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DACS Eightch

8-channel Volume Controller
By Hugh Robjohns

DACS Eightch

Simple, functional and attractively priced. What more do you need from a monitor controller?

The DACS Eightch (pronounced 'H') is a simple, eight-channel, VCA-based volume controller. It is intended to provide a very straightforward means of controlling monitoring speaker levels in a surround sound system, without duplicating a host of facilities that are already built in to your favourite DAW (like downmixing, speaker muting, and so on).

The Eightch, which at around 2.5kg is heavier than it appears, is housed in a shallow, 1U rackmount case that extends about 150mm behind the rack ears. The front panel is finished in an attractive and glossy dark-blue, with white and light-blue labelling.

Keeping Things Simple

What we have here is as simple as it looks, but it is built with the high-quality components and design attention for which DACS are renowned. The front panel has a mains on/off switch, a large, rotary volume control, and eight multi-turn trimmers, which are used to fine-tune the balance between channels. The rear panel has two rows of eight balanced, quarter-inch TRS jack sockets (for the line-level inputs and outputs), and a mains IEC inlet with integral fuse and voltage selector. That's it. The review model came without a manual (there isn't anything on the web site either) but even my dog could figure out how to make this box work!

Internally, the unit makes use of the THAT Ingenius balanced line receiver chips, which were developed by Bill Whitlock of Jensen Transformers. The advantage of these chips is that they use a clever bootstrapping technique to maximise the common-mode rejection, even with poorly balanced sources (much like good-quality transformers). They also provide a wider bandwidth and lower distortion than is easily achievable with conventional transformers, and they are smaller and considerably less expensive.

Pure and simple: the Eightch offers a good, clean signal path, with eight inputs and outputs on balanced TRS jacks at the rear, and a single volume control on the front.Pure and simple: the Eightch offers a good, clean signal path, with eight inputs and outputs on balanced TRS jacks at the rear, and a single volume control on the front.Photo: Mike CameronThe level-control stage is performed with more THAT chips — this time, the well-known 2181A VCA devices. These are individually trimmed by DACS during the burn-in phase at the factory, to ensure accurate tracking between channels. They are also temperature-sensitive and so some thought should be given to the installation of the Eightch — in other words, you should avoid placing it close to other hot-running equipment. The tracking is most accurate towards the top of the volume control's range, where it is claimed to be better than 0.1dB, reducing to about 0.5dB by -40dB of attenuation, and 1dB below that, which is a fine level of performance.

The manufacturer's notes recommend setting the monitoring gain structure through the box, so that the Eightch's volume control is at its maximum for the highest likely monitoring level. This optimises the channel-tracking accuracy, and even operating at more normal listening levels — say, 20dB down on maximum — the tracking will still be within 0.2dB, which is excellent. To adjust the gain structure, you could tweak the input sensitivity of your amplifiers or active monitor speakers, as necessary, or use the front-panel trimmers — although the latter only provide a modest trim range of ±6dB.

Finally, the outputs are driven by Analog Devices SSM 2142 line-driver ICs, which also provide some transformer-like functionality. The output level doesn't drop 6dB if connected to an unbalanced input device.

Performance

The Eightch is radically different from most other (if not all) multi-channel surround monitoring controllers that are currently available, in that it only provides level control. But if that suits your purposes — and in a majority of home studios it probably will — then, on the face of it, this is a cost-effective unit. Sonically, I can't fault the Eightch: it sounds clean and open, has a low noise-floor, and plenty of headroom, and completely negligible crosstalk between channels. However, being the fussy old codger that I am, I do have some reservations, all of which are of a practical nature.

The first is that, ideally, you need to place the unit so that the volume knob is within easy reach, and not all desks or workstations allow that. If you are old-fashioned enough to work at a console, or simply don't have racking in front of your working position, what you really need is a small, convenient remote-control box with a volume knob on it, that can be placed somewhere accessible. Sadly, the Eightch doesn't cater for one, even though the use of VCAs makes it a relatively simple facility to provide. Apparently, though, it is on the wish list for a future version.

My next concern is that of level-matching. A range of ±6dB is fine if the entire monitoring chain is closely matched, and you are simply tweaking gains for room matching of monitoring levels. However, at the home-studio level the chances are that the centre and rear speakers (and/or their amplifiers) will be of different designs and manufacturers than the main left and right channels. In this case, 6dB of trim may well not be enough, and I'd have preferred to see ±15dB as a minimum working range.

Finally, and most seriously of all, the volume knob is hugely non-linear. The bottom half of the range is entirely useless, and most of the level-controlling action happens in the top 20 degrees of rotation. This makes the control unpleasantly sensitive and difficult to use. Apparently, this issue is known to the boffins at DACS, and a new design is on the drawing table that includes a microprocessor to provide a more useful mapping between rotary control angle and VCA output volume. But this seems serious overkill to me; plenty of other manufacturers use these VCAs, apparently without the same control linearity problems. If the control-knob law problem can't be overcome easily, perhaps a quick and dirty solution would be to replace the continuous volume knob with a decent rotary switch, and then work out a range of fixed resistor values for each setting, to give a more linear and proportional range of volumes. The switch approach would also make it far easier to establish repeatable, known monitoring volumes, which is most important when mixing and mastering!

The bottom line is that the Eightch is a nice idea, which is well-priced and offers good sonic performance, but it is let down by some disappointing practical issues and limitations, so it may be a good idea to try before you buy. 

Alternatives

Most monitor controllers offer far more functionality, such as input-source selection, channel muting/soloing and downmixing, but at this end of the price range there is very little competition. The closest device I can think of in terms of price is the Coleman SR5.1 MkII, which is slightly more expensive but offers channel muting and downmixing and uses a rotary switch for accurately matched level control (with a more usable control law).

Pros

  • High-quality electronics.
  • Simple functionality at an attractive price.
  • Accurate channel tracking.

Cons

  • Very strange volume control law.
  • Limited channel trim range.
  • No remote control option.

Summary

An absolutely no-frills multi-channel volume controller, using hand-trimmed VCAs for accurate channel-matching.

information

£646.25 including VAT.

DACS +44 (0)191 438 2500.

+44 (0)191 438 2511.

info@dacs-audio.com

www.dacs-audio.com

Published September 2007