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Focusrite Octopre

Eight-channel Mic Preamp
By Sam Inglis

FOCUSRITE OCTOPRE

Focusrite's Platinum series expands further with an unusually affordable eight-channel mic preamp boasting dynamics on every channel.

The market for affordable multi-channel soundcards seems to have exploded over the last couple of years. A couple of hundred pounds will now buy you eight channels' worth of pristine 24-bit hardware and leave change for the bus-ride home. Frustratingly, though, the situation is not so rosy when it comes to choosing a mic preamp to use with your new soundcard. There's any number of high-quality single-channel units available, while the likes of Mackie's VLZ Pro series of mixers include multiple preamps of excellent quality — but perversely, if you want eight decent preamps in a rack unit without faders and knobs, you're hard pushed to find anything under the £1000 mark.

The latest addition to Focusrite's Platinum series aims to improve matters for the computer-based musician who needs to record multiple sources without breaking the bank. At £749, the Octopre occupies the same price point as the larger VLZ Pro mixers, and promises a feature set that puts many of its pricier competitors to shame. Its eight channels of preamplification each offer not only independently switched phantom power but compression and limiting too, while the first two 'Super Channels' can also be used as DIs for electric guitars and other instruments. Add to this balanced I/O throughout and two competitively priced 24-bit/96kHz digital output options, and the Octopre looks attractive.

Up Close

Like some of the other units in the Platinum range, the Octopre is manufactured in China, and it's pretty solidly built. Although only 1U high, it's quite deep, at 12.75 inches, and weighs a good 4.25kg, so the inclusion of rear rack ears as well as front-panel ones is sensible. The front panel itself is formed from a hefty and attractive piece of brushed aluminium, and the control layout is commendably clear given how many knobs, buttons and LEDs are crammed on to it. Incidentally, as you would expect from a device containing Class A circuitry, the Octopre gets quite hot when in use, so the manual sensibly recommends that you leave a blank 1U panel above it in a rack.

The Octopre spreads its tentacles to the world via eight XLR mic inputs and two Tascam D-Sub connectors. With the appropriate looms in place (not supplied with the unit), these provide eight balanced line outputs and eight balanced line inputs. Each channel can be switched individually between mic and line inputs — and, in the case of channels one and two, the instrument jacks on the front panel.

Not every mic preamp also includes line inputs, and not everyone will find a use for them. I was initially wondering if they could be used to provide insert points when using the Octopre's digital output options, but this is not possible (unless you simply want to treat the Octopre as a four-channel unit, taking the outputs from the first four channels into the line inputs on channels five to eight). For those who have the digital board fitted, however, the line inputs provide a way to record synths and samplers to digital-only soundcards, or simply to bypass a soundcard's own A-D converters. Focusrite also suggest that you might use them to employ the Octopre's dynamics on line-level sources or at mixdown.

Each channel features two knobs, which set the input level and the amount of dynamic processing to be applied. Between 0 and 60dB of gain is available for microphones, while when a channel's line input is selected the range of the Level control is -10 to +10dB. The presence of an input signal is acknowledged by a green Signal LED, while excessive levels are indicated by a red O/L LED. Every channel also has a button to switch +48V phantom power on or off, and another to engage a high-pass filter turning over at 75Hz. The first two 'Super Channels' add switches to invert the phase of the input signal, along with additional buttons to select the front-panel Instrument inputs. These are on high-impedance quarter-inch jacks, making them suitable for electric guitars and the like, and offer between +4 and +34dB of gain.

On the review unit, the two Super Channels and the even-numbered channels (ie. those on the bottom row) were whisper-quiet even with the gain fully up. Channels three, five and seven did produce audible hiss at extreme gain settings, but I must add that we are talking extreme here: it would only be a problem when using a ribbon or low-output dynamic mic on a quiet or distant source. In use, the mic amps sounded excellent at both low and high gain settings, and DI'ing an electric guitar via one of the Super Channels also gave very good results.

