (For unbalanced operation the maximum gain is reduced to 64dB, which is plenty for most applications.) Both unbalanced quarter-inch jacks and balanced XLR inputs and outputs are available, as are 48V phantom power, a switchable first-order 70Hz low-cut filter and a polarity-inversion facility. There is LED metering of both gain and any 'tube warmth' you decide to add. The output signal can be switched from line to instrument level, allowing you to feed line-level gear or guitar/bass amps alike.

The compressor has far fewer controls than is typical, and is set to a fixed 2:1 ratio, but it's not as limited in operation as this might suggest. It has a switchable release time, which is fixed at 150ms in the fast setting, and automatically variable between 20ms and 3s when set to slow. It can also be switched to function either as a compressor or a limiter, with attack times fixed at 20ms and 1ms respectively. The amount of compression is set using the threshold control, and a row of four LEDs shows the amount of gain reduction up to 15dB. Further LED metering follows the overall output level from -20dB right up to clipping. Sensibly, there's also a bypass switch for the whole compressor/limiter section.

The preamp has an EIN (Equivalent Input Noise) of -128dBu A-weighted, and the frequency response is a surprisingly wide 30Hz to 60kHz ±1dB balanced, with the non-transformer jack output going down to 20Hz. A maximum line output level of +19dBu is quoted. Although this is below what you'd expect from a top-of-the-line preamp, it should be enough to keep you out of trouble, even when feeding a digital interface that expects to see levels of +16dBu or so to reach digital full scale.

I have to admit that I wasn't expecting anything particularly subtle or refined from what, at the end of the day, is a very affordable mic preamp. However, having conducted a series of subjective tests with voice and acoustic guitar, the results were rather more classy than I'd anticipated. Used as a straight mic preamp in its clean range, with no added compression, I obtained clean, clear signals from both capacitor and dynamic microphones. But even with the gain pushing those amber LEDs into the 'warm' range, the added coloration was still nicely subtle, bringing a pleasant weight and thickness but not at the expense of clarity.

Bringing in the compressor evens out the sound level, just as you'd expect, and at lower gain-reduction settings the compression is reasonably transparent and natural-sounding. Add more and you start getting into 'compression as an effect' territory, but even then the result remains musical, with an endearingly springy 'opto' character. Using the fast release setting on acoustic guitar can create a very 'produced' kind of sound, combining thickening and levelling in a way that adds a sense of excitement and power. Meanwhile, although you can hear the compressor doing its thing in a more obvious way in the fast mode, it never seems to get offensive. The fast compression mode also works well to add sustain to bass guitar, and is particularly good for enhancing that fretless whine, while the slower mode keeps the gain under control in a far less obvious way.

As ever, you must keep background noise to a minimum when using heavy compression (such noise is raised in level between notes or phrases), but for typical close-miked applications the noise of the mic preamp was never an issue for me, and both room sounds and computer-fan noise will typically be of far greater concern for home-studio recordists.

The only thing this preamp doesn't do is overload gracefully; clipping the input produces a harsh and spitty sound, so this isn't a preamp you can use to create musical 'dirt' once you've passed the 'warm' stage. As long as you keep an eye on that input clip LED, though, you'll find the ART Tube MP/C makes a versatile and musical-sounding front end. 

£101.99 including VAT.$99.

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