ART Pro MPA & Dual MP

ART have followed up their single‑channel Tube MP valve mic preamp by developing two more tube designs for both the project studio owner and the audio professional. Paul White huddles round to warm his signals...

Almost everybody who has had a chance to compare different microphones will agree that valve or tube mics have a certain sonic quality that solid‑state mics generally lack, and it's also generally accepted that valves, when driven hard, add a type of distortion that we find musically pleasing. With early mics, the 'alleged' magic sound of valves wasn't an issue, because valve technology was all that exsisted — transistors were still waiting to be invented. Whatever the valves contributed, you had to accept, because there was no way to adjust the amount of tube overdrive, short of changing the distance between the mic and the subject being recorded. It's only when solid‑state electronics became commonplace that people realised there was such a thing as 'the valve sound'.

Reissue valve microphones tend to be expensive, though there are some affordable new designs, such as those built by Groove Tubes (see review in SOS October '95). However, if you're happy with the mics you've already got, using a valve mic preamp is another way of injecting a little vintage warmth into your work. What I find particularly interesting is that every valve circuit has its own tonal character — valve preamps from different manufacturers will never create quite the same effect. The sound changes depending on the types of valves used, how they are biased, the HT voltage applied to the anode, and whether or not transformer input stages are used.

Call In The MPs

ART's approach in both these mains‑powered valve mic preamps is to use a solid‑state circuit at the front end to provide a degree of quiet gain, and then follow that with a valve circuit that can be driven into varying degrees of distortion as required by the user. The multiple transistor input stage of the Pro MPA is more sophisticated than that of the Dual MP, and is largely responsible for the unit's very low noise figures (though as these are quoted with a shorted input rather than with a typical mic load, it's very difficult to make exact comparisons). Both models also include high‑impedance jack inputs allowing instruments such as basses, guitars or line sources to be DI'd with the benefits of valve 'flavouring'. There's sufficient gain range to accommodate line‑level signals, so it's quite possible to use these units via a console's master insert points to warm up a whole mix.

The valves used are 12AX7 (ECC83) dual triodes (one per channel), and a relatively low anode voltage (sub‑100V), is applied, which invariably produces a different characteristic compared to the same tube run from a 250‑300V HT line. It does, however, mean the power supply voltages can be stabilised, which helps minimise noise and hum. On both models, a LED meter monitors the tube drive, so that you get a visual indication of the amount of clipping taking place. A simple 4‑LED meter is used on the 1U Dual MP, whereas the 2U Pro MPA incorporates a 10‑LED bargraph.

The Dual MP

The Dual MP offers similar facilities to other mic preamps, insofar as it has both input and output gain controls, phantom power and phase invert facilities, but there's also a Norm/+20dB gain switch which operates before the tube stage. Most mic preamps have an input stage with around 60dB of gain variation, but in this design, the input stage has over 40dB of gain, with the other 20 being provided by the valve stage. A similar gain structure is employed in the Pro MPA, and the maximum gain depends on which connectors you use to get in and out of the units.

Balanced XLRs are used for both the mic inputs and outputs, and these are to be found on the rear panel along with unbalanced output jacks — a useful option for anyone plugging into an unbalanced insert return. High‑impedance, unbalanced line jacks are located on the front panel.

To set the degree of valve coloration, the Input Gain is used in conjunction with the 20dB switch, while keeping an eye on the four drive LEDs. Green indicates a clean sound, while the two Yellows show that a degree of warmth is to be expected. If the meter stays in the Red, it indicates clipping, which is perhaps best avoided unless you're using the unit as a valve fuzz box for your guitar. In normal use, the red LED will probably flash only on signal peaks.

The Pro Mpa

Though the circuitry employed in the Pro MPA is rather more sophisticated than that found in the Dual MP, the general facilities are broadly similar, the obvious exception being that the Pro MPA has two large, back‑lit moving‑coil meters to monitor the output levels, in addition to the LED bargraph, which shows the tube drive conditions. There's also a high‑pass filter before the gain stage, which provides a gentle 6dB/octave slope with a continuously variable frequency of 7Hz to 150Hz. There's no Bypass for this filter, but if you set it down at 7Hz, it's as good as bypassed for all practical purposes.

All the Pro MPA's audio connections are on the rear panel, and once again, there's a choice between XLRs or unbalanced jacks with jack sockets providing high‑impedance inputs for use with electric guitars and basses. Status LEDs are provided for all the buttons.

A glance inside the box reveals that the input stages are placed right behind the input XLRs as opposed to on the main circuit board (where they are on the Dual MP). This is a good move, as it minimises the risk of interference from inside the box being picked up by the input electronics. A further plus point is that the valves are fitted with screening cans in the Pro MPA.

Warmed Up

Once plugged in, these preamps are much the same to use as the mic input stage on a typical mixing console, the only additional consideration being how much valve drive to apply. With most sources, including vocals, I found that little effect was audible until the drive was set to a level that resulted in the clip lights coming on quite strongly on the signal peaks, but then you have to be careful that further level excesses don't drive the unit into audible clipping. This being the case, it would perhaps have been a nice idea to include a rear panel insert to allow a compressor to be connected before the valve stage.

Operation is simple — the drive indicator makes it easy to set up the required degree of valve warmth, and no vices of any kind were evident. Once the levels are adjusted correctly, the tonal changes brought about are very subtle, and you have to listen quite carefully to notice exactly what the difference is. On vocals, the effect is to thicken the lower mid‑range, which emphasises throaty or chesty sounds. There also seems to be a slight loss of transparency, which is replaced by a greater sense of directness or proximity, but I stress again that these effects are on a very small scale — they don't leap out of the speakers at you.

On mixes which include drums and bass guitar, the effect can be to create a richer, rounder bass sound and, at the same time, make drums appear more even in level, almost as though they've been compressed slightly. Once again, I got the impression that some of the 'air' was sucked out of the high end, but in a way that made the sound seem more focused and upfront.

When they are used to DI instruments, either unit works fine with passive guitars and basses, and using the valve drive to warm up the tone helps smooth out the sound. For obvious distortion effects, you can crank the drive up to full or even put the two channels in series, but you definitely need a speaker simulator at the end of the chain to make the overdrive sound usable. With most sounds, you can push the peaks well into the red before any distortion becomes evident and used with a little care, it is possible to add a little valve warmth to mics, instruments, complete mixes or even to synths.

I found little subjective difference between the sound of the two units, though the more expensive Pro MPA is a touch quieter, and does have a more comprehensive control layout, as well as the useful high‑pass filter.

Summary

There's no doubt that both units are well designed and nicely engineered. Both are quiet enough for the majority of pop applications with the Pro model being particularly good for a tube product. However, while everything about the units works perfectly, I have to reiterate that the amount of tonal change you can expect (short of out‑and‑out overdrive distortion) is subtle. Valve preamps are most definitely not a case of 'the emperors's new clothes', but at the same time, they don't give you an effect that you can lay on with a trowel either. If you're not sure what to expect from a valve preamp, rent one for a day or two before you sign that credit card slip — that way, you'll know exactly what you're getting. This is true of any valve mic preamp, not just these two — the valve sound is one of the most contentious issues in audio, and I don't expect it to go away for some considerable time!