This two‑channel tube preamp offers a range of useful features, and all for a very modest asking price. Is it too good to be true?
ART have a long‑standing reputation for delivering good, affordable audio processors, and their Pro MPA2 seems set to continue that tradition. Essentially, this device is a 2U-high, two‑channel mic preamp with switchable 48V phantom power and continuously variable input impedance. The latter is something that can have a significant effect on the sound of the connected microphone; in particular, many dynamic models, including passive ribbons, prefer to see a higher impedance.
The Pro MPA2 can be set up for dual‑mono, conventional stereo or M/S (Mid/Side) operation. In this last role, channel one is used for the 'Mid' mic and channel two for the 'Sides'. Further tonal variety is offered by the ability to run the 12AX7 valves at low or high anode voltages.
The Pro MPA2 certainly feels solid — it was rather heavier than I expected it to be when I lifted it — and the front panel looks stylish, with its detented, somewhat retro‑styled knobs and large, backlit, moving‑coil VU output meters that have adjustable calibration trims. Further LED bar-graph meters, labelled Tube Warmth and set between the VU meters, show the amount of tube drive being used. While the mic inputs and balanced XLR and TRS jack outputs are on the rear panel, a high‑impedance instrument input is conveniently positioned on the front panel, this taking precedence over the mic input when connected. A rear‑panel button allows the output level to be set to +4dBu or ‑10dBv, and the meter calibration automatically follows this setting.
The mic input is a solid-state design, with a rotary control to set the initial gain. This has only a 40dB range, but an extra 20dB of gain can be applied before the tube stage. Having an extra gain boost selectable in this way helps the preamp to run at lower gains while still maintaining a high signal‑to‑noise ratio, as many affordable mic preamps only deliver their quoted EIN figure when set close to maximum gain. A combination of the input gain and this 20dB 'boost' can be used to drive the tube stage as hard as you feel appropriate. A further 10dB is available via the output gain control, so there's an overall maximum gain of 70dB. (The manual claims 75dB with everything flat out, but I'm not sure where the extra 5dB comes from!)
Next to the input gain control is the one that governs input impedance, which ranges from 150Ω to 2.4kΩ. To keep the lows under control, there's a smooth, 6dB/octave low‑cut filter that can be swept from 10Hz to 200Hz. Four illuminated buttons beneath the VU meter are used to select 'normal' or +20dB of gain, phantom power, high or normal 'plate' voltage, and polarity inversion.
A further button mounted between the channels selects the M/S decoding matrix, so that an M/S mic pair fed through the unit will produce a standard stereo signal at the output. At the extreme right of the unit is a 'stereo' button that allows the rightmost output‑level control to function instead as a stereo balance, while the leftmost output‑level control operates as a master stereo level control.
The technical spec looks suitably impressive on paper, with a frequency response extending from 15Hz to 48kHz (+0, ‑1dB) when the valve anode‑voltage is set to normal. An equivalent input noise for the mic input is quoted as ‑129dBu A‑weighted, translating to a dynamic range of better than 110dB, again A‑weighted. Both mic inputs can accommodate signal levels of up to +10dBu before clipping, whereas the maximum instrument level is +17dBu (800kΩ impedance). The output stage can handle up to +27dB on the XLRs, and has a low 47Ω impedance, which is suitable for driving long cables.
So how does it sound? My first test was with a capacitor mic and, as expected, the input impedance made very little difference. (The active circuitry in the mic buffers the signal, making it less dependent on load impedance.) I set the input gain near maximum and used the output level control to achieve a decent recording level without switching in the 20dB boost. Only one or two of the tube drive LEDs came on, and this arrangement produced the most transparent, natural‑sounding result.
Engaging the boost and adjusting the input gain to push the tube-drive LED meter around halfway introduced a subtle warmth that I found quite usable, but as I pushed things harder to get the meter bouncing around close to the top, the sound became noticeably overdriven. This degree of drive would probably only be used on certain instruments, or maybe as an obvious effect on some pop and urban vocal styles.
Bringing the drive down to a low level and switching on the high tube-voltage brought about a subtle thickening of the sound, which, again, I found musically useful. In combination with a higher drive level, the sound again became progressively warm, then thick, then quite distorted as the pre‑tube gain was increased. At modest drive levels, this can be useful in bolstering a weak vocal, but I'm not a fan of vocals driven to the point where the distortion is obvious, so probably wouldn't venture much more than halfway up the drive LED meter.
Using a passive ribbon mic, the output level got significantly louder at the higher end of the impedance dial, along with a subtle brightening of tonality. The variable input impedance will be of particular benefit for anyone working with such microphones.
Despite its relatively low cost, the ART Pro MPA2 delivers on both sound quality and versatility. It doesn't match the transparency of some of the more costly high‑end preamps, but it gets close enough for most home studio applications — and out-performs many audio interface on‑board preamps — while also having the ability to produce very controllable tube character. The two tube‑voltage options provide another subtle but useful variation in tonality, and the variable input impedance is invaluable for use with passive ribbon mics or other insensitive dynamic models.
I was also pleased to see the M/S facility built‑in, as M/S recording is a very useful technique that's often neglected in the smaller studio; having on‑board decoding means that anyone with a suitable figure‑of‑eight mic plus one other mic can have a go and hear the benefits of this approach for themselves.
You may not feel you need an additional mic preamp if you have some built into your audio interface already, but I find that many of the mic amps built into audio interfaces, especially at the more affordable end of the market, are susceptible to picking up digital interference from the Firewire or USB circuitry when run at high gain settings — and this, again, is a particular problem when using passive ribbon mics. The MPA Pro 2 would be an ideal remedy for that particular disease.
In a nutshell, the Pro MPA2 is great value for money. Not only does it sound pretty good, but it's one of the few budget tube preamps to run the tubes at a suitable voltage, and it's extremely versatile, with several useful tricks tucked up its sleeve that many of its competitors lack.
The SPL Gold Mike costs only slightly more than the Pro MPA2 and is still top of my favourites list for affordable tube preamps, though it doesn't have as many features as the Pro MPA2. Specifically, there's no variable impedance and no M/S decoding. TL Audio's mic preamps also perform consistently well, but they tend to be more expensive.
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Test plots to accompany the article.
Audio files to accompany the article.
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