Audio & Design DMA2

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Digital Mic Preamp

Published in SOS January 2008
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Reviews : Preamp

Photos: Mike Cameron

Audio & Design Reading (A&DR) are one of those small British specialist manufacturing companies who have been around seemingly for ever, but always in a fairly low-key way. The company started in the mid-1970s, making highly regarded solid-state limiters and compressors — such as the Compex — for the professional recording studio market and broadcasters. Amongst their eclectic product range were also several very specialised Ambisonic surround sound products, including UHJ transcoders, pan and rotate units and more.

With the introduction of digital recording in the early 1980s, the company started making specialist digital interface products as well as modifying semi-pro equipment for professional use. In fact, I bought an A&DR-modified Sony PCM701 digital processor in 1986, which was my first venture into the world of digital audio.

These days, in addition to the manufacture of a small range of bespoke products, the company has evolved to become more of a consultancy and systems installer for the Broadcast IT market, with recent projects including the installation of a multi-terabyte archive system for the BBC World Service. The manufacturing side mainly comprises a range of broadcast transmission limiters, grade 1 video and AES digital clock reference generators, sample-rate converters, digital audio distribution and management processors, compact desktop digital mixers and high-quality digital mic-amps. It is the last of these that forms the subject of this review.

Technicalities

The DMA 2 is a two-channel microphone preamplifier with built in 24-bit, 96kHz A-D conversion. It is designed to offer all the essential facilities in a convenient package without any unnecessary frills. The sound is clean and transparent, rather than coloured or tailored — so this is a product aimed at cost-conscious professionals who want precision, reliability and accuracy.

Housed in a simple, 1U rackmounting box, the DMA2' has front-panel controls that are all push-buttons with associated LED status lights. The case itself measures only 153mm deep and weighs just 1.65kg, and the unit is powered by an internal universal mains supply, which accepts mains voltages between 90 and 250V AC.

The rear panel features a pair of female XLR sockets for the microphone inputs, plus another for an external AES11 reference clock input. The digital output is provided on a male XLR connector. There are no analogue outputs and no word clock input or output. An RS232 serial port is provided for full remote control and there is an option for an RS485 interface if required. The mains on-off switch is adjacent to the fused IEC mains inlet.

Internally, the unit is built to high standards, with a main circuit board covering most of the floor of the box and a subsidiary one behind the front panel to carry the switches and LEDs. The surprisingly large switched-mode power supply is in a separate case at the right-hand side.

The circuitry is a mixture of conventional and surface-mount components, with the analogue stages being based around very low-noise SSM2019 preamp chips, supplemented with a couple of discrete transistors at the front end. The gain control and audio switching is performed using a combination of CMOS analogue switches and a digital volume control chip, and the A-D converter is a Crystal device.

The technical specifications are impressive, with an effective input noise of -124dB (with a 200(omega) source) at the maximum gain setting of 70dB. Distortion measures 0.003 percent at 30dB of gain, and the digital output has less than 0.5ns of jitter when running from the internal crystal clocks.

Operation

The front-panel controls are very logical and straightforward. Each of the two mic-preamp sections features five push-buttons and an array of LEDs. The preamp gain is set using a pair of up/down buttons and an associated ladder of LEDs shows the nominal input level in 5dB steps from -70 to +10dB.

The three other buttons switch in a high-pass filter (18dB per octave from 100Hz), phantom power and a polarity reversal — and each has a different colour of status LED, so that the configuration is obvious even from a distance.

This is a dedicated mic preamp, with mic inputs via the usual XLR sockets but no line inputs. The digital outputs are offered on AES-EBU XLRs and there are no analogue outs. An RS232 serial port is also included on the rear panel, allowing full remote control. This is a dedicated mic preamp, with mic inputs via the usual XLR sockets but no line inputs. The digital outputs are offered on AES-EBU XLRs and there are no analogue outs. An RS232 serial port is also included on the rear panel, allowing full remote control. This is a dedicated mic preamp, with mic inputs via the usual XLR sockets but no line inputs. The digital outputs are offered on AES-EBU XLRs and there are no analogue outs. An RS232 serial port is also included on the rear panel, allowing full remote control.

There is no conventional level metering, just a trio of LEDs set to illuminate at -18, -10 and 0dBFS, corresponding to the EBU recommendations for nominal level, maximum permitted level and clipping. Clearly, as this unit provides only a digital output, the intention is to use the recorder's own metering for a more detailed analysis of signal levels.

