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Benchmark MPA1

Dual-channel Microphone Preamplifier
Published December 2009
By Hugh Robjohns

Benchmark strive to make their products exactly that: something against which the competition should be judged. So how does their new preamp measure up?

Benchmark MPA1

The American manufacturers Benchmark have been making and selling high-end audio products since the mid 1980s, and their DAC1 converter (SOS July 2005) has rightly become something of a standard bearer for reference‑grade D‑A converters — indeed, I use one within my own reference system. The similarly superb and partnering ADC1, sharing the same half rack-width form factor as the DAC1, was reviewed in SOS July 2006, and even back then there was talk of a matching mic preamp to extend the family. At last, the MPA1 has arrived, in a half rack-width case to match the ADC1 and marketed as a very clean and neutral high‑end, high‑quality two‑channel microphone preamplifier. This is not the company's first preamp design, though: the MPS400 and its successor, the MPS420, received many plaudits before the current and very clever PRE420 four‑channel preamp/mixer was launched. Benchmark also make a range of preamp modules for customisable rack-frame systems.

Overview

The MPA1 is part of the Benchmark 'System 1' range, and has been designed with the same meticulous attention to detail, both in design and construction, as the rest of the range. It is claimed to provide the highest standards of sonic transparency for a dual‑channel mic preamp, with ultra‑low distortion and extremely low noise, making it a great device for compact stereo recording applications — and not least because it incorporates an HPA2 headphone amplifier to provide the highest quality local signal monitoring. This headphone driver is also built into the DAC1 converter, and is very highly regarded as a reference-quality source.

As is usually the case, I'll start my description with the rear panel, which simply carries two pairs of XLRs for the dual inputs and outputs, plus a fused IEC mains power inlet that can be configured for nominal 110 or 220V operation. However, the internal power supply is very accommodating, with the 110 setting accepting supplies ranging from 105‑140VAC, and the 220 setting from 200 to 285VAC.

The front panel is a little busier, but clearly laid out and logically marked, with white legends on a black background. The two preamp channels are identically equipped, with recessed toggle switches to select phantom power, high‑pass filter (12dB/octave from 40Hz) and polarity inversion. The phantom switch is more recessed than the other two, to make accidental operation extremely unlikely. An endless rotary switch, marked from 0 to 24dB in 2dB steps, adjusts the gain, with three revolutions required to reach the maximum of 74dB. The current gain setting is indicated by four adjacent LEDs, which illuminate to show when the minimum (0dB) or maximum (74dB) positions are reached, along with range steps of 24 and 48dB. The current gain level is revealed by adding the switch value to the number indicated by the LEDs, so if the switch is set to 16 and the 24dB LED is on, the total gain is 40dB. It's an unusual arrangement, but one that works very well in practice.

To the right of the second preamp section is a simple, four‑LED, dual‑bargraph, peak‑reading meter. The bottom green LED lights when the signal level exceeds ‑20dBu and the second green LED when the signal rises above +4dBu. The yellow LED doesn't illuminate until +24dBu is reached, and the top red LED is at +28dBu, one decibel below clipping. It's very easy to set appropriate output levels with this arrangement, adjusting the channel gains to get both green lights on happily, with the yellow light warning of potentially scary transient peaks.

The right-hand side of the front panel is given over to the HPA2 headphone output facility, with a simple volume control and quarter‑inch socket. This is a high‑current, high‑output amplifier, which can drive any type of headphone down to 30Ω very easily, with plenty of level for higher impedance types.

Technology

The half-rack format of the MPA1 means you can fit four channels into a single rack space.The half-rack format of the MPA1 means you can fit four channels into a single rack space.

The input stage is transformerless, and uses precision‑matched transistor quad‑arrays to achieve very low noise and wide bandwidth (‑3dB at 500kHz). The input impedance is high at 8.1Ω (most standard preamps present around 2.4kΩ), but this is similar to the 'high' settings found on many adjustable‑impedance preamps, and some classic designs. It reduces the loading on dynamic mics and is generally associated with a more open and spacious sound.

A high supply voltage enables a very impressive +29dBu clipping point, while servo control minimises DC offset and maximises headroom. Dual‑capacitor AC coupling is used to maximise signal quality and transparency, while providing an unusually wide bandwidth (it's completely flat to below 10Hz), with extremely linear subsonic phase accuracy. Common-mode rejection is calibrated at three different frequencies, and two‑stage RF filtering guards against external interference. The unusual gain‑control arrangement is based on a 38‑step, relay‑switched resistor ladder, providing 2dB steps from 0‑74dB of gain, with inter‑channel gain‑matching of an excellent ±0.03dB, and no switching clicks on the audio. The input stage is fully protected from phantom power surges, caused by hot‑plugging and cable faults, with high‑speed Schotky and high‑current TVS diodes.

