Review: A.R.T. Dual MP Two Channel Microphone Preamp

by Bruce Wahler

The following are facts and subjective impressions about the Applied Research and Technology (A.R.T.) Dual MP preamp, which I have owned and used for about three months. I use the Dual MP to fatten the sound of a Voce V3 through a Leslie model 251 with a MOSFET amplifier. As such, this review will concentrate on the unit's capability to warm a solid-state sound, and its ability to provide an overloaded sound at lower volumes.

Overview

The Dual MP is two-channel tube preamp, designed for use in connecting a microphone to a recording console. The idea is to allow the tube's natural compression and harmonic enhancement to "warm" the sound, prior to recording in the digital domain. In addition, the Dual MP is promoted as a direct-input DI adapter for connecting instruments such as bass and piano to a mixing or recording console, without the need for microphones. The Dual MP uses both solid-state and tube technologies to provide its sound coloring.

The Dual MP is essentially two of A.R.T.'s popular Tube MP preamps in one case. I chose the Dual MP for several reasons:

The Dual MP is rack-mountable, while the Tube MP is not;
The Dual MP has a build-in power supply, while the Tube MP needs an external power supply;
Past experience has shown me that a single preamp tube cannot create a good copy of the subtle overload effects seen in a standard Leslie model 122 or model 147 amplifier.

The durable, lightweight unit is mountable in a single 19" rack space, and needs no special mounting considerations for heat.

As with any signal processor I've ever seen, there's some room for improvements in the Dual MP's design, but I believe it is a very effective re-creator of the classic "tube sound", at a reasonable price. This review criticizes the Dual MP on certain points, but it's a great choice for keyboard players who need a higher power Leslie, but miss the classic sound of the 40W Leslie tube amp.

Compared to the myriad of tube preamps made for guitar or bass, the Dual MP sounds less distorted and "buzzy", and its controls allow for a wider range of the overload characteristics that keyboard players want. It's not a perfect replacement for a stock 122 amplifier, but it's very close.

Note: All comments in this review are based on using the Dual MP as an organ/keyboard preamp. Obviously, many of the unit's features were designed with recording use in mind, and in certain cases, the inclusion of such features may even be contrary to the needs of keyboard sound processing. No evaluation of the unit's suitability for microphone amplification or recording needs is provided.

Physical Description

The Dual MP occupies a single rack space (19" x 1.75"). Like all of the best rack modules, the Dual MP is fully "wall wart" free; i.e., the power supply is internal to the unit. At around 6 or 7 pounds, the unit is very light.

Since the Dual MP is really two separate preamps in one package, the front panel is divided into two identical sections. Each features two control knobs: Input and Output, as well as switches for +20dB Gain (boost), Phantom Power, and Phase Reversal. The unit provides 1/4" jacks for input on the front panel, and balanced XLR inputs, 1/4" outputs, and balanced XLR ouputs on the back panel. The power cord is not removable, but is surprisingly long for a rack mount device.

Each section of the Dual MP uses a single 12AX7 tube, which has been hand-selected to provide the desired overload response in the Dual MP's unorthodox tube circuits. (We'll discuss this later on in the review). The tubes are not easy to replace, but the A.R.T. technician I spoke with claimed that the furnished tubes should last for many years.

The Dual MP is manufactured in the USA.

Signal Routing

The MIDI keyboard, modules, etc. are plugged either into the 1/4" jack on the front, or the XLR input on the rear panel. (It is possible to use both inputs at once, although the 1/4" signal is attenuated when you do this.) Either of the outputs is sent to the rest of the chain. Both the input and output signals are buffered with solid-state circuits, and aren't affected by low-impedance circuits like classic tube circuits are.

Personally, I wish all of the jacks were on the back, but I understand that the real intent of the Dual MP was microphone signal processing, so the jack on the front makes sense, from that point of view. Other preamps provide no jacks on the back, so I guess the Dual MP's setup is better than some. I ended up plugging in an impedance matching transformer from Radio Shack into the XLR input, so that I could use the rear jacks only. If you are using the Dual MP with a Hammond console organ, the balanced XLR inputs could actually be a plus, although they will have to be attenuated before use.

Adjusting the Sound

Unlike most musical instrument preamps, the Dual MP has no EQ controls or tube bias adjustments. Any tonal coloration is created solely by overdriving the 12AX7 tubes.

Each channel has a four-LED "Sonic Character LED Display" - Clean, Warm (2 LEDs), and Clip - which as best as I can tell provides a measure of the distortion being produced by the tube. A.R.T. states that the circuitry has been designed so that the solid-state circuits will not overload, no matter how distorted the tube electronics get. My experimentation seems to bear this out. The Input control is adjusted until the desired level of warmth or overload is seen, and the Output control is used to set the proper signal level for the next preamp or amplifier stage. Changes in the Output setting will not change the warmth or distortion of the Dual MP, except for the degree that they provide a "hot" signal for the next stage in the amplification chain.