Both XLR mic and line inputs can be accommodated by the Octopre's eight channels, and the first two channels also feature a front-panel instrument jack socket. The review model came fitted with the ADAT plus S/PDIF or AES-EBU option.Both XLR mic and line inputs can be accommodated by the Octopre's eight channels, and the first two channels also feature a front-panel instrument jack socket. The review model came fitted with the ADAT plus S/PDIF or AES-EBU option.

Dynamics

The idea of fitting not only eight preamps but eight compressors and limiters into a 1U rack unit provokes the obvious question 'How did they fit all the controls on?' The answer, not surprisingly, is 'They didn't.' Each channel has only one knob devoted to controlling its dynamics. At its anti-clockwise extreme, helpfully marked 'Off', both the compressor and limiter are out of circuit. Rotate it slightly, and it clicks to indicate that the limiter has been switched in; turn it further towards the legend 'More', and you begin to add more in the way of audible compression as well.

The manual, which is sketchy at best, has very little to say about how the channel dynamics work, but applying the thumbscrews to Focusrite's technical staff yielded some more information. The limiter that is engaged when you first switch in the compression is a fast device designed to catch peaks before they can cause clipping at the A-D stage. This is always in circuit unless the Dynamics knob is in the 'Off' position, but turning the knob further clockwise effectively introduces an additional compression circuit before the limiter. This, apparently, is a new two-band optical design with a crossover point fixed around 2kHz: high frequencies are controlled using fast attack and release times, while the low end is treated in a more relaxed fashion, for a warmer, distortion-free result. Turning the Dynamics knob further to the right affects both the threshold and ratio, but there's no control over the time constants. Nor is there any way of stereo-linking any two channels' dynamics.

I can see Octopre owners making a good deal of use of its dynamics. Limiting is a handy safety measure on any preamp circuit feeding an A-D converter, while analogue compression often has a different character to that of digital dynamics processing, and the Octopre's compressors are more versatile than their single control would suggest. At low settings they are relatively transparent and would be useful for ironing out uneven playing on, say, acoustic guitar; more extreme settings are flattering on vocals and add a healthy 'chunk' to clean electric guitars.

It should be borne in mind that the limiter is an analogue circuit, and that it doesn't always protect even the Octopre's own A-Ds from clipping. I tested the unit with a Digidesign 888/24 I/O unit, connected via AES-EBU; with an electric guitar feeding one of the Octopre's Instrument inputs, I set it up so that the loudest strums just tickled the limiter. Even when its red LED lit to show that limiting was taking place, the 'over' warnings on Pro Tools' channel input meters still illuminated, indicating that at least three consecutive peak-rate samples had been received. Sometimes Pro Tools even reported overs with transients that did not light the 'Lim' LED. The resulting clipped transients were usually unnoticeable, however, and turning the Dynamics knob further to introduce a modest amount of compression was always enough to stop the clipping.

In Conclusion

With the Octopre, Focusrite have taken advantage of a gap in the market, and the unit has surprisingly little competition. PreSonus's DigiMax is probably the closest in terms of specification: it too provides eight channels of mic preamplification, each having its own compressor/limiter, with the first two channels offering instrument inputs and phase-reverse switches. Digital I/O is fitted as standard but is restricted to rates of 48kHz and below, and consists of a single ADAT output, word clock in and out, and eight more outputs switchable between AES and S/PDIF formats.

However, at £1763 the DigiMax is a lot more expensive than the Octopre, as are products such as CLM Dynamics' DB8000S and Oram's Octasonic. PreSonus also make the Digimax LT, which is closer in price at £1116, but it lacks its big brother's dynamics, instrument inputs and AES-format digital I/O. I know of no other product that offers the same balance of features at the Octopre's price, and it must be considered excellent value for money. As a front end for a digital recording system it's much cheaper than comparable rackmounting alternatives, while compared with the option of using a budget analogue mixer for this purpose it offers substantial advantages in terms of size, expandability, and (in many cases) sound quality, as well as offering built-in dynamics and A-D conversion. I expect Focusrite to sell bucketloads of them.