Internal construction is to very high standards, with a shielded torroidal transformer at the centre rear. The rest of the floor area is occupied by a single PCB, mainly carrying surface-mount components. An internal 'handbag' link can be removed to disable phantom power (from both channels), which is useful if you intend to use the MPA1 with ribbon mics, or just don't require phantom for some reason.

In Use

Everything feels very solid and professional, with high‑quality control knobs and switches. The gain‑setting arrangement takes a little getting used to, but other than that, operation is very simple and straightforward. Given its cost and the claims made for it, I compared the MPA1 against my own GML 8304 mic preamps and another preamp on review, the AEA RP48Q (see SOS November 2009). These seemed the most relevant competitors, as both are transformerless solid‑state designs and aim to provide the finest of neutral, clean and transparent sounds, albeit using slightly different technology (the GML is all discrete, with no capacitors in the signal path, while the AEA is FET‑based).

The initial impression on first hearing this preamp is of masses of clean, precise detail. The sound character is very natural, with a perfect balance throughout the frequency range, from the deepest lows to the airiest of highs. Nothing seemed prominent, exaggerated or exposed; just very natural, but with that slightly larger‑than‑life character that so often marks the true high‑end designs from the ordinary. My first tests used Sennheiser MKH40s as a stereo pair in front of a singing acoustic guitar player, feeding straight into an HHB CDR recorder, and I was rewarded with a lovely and very natural rendition of the performance, complete with a very stable and detailed enveloping acoustic. In comparison with the same mics into the GML, the latter seemed to sound a tad larger and more 3D, somehow, but it was a very subtle difference. Detail and clarity were very, very similar on both.

Being able to adjust the gain in precise 2dB steps, with perfect channel matching, made stereo working direct into the CD recorder a doddle, and allowed the headroom margin to be reasonably minimised without too many will‑it/won't‑it clip worries. This was aided by the superbly clear and detailed headphone output, which justifies a significant proportion of the cost of this unit all on its own!

With plenty of gain on hand (and with phantom turned off and allowed to expire before replugging), I also quickly hooked up a pair of AEA R92 ribbon mics. The results weren't as good, although that was entirely down to their positioning and the acoustic: the preamp performed flawlessly, providing more than enough gain and allowing the ribbons to deliver the full and balanced sound they're capable of, thanks to the relatively high input impedance. There was, in fact, very little to tell the difference when comparing the MPA1 with the RP48Q.

For some vocal tests, I set up a Gefell M930 on one channel and an AKG C414B ULS on the other, and was immediately very pleased with the results from both. The AKG can often sound a little thin and clinical on some vocals, but here it sounded very natural and well balanced, with a bright, open, detailed top end, and a realistic weight to the lower ranges. The Gefell provided a warmer but still very usable sound, equally blessed with an airy but not sibilant top end. In both cases I felt that I was hearing exactly what the microphone was delivering: all the detail and clarity was precisely laid out, without any hint of congestion or coloration. The preamp was simply making the signal louder without adding any character of its own — and that's precisely what I (generally) want from my preamps.

The high‑pass filter has an unusually low turnover at 40Hz, and I'd have liked a higher option too (perhaps 80 or 100Hz), but it was effective at helping to reduce unwanted stand‑vibration noise and air‑con rumbles. The provision of a polarity inversion is also useful to have, if not essential in the kind of applications where this mic preamp is likely to be used. The half‑rack design makes it practical to fit four channels in 1U of rack space, or to partner the MPA1 with the ADC1 in a single rack space, or for racking to make a very high‑quality front‑end for a digital system.

As with all Benchmark products, the manual is well written and includes an exhaustive set of test measurements and plots that reveal the product's genuine performance specifications. Would that more manufacturers adopted such a transparent and engineering‑based approach to back up their claims!

Impressions

I had high expectations of the MPA1, based on my experience of the DAC1, and I wasn't disappointed. There aren't many preamps that can challenge the GMLs for clean, neutral and transparent operation. The AEA designs are quite close, but this Benchmark is closer, and also better looking, better equipped, more practical to use and less expensive. I had been toying with the idea of adding more GMLs, but in all honesty I think I've just found a more versatile solution that gives nothing away.  

Alternatives

Ultra‑clean and transparent preamps aren't everyone's cup of tea, but GML probably sets the gold standard at this quality level, with the specialist ribbon preamps from AEA following closely behind.

Published December 2009

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