Disabling the +20dB gain boost (6-40dB gain, rather than 26-60dB) seems to provide a more natural sounding overload. Similar results can be reached by kicking in the extra gain and then backing down the Input control accordingly, but I find that the latter combination results in a rough transition where the signal just begins to distort. It's hard to explain the problem in print, but if the Dual MP is used in the higher-gain mode, and set such that a stage just starts to "growl", the distortion will tend to pulse or beat as the frequencies of the notes beat against each other. This is not noticeable with single notes or octaves, but rather annoying when playing chords. I cannot duplicate the problem on the lower gain setting.

Getting the sound right is a matter of both taste and experimentation. I played around with single stage operation, as well as all three dual-stage combinations: high-gain first stage with lower gain in the second stage; low first stage gain fed into a high-gain second stage, and two stages of relatively equal gain. I didn't find a single stage up to creating the warm, but slightly overloaded, sound that I want. All of the dual-stage scenarios, on the other hand, were quite useable, and the differences are subtle. My preferences for use with a Voce V3, in order to get a slight growl that disappears as the expression pedal is pulled back, are as follows. Your results may vary, especially if you use a different sound source:

Set the drawbars to 888888888. Push the expression pedal all the way down and play the lowest two C's on the organ.
Adjust the first preamp stage until the red Clip light just begins to flicker, then back off until the flickering completely disappears.
Adjust the Ouput of stage 1 and/or the Input of stage 2 until the Clip light flickers 2-3 times per second.
If you like less growl, back the Input of stage one off about 1 "clock position"; ex: from 3 o'clock to 2 o'clock. If you like extra distortion, turn the Input up an extra clock position.
Adjust the Output of stage 2 to a level that works well with your amplifier or mixing board.

This method will provide a warm but essentially clean sound when playing chords at low and medium expression pedal settings.

I found the final result to be a little heavy on the mid-bass, compared to a real 122/147. A.R.T. alludes to this increase in low-end presence in the manual, and considers it part of tube "warmth". For my liking, I use EQ'ing to knock 4-5dB off at around 200Hz, which seems to bring the sound back to 122 territory.

Under the Hood

Unlike classic tube systems, the Dual MP has no hazardous voltages running around inside it. In fact, you'll find the unit to be surprisingly cool to the touch, even after several hours of use. How do they do it? By running the 12AX7's at only about 50 volts! Since this is well below normal operating voltages, the technician I spoke with told me that A.R.T. hand-selects the tubes for suitable response (and probably for matching to their LED display values as well). What this means to the user is two things:

The tubes will probably last many times longer than normally expected. Many of the reasons that tubes "wear out" are related to the high temperatures and voltages that they are subjected to during warm-up and normal use;
They cannot be replaced with just any 12AX7 tube, so experimentation with other 12A_7 varieties (12AU7, 12AT7, etc.) is probably out of the question.

Luckily, the whole mess seems to work just fine, and the response of the factory-installed tubes provides the smooth, even sound I was looking for. Given this, I did not try to substitute other tube types.

Sound

How does the Dual MP sound? Pretty much like a tube amp should. It adds warmth and growl at higher input levels, and completely disappears at very low levels. A single stage can be used to provide warmth only, and two stages serves to provide a good imitation of the sound of a classic tube amp. There are some subtle differences between the Dual MP's sound and that of any given tube amp, but most of these issues can be solved with EQ.

I personally find that the Dual MP sounds best with a MOSFET power amp, rather than a bipolar one. MOSFETs have a lot in common, electrically, with tubes and so MOSFET amps are already a little warmer and smoother than their bipolar counterparts. In this scenario, the tube preamp is used to add overload and distortion, rather than to overcome the inherent sound of the amp.

One big advantage I find is that my rig - all adjusted and EQ'ed - sounds pretty much like a 122 amplifier on "8" at conversational volumes, or at levels that can keep up with the average rock guitarist. I can adjust the output level to suit the gig, rather than to get the sound I want.

Cost and Portability

The Dual MP's advantages include portability, light weight, and lack of maintenance. Since the final system is modular, my gear is more portable than most Leslies. The tubes should last a long, long time.

When considering the cost of the Dual MP, however, you need to factor in the entire amplification system: preamp, amp, and Leslie or clone. If you already own a tube Leslie, and find the output volume sufficient for your use, the Dual MP probably sounds like an unnecessary complication of your setup. If, on the other hand, you normally use microphones or multiple units to get your Leslie or Pro-3 up to necessary volume, the Dual MP can be a life (and back) saver. The unit retails for US $329. Street prices vary quite a bit; but a price of about US $250 seems about right.

Conclusion

The Dual MP is a great product for those players who want to get the classic tube sound out of their solid-state rigs, or through a PA system or recording console. As with any product, the Dual MP has its good and bad points, but the unit offers excellent sound at a very reasonable price, and it weighs a small fraction of a tube amplifier. It's definitely worth checking out.

Copyright (c) 1997 by Bruce A. Wahler. Reprinting of this document for non-commercial purposes is freely allowed. Use at your own risk.

No laboratory animals were used during the evaluation of this product!

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