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Audient Mico

Dual Microphone Preamplifier
Published May 2009
By Hugh Robjohns

Audient Mico

With two channels of mic preamp, a phase‑alignment control, digital outputs and clean signal path, the Mico offers you an awful lot of Audient for the money.

The Audient 8024 console (in two variants) has become one of the most highly respected professional studio consoles on the market, and a popular choice for smaller commercial studios that can't quite justify the SSL and Neve high ground. Its specifications, sound quality and ergonomics are all superb — although we'd expect nothing less from Dave Dearden's drawing table — and, in particular, its microphone preamps are extremely good: clean, quiet and neutral, with plenty of headroom and excellent resolution. It's not surprising, then, that there's demand for these preamps in a stand-alone outboard form. Up until now, though, the only option was the ASP008 eight‑channel rackmounting unit — which is excellent, but overkill if you only need a couple of channels. This is where the Mico comes in: two channels of ASP mic preamplification, with some useful and interesting bells and whistles thrown in to make the product more versatile and attractive to its potential market.


The unit is a half‑rack size, measuring 430 x 270 x 80mm and weighing just 2.5kg. An external in‑line, switched‑mode power supply provides 12V DC, so the Mico can be battery powered if necessary (it accepts 9‑15V DC). There are two channels, both able to accommodate mic or line inputs, and a built‑in A‑D converter to make it easy to feed a digital recorder directly. The basic mic-amp design is essentially the same as that in the ASP8024 console and rackmount units, but with some incremental improvements that have resulted in a slightly lower THD+N distortion figure (0.0015 percent instead of 0.007 percent), as well as some additional facilities well suited to a home studio application. Following user (and reviewer) feedback, the Mico has recently undergone a slight revision to improve the linearity of the gain control, and it is the revised model I'm describing below.

Each channel is controlled via one rotary knob and five illuminated buttons. These provide variable gain (18 to 66dB, with 36dB in the mid‑point), phantom power, a 20dB pad, a switchable high‑pass filter (40, 80 and 120Hz turnovers and off), plus polarity inversion. A four-LED bar-graph meter is minimal, but workable — the LEDs are calibrated for ‑36, ‑12, ‑6dBFS and Over, all referring to the digital output, which is calibrated so that 0dBFS equates with +18dBu. In other words, for normal signal levels the bottom LED will be on all the time, the ‑12dBFS will flick on peaks, and the ‑6dBFS should barely light at all. In practice, it is actually very easy to optimise levels with sensible headroom margins using this simple meter. Both channels share these same, identical preamp controls and facilities, although channel one also has a front-panel DI input socket. Plugging in a quarter‑inch jack plug overrides whatever is connected to the rear input sockets.

In addition to the controls already described, both channels also have a rotary control and one more illuminated button — but their functions are different for the two channels. Channel one is equipped with a 'harmonics' (HMX) control, which has been translated from Audient's Black Series preamp. The knob sets the amount of harmonic coloration, while the push button switches the system on or off. This HMX system is designed to progressively introduce harmonic distortion with a character similar to that of classic valve preamp designs, and it adds a nice degree of 'body' and richness to a source — particularly through the mid-range — that just helps to bring the sound alive. It is useful on vocalists, and absolutely perfect for DI instruments, which is why the function was added to this channel.

As well as analogue outputs on balanced XLRs, the Mico gives you AES and both S/PDIF formats.As well as analogue outputs on balanced XLRs, the Mico gives you AES and both S/PDIF formats.

The second channel's rotary control is labelled Variphase and uses an all‑pass filter to provide a continuously variable phase-shift of up to 180 degrees. The idea is to allow phase alignment of one channel with the other (for example, a DI'd guitar with the signal from a mic in front of the cab), although it should be realised that 'phase alignment' isn't the same thing as 'time alignment' — I'll come back to that in a moment. Again, a bypass button is provided to disable the facility or check what effect it is having.

Tucked away between the two sets of channel controls are three mode LEDs which indicate the digital sample rate (44.1, 48, 96kHz or external). Oddly, the controls to configure the sample rate are located at the back of the unit.

The majority of the rear panel is taken up with five XLR connectors: two Combi XLRs for the mic/line inputs, two male XLRs for the balanced line-level outputs, and a further male XLR providing an AES3 digital output. This is supplemented with an RCA/phono socket and TOSlink optical port to provide S/PDIF digital outputs. All three can be used simultaneously if desired.

Alongside these output connectors is a BNC socket and a recessed six‑way DIP switch. The BNC accepts an external word clock (44.1 up to 192kHz) to drive the internal A‑D converter, while the DIP switches configure the unit's operation. The first two switches select the internal sample-rate options (44.1, 48 or 96kHz), while the last two determine whether the internal or external word clock is used, and add a 75Ω termination to the BNC socket. The middle two switches currently have no function... although I have an idea for one of them which I'll explain in a moment. The analogue‑to‑digital converter sounds very clean and detailed, with a noise floor around ‑95dBFS, and it sounds very smooth.

