Focusrite Green

Some mic preamps are used for the audible effect they produce on recordings, while others are chosen simply to amplify mic signals as cleanly as possible. Paul White checks out the third unit in Focusrite's affordable Green range, and finds a quality mic amplifier that falls into the latter category...

Over the past few months (see SOS November '96 and January '97), we've looked at the members of the Focusrite Green range of processors, which, although not aimed at the budget end of the market, does enable serious users to buy into top‑end performance for considerably less than they would have to pay for a Focusrite Red equivalent. The reduction in cost has been made possible by re‑examining traditional construction methods; the Green range uses surface‑mount components on a single circuit board, and though the case is undoubtedly an expensive component to fabricate, it has the advantage of simple assembly and good RF screening. Some cost saving has also been made by leaving off some of the more esoteric features, and by adopting transformerless circuitry, though the quality of the audio path is not compromised.

I'm actually starting to get used to the rather weird front‑panel design; and straight away, you can see a difference in construction to traditional folded steel boxes. Here, the sculpted section of the panel and the main section of the case are moulded in one piece using aircraft‑grade aluminium. The green front panel and circuit board form a sub‑assembly, which is then inserted from the rear of the case.

First Principles

The Focusrite Dual Mic Preamp is conceptually very simple; there are two identical high‑grade microphone preamplifiers in a mains‑powered package. The mains voltage may be switched from 240V to 120V by means of a recessed slide switch on the rear panel. In addition to the expected gain, phantom power, phase and high‑pass filter switches, there's also a mute button and a rear‑panel jack to allow remote muting; a front panel LED indicates when the channel is muted. The high‑pass filter operates at 75Hz with a 12dB/octave slope, and is designed so as not to compromise the phase or transient performance of the unit when switched in. Each of the front‑panel buttons has a built‑in amber status LED, and there is no pad switch, though with the amount of headroom and gain range provided by this circuit, it is very unlikely that excessive signal level will cause problems. What's more, using a pad invariably compromises the signal‑to‑noise performance of a mic preamp.

One other obvious cost‑cutting measure is evident in the area of metering, where only a simple overload LED is provided. I would have liked to have seen more comprehensive metering, but in reality, this unit will probably be used mainly for feeding mic signals directly to a tape machine or hard disk recording system, in which case the meters on the recording machine would be used. The LED doesn't come on until a level of +20dBu is reached, and even then, there's another 6dB in hand before the onset of clipping. A lone green power LED keeps solitary vigil at the far end of the front panel.

Audio connections to the preamp are by means of balanced XLRs only, though I feel a line‑level input would not have gone amiss for high‑quality DI applications. Remote muting is via a TRS jack, and requires a 9V source (such as a battery), in series with a 47Ω resistor. A wiring diagram is provided in the handbook.

Inside Story

Opening the case requires only the removal of four screws on the rear panel, upon which the whole assembly can be slid out of the sleeve for inspection. I have to say that the attention to design detail is impressive ‑‑ even the torroidal mains transformer is mounted in a screened enclosure and potted in a soft silicone compound to eliminate mechanical noise from the windings. At the front end of each channel is a pair of the same very low‑noise (and very tiny) Linear Technologies LT1028 mic amp chips used in the other Green units, in a circuit providing continuously variable gain from 10 to 60dB.

Muting is carried out using an opto‑isolator, and a miniature encapsulated relay. Very high‑grade components are used throughout — it's vitally important that the power supply is absolutely clean, not just for powering the sensitive circuitry, but also to provide an impeccable source of phantom power. A poorly‑designed phantom power supply can wipe out many of the performance gains in other areas of the mic preamp circuitry at a stroke.

Part of the Focusrite philosophy has been to build products that have a very wide frequency response, and the Dual Mic Preamp is only 3dB down at 50kHz with maximum gain. At the minimum 10dB gain setting, the response is 3dB at 150kHz. Even though nobody can hear these frequencies, maintaining a flat frequency response helps ensure a flat phase response — all the indications suggest that rolling off the frequency response too soon after 20kHz causes phase anomalies that are audible by most listeners.

Distortion is typically less than 0.001% (@1kHz 0dBu, 22dB gain), and the equivalent input noise (EIN) is quoted as ‑128dBu. This isn't significantly different from the figure you'd see quoted for a decent budget mixing console — but the figures don't tell the whole story. Simpler mic preamps only deliver good noise performance at their maximum gain settings, but most work is done with the gain setting somewhere between the two extremes, where the noise performance can be significantly worse. More sophisticated designs, such as the one used here, maintain a good noise performance at all gain settings.

Conclusions

The Focusrite Dual Mic Preamp is significantly cheaper than its more sophisticated Red equivalent, yet because of its transformerless design, it may arguably be even more accurate, though it does miss out on whatever magic good transformers are said to impart. This is, however, not one of those preamps you buy to warm up your sound or to change it in some other subtle way — this is a preamp you buy because you have a good mic and you don't want the sound messed about with. What you're buying is pure, clean gain — nothing more and nothing less. Yes, you could buy a small mixer for the price of this unit, but making such a comparison would be to miss the point. The real benefit of something like the Dual Mic Preamp is that it provides a minimum, clean signal path from microphone to recording device. The addition of remote muting could be useful in live recording applications, or in broadcast, where the mute could, for example, be driven from a fader start switch, but in the studio all you really need is the clean gain and exemplary phantom powering provided by the unit. Despite its unorthodox appearance, there's nothing at all frivolous about the Focusrite Dual Mic Preamp, and if you're equally serious about capturing sound as accurately as possible, it's a device you should consider investigating further.