Octopre Digital Options

Two digital output options are available for the Octopre. Both provide a BNC word clock input and two ADAT optical outputs, while the second, which was fitted to the review unit, adds a nine-pin D-Sub connector supplying the Octopre's eight outputs in electrical digital format. A rear-panel button switches these outputs globally between AES-EBU and S/PDIF voltage levels.

At 44.1 and 48kHz, the two ADAT outputs are identical, each carrying all eight channels. However, the Octopre's A-D converters can also spit out 88.2 and 96kHz signals, which is why it's equipped with two optical outputs: the lightpipe bandwidth is insufficient for eight channels of 96kHz audio, so at high sample rates they are split across the two. The AES digital outputs can also supply eight channels of 96kHz audio, using the 'double fast' protocol. Both ADAT ports can be used simultaneously with the analogue outputs and the AES or S/PDIF outputs, if fitted, which will please those needing to run a backup multitrack recorder at live gigs and the like.

Two front-panel buttons are associated with the digital options: the first determines whether the signals are output at 24-bit or dithered to 20- or 16-bit, while the second selects the output sample rate (assuming the Octopre is not being clocked externally). Annoyingly, this defaults to 88.2kHz every time you power the unit up, regardless of the sample rate you used last time.

At £149 and £249 respectively, both of the Octopre's digital options offer good value for money. There are now innumerable soundcards available with ADAT interfaces, so many Octopre buyers will be happy with the cheaper of the two, while the second digital board, with its AES-EBU I/O, seems to be targeted primarily at Pro Tools users. Grumbling about the A-D converters of Digidesign's 888 interfaces has become a popular pastime, but existing alternatives from the likes of Apogee and Prism have been costly, so the availability of a high-quality, affordable front end will be welcomed. Given that Focusrite have forged a close alliance with Digidesign, thanks to jointly designed products such as the M Box and Control 24, the Octopre is sure to be seen as an 'official' lower-cost counterpart to Digi's own forthcoming high-end eight-channel preamp, the Pre.

I was interested to see if I could discern any difference between the A-D converters on my 888/24 and those of the Octopre, so I fed channel eight's digital output into one of the 888's AES sockets and its analogue out into an 888 analogue input, having calibrated the latter to match the levels within Pro Tools. I then recorded sources via both paths simultaneously so that I could A/B the results.

On typical close-miked sources, such as acoustic guitars, I couldn't reliably hear any difference in terms of frequency balance or clarity between the two, and certainly wouldn't have been able to tell them apart in a blind test. I was more surprised by the results I got when I recorded silence and very quiet signals through both units, as these tests revealed that the Octopre's converters were actually noisier than the 888/24's. Using Nuendo's Statistics function to analyse recordings made through the Octopre's line input at unity gain, I measured background noise figures of around -110dB for the 888, compared with -105dB for the Octopre's digital output. This, however, is still a decent result, and will better the performance of many soundcard A-D converters and some other Digidesign interfaces. It should also be more than enough to accommodate the dynamic range of the analogue circuitry in the Octopre, which is what matters! I asked Focusrite about the results, and they claimed that the digital board in the review unit was an early production model; they apparently measure the board's dynamic range as 110dB.

Pros

  • Impressive and versatile feature set, with dynamics on every channel.
  • Well-conceived digital options can output all eight channels at up to 96kHz.
  • Clear and obvious control layout.
  • Good value for money.

Cons

  • The manual could be a lot more informative.
  • No control over compressor attack or release times.

Summary

The current gap in the market for affordable multi-channel mic preamps has given Focusrite an opportunity and they've grasped it with both hands. The Octopre makes an excellent companion to almost any digital recording system.

information

Octopre £749; ADAT digital option £149; AES-EBU, S/PDIF and ADAT option £249. Prices include VAT.

Focusrite +44 (0)1494 462246.

www.focusrite.com

Published September 2002