On Test

The Mico's controls are well laid out and logical to use, as is the rear-panel connectivity. Everything works as you would expect, and the only real niggle concerning the control layout is the rear-panel location of the sample-rate switches. That wouldn't be a problem if the unit is used on a desktop (as I suspect most would), but if you do choose to rackmount the thing, it would become a frustration if you need to select different rates for each job. There's no power on‑off switch — if the external power unit is plugged in, the Mico is powered — and the only indication that the unit is running is that one of the three sample-rate lights will be illuminated.

Thinking about rackmounting, I came up with a second possible issue. If you had two Mico preamps, perhaps mounted alongside each other on a rack shelf, you would want their digital outputs to be synchronised with each other to feed a digital recorder. But how? OK, both units will accept an external word-clock reference, but where would that come from? You could use an external master clock, but that's an additional expense and another unit to build into the rack. You might be able to generate a clock from the recorder, but chances are you'd get better jitter performance by using the Mico's internal clock. What would make life really easy and convenient is if the Mico's clock connector could be switched to either accept an external clock input, or provide its internal clock as an output. You could then link two Micos with a short BNC‑BNC lead, with one unit set to output its own clock, while the other is switched to accept the external clock (and terminate the line). Hey Presto! A pair of perfectly synchronised Mico's with a single lead and a flip of a DIP switch. I put the idea to Audient, and it was well received, although apparently it would take a significant PCB revision — so it won't be happening anytime soon.

Returning to slightly more real‑world scenarios, all of the basic preamp functions work perfectly. Phantom power is maintained above 46V, regardless of the current being drawn by the microphones, and the microphone input impedance of 1.8kΩ, while lower than some preamps, is perfectly acceptable. The high‑pass filter frequencies are well chosen and very useful too.

The HMX control widens the available sonic palette, enabling you to add a tube‑like quality to the signal from a solid‑state microphone.The HMX control widens the available sonic palette, enabling you to add a tube‑like quality to the signal from a solid‑state microphone.However, I found the gain control law on the original model suffered a 'dead zone' from about 11 o'clock through about 3 o'clock, in which the gain barely changed at all, followed by a mad rush of gain at the end — over 15dB in less than five degrees. The problem was essentially due to the characteristics of the potentiometer being used (which was different to those used in the consoles and rackmount preamps). After I discussed the issue with Dave Dearden, various alternative configurations were tried and tested before settling on the version described above, which has a slightly different gain structure but far better control linearity, and is more easily adjusted at high gain settings as a result. The only obvious differences between the original and revised models are the pad attenuation and the gain control scaling. The original had a 10dB pad, with a gain range starting at 6dB and reaching 30dB in the middle. The revised version has a 20dB pad, and the gain range starts at 18dB, with 33dB in the middle.

The HMX control is a very nice feature, which adds character in a subtle but musical way. The working range spans 'barely audible' to 'a bit more than usually necessary,' but is never completely overblown or grossly exaggerated. It works very well to liven up a DI guitar input, and adds some interesting character to vocals (both male and female), which will appeal greatly to those running home studios with a limited choice of microphones. You can create much of the sonic character of a good valve mic (or valve preamp) with this HMX control, and I liked it a lot.

I was initially more dubious about the Variphase control. It is not uncommon to record some sources with more than one microphone (acoustic guitars, snare drums, and so on), or with a microphone and a DI (acoustic and electric guitars, for example). The fundamental problem with this approach is that these two signals may not be time aligned because the different microphone distances from the source (or mic and DI) capture the sound at different times — and that can cause phase cancellations that colour the sound. This problem can be fixed if the material is recorded into a DAW, simply by sliding one track along the time‑line relative to the other, but it is time‑consuming and fiddly to do.

What Audient are offering with the Variphase control is not the ability to time‑align the two channels — there is no built‑in delay facility here — but to adjust the phase of one channel relative to the other. The control spans 180 degrees, and by switching in the polarity inversion you can access the 180 to 360 degree range too. It's an interesting effect — like a manual phase shifter, in fact — so by adjusting the control you can effectively decide which frequency components to align and emphasise, and which to sacrifice through partial cancellation. This approach enables you to compensate to some extent for the dominant phase cancellations that would otherwise colour the sound in an objectionable way. The facility doesn't replace true time‑alignment, but it may make accurate time‑alignment unnecessary. Whereas correct time‑alignment would bring all frequencies into phase alignment, this Variphase control only brings some frequencies into phase alignment (you choose which by adjusting the control), and in practice with DI and miked guitar I found I was able to get some very pleasing results, with nice colorations rather than nasty ones.

Righteous Revision?

With the gain-structure revisions, everything about the Mico is now excellent. The mic amps are quiet, clean, transparent and detailed, and a worthy step up from the average. With the provision of a DI input, the HMX feature, the Variphase facility, and a built-in, high-quality A‑D converter, I have to say that this is a bargain.  


Given the unique feature set of the Mico, there are no directly equivalent alternatives, although there are several good mic preamps at a similar price point. A favourite of mine is the DAV BG1 (or the BG1u for a rackmount alternative with a DI input), but this lacks the A‑D converter, as well as the HMX and Variphase features.

Published May 2009

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