Located between the two preamp sections are two more buttons. The first switches the sample rate and the second configures the output channel assignment. The sample-rate switching display is slightly unusual. With four LEDs available, most equipment would provide lights for the 44.1 and 48kHz modes, plus a double-rate light and an external input light. However, the DMA2 has lights for external input, 44.1, 48 and 96kHz — because those were the only rates provided in the original model. Subsequently, an 88.2kHz option was added, and this is now indicated by illuminating the 48 and 96 LEDs together. Admittedly, the front-panel graphics make this arrangement perfectly clear, but it is a little unusual.

The Channel Assignment button switches the output formatting. Normally, preamp 1 feeds the left channel of the digital output, and preamp 2 the right — normal stereo. However, it is also possible to switch the unit to provide mono (both inputs summed and routed to both outputs), or to route either preamp's signal to both outputs (with the other input muted). These facilities betray a broadcast requirement and will be rarely used by most, although the ability to generate a true mono signal is useful when lining up a stereo pair, especially given the polarity-reversal facility in the preamps.

The final front-panel facility is a slide switch to disable the front-panel buttons completely, as a security function. To avoid accidental operation, if a button hasn't been pressed for a while, the first press of any button causes its corresponding LED to flash, but the function is only activated if the button is pressed a second time. If pressed within the 'timeout window', other buttons can be activated and their functions will change immediately — but once the timeout expires the next press will trigger the warning flash again. If a second push doesn't happen within a couple of seconds, the flashing will stop and the function remains in its original state. The only exception to this arrangement is the gain-control switching, which responds instantly at all times.

Every operational function can be controlled remotely via the RS232 port using a simple ASCII-based interface. RS232 can be extended over about 15 metres, but for more distant remote control an optional RS485 board can be installed, allowing connection over more than 100 metres.

In Use

The DMA2 is very simple to set up and use. The nice thing about the gain switching is that it is very accurate and totally repeatable. The double button-press business sounds a lot more complicated than it really is — and in practice it doesn't seem to get in the way at all.

In terms of performance the DMA2 really impressed me. It is a quiet, clean and very neutral-sounding preamp, with a huge amount of gain available. I'd compare the sound of the DMA2 with the likes of the Audient ASP008, or the DACS MicAmp — and it costs significantly less than either (although it has only a quarter of the channel count of the Audient, of course), while also providing the A-D conversion.

The gain markings seem to correspond accurately to the input level needed to achieve full modulation — which means that the highest input it will tolerate is +10dBu. This is fine for a microphone source, obviously — even a condenser in front of something very loud indeed — but it won't accept a full professional-level line input, so you can't use the DMA2 as a line-level A-D converter without padding the line source down first — although, having said that, you will probably get away with a semi-pro line source working at a nominal -10dBV.

Everything works as expected: the high-pass filter is nicely judged, and useful in reducing low-frequency acoustic or mechanical rumbles; the polarity inversion does what it says on the box; and although the phantom power measured a little low at 45V, it is still within spec... just. The digital output is stable and the clock rates are accurate. The only disappointment here is the absence of word clock in and out, which makes it difficult to synchronise multiple DMA2s: you'd need to get a master clock unit that provided duplicate AES11 reference outputs, such as the Drawmer M-Clock, for example, and clock each DMA2 via their AES11 reference inputs.

In terms of sonic quality, this unit impressed me — but it does not produce an 'impressive sound.' By this I mean that it is clean, transparent and natural-sounding. It does not endow its outputs with the 'larger than life' character that so many aspiring high-end preamps do. Personally, that suits me down to the ground — I'd rather have an accurate, clean recording that I can tweak and shape later in post-production than a coloured master recording... but I accept that this approach doesn't suit everyone. At the current price, the DMA2 has to be considered something of a steal. 

Alternatives There are a few other two-channel preamps with integral A-D converters: the slightly cheaper Sonifex RBDMA2 has similar facilities; the Audient MICO offers word clock and multiple digital output formats for a smilar price to A&DR's preamp; Apogee's Mini-Me is a bit more expensive; and the Neve 1073DPD is rather more expensive! Of these, only the Neve offers the repeatable switched gain facilities of the DMA2, and none has the remote-control features. Alternatively, in cost-per-channel terms, the four-channel Focusrite ISA428 is only slightly more expensive, and provides several additional facilities.