M Audio Octane

Mic Preamp & A-D Converter

Published in SOS September 2004
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Reviews : Preamp

Photos: Mark Ewing

The M Audio Octane seems to be one of a growing range of products aimed at extending the I/O capability of any digital recording system that happens to have ADAT optical connectors built in. Very simply, this 2U box houses eight mic/line preamps that feed both individual analogue outputs and an ADAT digital lightpipe connector, the latter via onboard 24-bit converters. There are many hardware and software DAWs with ADAT input capability, so the idea is that you can turn that unused ADAT port into eight more useful analogue input feeds. Unlike the Behringer ADA8000, which offers similar functionality (but without the separate analogue outs), the M Audio Octane is strictly a one-way device — the unit won't give you extra outputs as well. However, most people now mix within their DAWs, so that's probably not a serious restriction for the majority of potential buyers.

Flexible Front-panel Design

The Octane is housed in a very stylish 2U rack case with a heat sink on the back, and draws power from the included 18V AC mains adaptor. A power switch and LED are located on the front panel, but of course any PSU of this type should be unplugged or turned off when the unit is being left idle for long periods. Phantom power is switchable onto the mic inputs in two groups of four via buttons to the right of the front panel, and this supplies a full 48V. All connections other than two instrument jacks are on the rear panel, and all the controls are on the front, which keeps everything very tidy.

To offer the maximum flexibility without adding to the cost, only channels one and two have the aforementioned front panel instrument input as well as a gain control for that input. The main mic input controls operate similarly across all the channels; there's a gain control, a 20dB pad button, and a three-segment LED level meter, where the top LED indicates the onset of clipping. Channel one also has a switchable low-cut filter, and all the even-numbered channels have phase-reverse buttons.

Channels seven and eight are special cases because, in addition to functioning as regular mic/line channels, they can be switched to decode the feeds from an M&S microphone pair into stereo via an internal sum-and-difference matrix. When M&S decoding is switched in, the Width control may be used to adjust the width of the stereo image. These two channels are set up so that the Middle signal feeds channel seven and the Sides signal channel eight. This is a nice feature if you happen to have a figure-of-eight mic you can use, because recording in M&S stereo can produce excellent results.

Because the conventional ADAT format was designed to operate at 44.1kHz or 48kHz, those are the only two sample rates supported here. A rotary selector switch chooses one of the two internal sample rates or external clock (via the rear-panel word-clock BNC socket) and a green LED shows when the unit is locked to an external clock. Both word-clock ins and outs are provided, but there's no provision to sync to S/PDIF or ADAT inputs, as there are no digital inputs on this unit. That's no problem if your DAW interface can provide word clock, but it means you have to use the Octane as the master clock for your system if you don't have a source of word clock to feed it. However, if you're using the Octane as your main high-quality input device, then running it as your clock master may well be the best option for minimising clocking jitter anyway.

What Is M&S Miking? The M&S mic technique is an interesting way of recording in stereo. A cardioid mic is pointed directly at the centre of the sound source, and a figure-of-eight mic is set up in a coincident configuration so that its null points in the same direction. The cardioid mic provides the Middle signal, essentially just a mono recording of the sound source. The figure-of-eight mic collects the Sides signal, which can be combined with the Middle signal to create a stereo signal. However, combining the Middle and Sides signals to create traditional stereo requires a bit of extra routing and the use of a mixer with phase-inversion facilities, so M Audio have made things simpler here by including the necessary M&S matrixing within the Octane. This means that you can set up an M&S rig, plug the two mics into channels seven and eight, and then simply hit the M&S Matrix button to output a normal stereo signal for recording purposes. The reason why many engineers go to the trouble of recording M&S stereo is that it has some practical advantages. Firstly, by adjusting the levels of the Middle and Sides signals, you can adjust the stereo width of the combined stereo signal, which can be very useful in some circumstances — the Octane's Width control works in this way. Also, when M&S stereo recordings are collapsed to mono, the Sides signal completely cancels itself out, leaving the Middle signal intact — other stereo recording techniques can be compromised by phase cancellations when collapsed to mono. Mike Senior

Connections

The rear panel has balanced XLR inputs on all eight channels, and in conjunction with the pad switches these can also accommodate line levels. However, you must be very careful not to apply phantom power to any inputs that have line sources connected via the XLR. A safer bet is to use the rear-panel TRS A-D Line Input jacks, where inserting a jack plug overrides the XLR input. The instrument inputs on channels one and two also override the XLRs and have a very high input impedance of 3M(omega) so as not to load passive guitar pickups.

Each channel also has a direct analogue output, again on a TRS jack, that may be used balanced or unbalanced. Plugging into these doesn't disconnect anything, so they can be used as regular outputs to feed some other analogue system or they can be used in combination with the inputs to provide semi-normalised insert points. Of course if you do decide to connect the line inputs to a patchbay, the XLR inputs will be bypassed, so you need to plan your wiring needs in advance and ideally allow easy access to the rear panel for when those inevitable non-standard jobs come along. Both the word-clock connectors are standard BNC sockets.

High-octane Performance?

The success of a product like the Octane relies largely on the quality of its mic preamps. These provide up to 50dB of gain, which is a little short of the 60dB provided by most preamps and consoles, but it's more than enough for capacitor mics, DI boxes, or dynamic mics used up close. The frequency response is flat from 20Hz to 20kHz, within a little over a tenth of a decibel, and the signal-to-noise ratio is 133dBA at a mid-gain setting. The balanced line inputs have a fairly standard 20k(omega) input impedance, and the line output impedance is 600(omega). Channel one's bass-cut switch has a 12dB/octave slope and an 80Hz cutoff frequency, which is useful if your mic doesn't have a roll-off switch and there's some low-frequency background noise you'd like to get rid of. As with most such equipment, there is plenty of headroom to accommodate the peak levels needed by digital systems.

The Octane was checked out using my MOTU 828 audio interface, with the Octane used as the master clock. After setting my MOTU to external sync, the Octane was recognised without any fuss and it functioned perfectly. The instrument inputs checked out fine and these were also good for recording the outputs from my Line 6 PodXT, so they seem to be able to handle modest line levels. Other than the limitation of 50dB of gain, the mic amps worked very well, imparting no obvious coloration to the sound. However, they do suffer from a problem I've come across on several other mic amps in recent weeks and that is that the gain is all bunched together at the top of the control's range, and I think I'm right in saying that a special pot is needed to avoid this — regular pots with a logarithmic law just don't do what you need. Because you need to be working in the high gain range for most studio vocals through a typical capacitor mic, setting the gain with any precision is trickier here than it should be.

The M Audio Octane is a straightforward and well-thought-out product that has no obvious shortcomings other than perhaps its less than generous maximum preamp gain of 50dB and the bunched-up gain-control law. It is cheaper than the nearest equivalent Presonus model, but still over twice the UK price of Behringer's offering in this area. However, the Octane does have the M&S decoding matrix and the instrument inputs on the front panel, as well as analogue inserts/outputs, so it offers a comprehensive solution to most people's input expansion needs. It will of course also work with a hardware ADAT recorder, which makes it a useful front end for location recording.