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DAV Electronics BG4

Compressor/Limiter
By Hugh Robjohns

DAV Electronics BG4Photo: Mark Ewing

A premium compression sound lurks behind the no-frills cosmetics of this new Broadhurst Gardens processor.

The DAV Electronics range of Broadhurst Gardens audio processors has already made a couple of appearances in the pages of Sound On Sound. The BG1 dual-channel mic preamp was reviewed in SOS December 2003 and the BG5 channel strip appeared in August 2004. The latter unit combined elements from several BG processors including the BG1 preamp, BG3 equaliser, and BG4 limiter, and the last of these is now the subject of this review.

The Decca Recording Company was, at one time, based at Broadhurst Gardens in West Hampstead, where Mick Hinton — the owner and designer of DAV Electronics — was an employee. Mick has used the prestigious address on the BG product range to signify the heritage of these products, which are based directly on original circuit designs and principles used by Decca (albeit updated to use more modern components). A common characteristic of the BG processors is a slightly 'larger than life' sound quality, and all the units boast flawless technical specifications and switched control functions to enable all settings to be recreated precisely.

As I have said in previous BG reviews, the overriding impression of all of the BG units is of equipment designed to do the required job very well, but without being flash or equipped with 'marketing features'. These are products undoubtedly aimed at high-end acoustic music applications and for serious engineers — and the company already claims such high-profile users as the former Sony/Whitfield Street Studios, The Decca Record Company, Townhouse Studios, Metropolis Studios, the Master Room, and many others.

Construction

The BG4 dual compressor/limiter is contained in a 1U rackmounting box with a modest 250mm case depth behind the rack ears. As with other BG products, the case itself is unpainted steel, with a matt-black front panel carrying white and light-blue control legends. Each unit is constructed by hand, but to high standards, although a peek inside the box does betray a slightly old-fashioned approach to wiring and fabrication methods. Also, just as with the BG5, there was no handbook supplied.

The rear-panel connections comprise two pairs of XLR connectors for the line-level stereo inputs and outputs, plus an IEC mains inlet which incorporates a fuse holder and mains voltage selector. Internally, the majority of the electronics are housed on a single large PCB, with two smaller daughterboards mounted vertically behind the front-panel controls. All components are standard sized — no surface-mount technology here — and everything is well spaced out to avoid unwanted interaction and crosstalk.

The circuit design is apparently derived from bespoke equipment developed and used by Decca from the late '70s, and which acquired a fine reputation for having unusually low noise and distortion. In this updated version, a CA3083 high-current transistor array is employed in each channel as the basis of the voltage-controlled amplifier (VCA), while the supporting circuitry for each channel includes a CA3140, an LM1458, and a TL072 — all three being op amps with distinct properties obviously carefully chosen for their specific role. The audio path and output buffering are handled with four OP275 high-quality op amps, and the ubiquitous SSM2017 provides the balanced input (this chip being socketed for easy replacement).

Controls

The controls are arranged in separate but identical groups for each channel, enabling either independent two-channel or linked-stereo operation, as required. Each channel is equipped with a small black button (and associated red LED) to select the operating mode (compression or limiting), and a quintet of rotary switches. The compression ratio switches between a nominal 2:1 (apparently with a soft-knee characteristic) for the compression mode and 10:1 for the limiter mode. The limiter isn't a 'brick wall' design, and when driven by loud transients there is some overshoot. While this may have been acceptable thirty years ago in the heyday of analogue recording, digital recorders are considerably less tolerant, so it would be wise to set the limiter threshold to leave a few decibels of headroom, thereby avoiding possible A-D overloads.

DAV Electronics BG4Photo: Mark Ewing

The dynamics section itself is manipulated by three rotary controls. Gain Make Up is the first and provides up to 16dB of gain in 2dB increments. The second control adjusts the recovery time between 0.3s, 1s, or 3s, while the third sets the Threshold from zero to +20dBu in twelve steps. The first increment is from 0dBu to +1dBu, followed by 2dB intervals up to +19dB, with +20dBu as the final position.

When I encountered this threshold control on the BG5 Channel unit I initially assumed from the panel legend that the scale went from -20dBu to 0dBu — meaning that you would have to turn the control anti-clockwise to lower the threshold and thus increase the compression. In fact, the control is scaled from +20dBu to 0dBu, and you have to turn it clockwise to lower the threshold and increase the compression. With hindsight, the latter makes perfect sense, but I was thrown completely the first time I used it!

The last two controls weren't incorporated in the BG5 channel's incarnation of this design. The first selects a side-chain filter type (low pass, high pass, or wide band), and the second determines the turnover frequency (0.1, 0.25, 0.5, 1.0, 2.5, or 5.0kHz). The final element is a green bar-graph meter to show the gain reduction — no input or output level metering is provided at all. The meter is scaled to show a maximum of 12dB of gain reduction with 1dB increments to 4dB, and 2dB increments thereafter.

In the centre of the unit are two black buttons (with associated red LEDs again) that affect both channels — although the panel layout suggests they relate to the right-hand channel. The first activates the stereo-link mode, which couples the side-chains of both channels to ensure stereo images remain stable during compression or limiting. The second switch is labelled Limiters Off, and this disables the VCAs — a kind of bypass mode, if you like. The fact that this affects both channels simultaneously makes it slightly harder to use this machine as a pair of independent dynamics processors, but in practice I doubt this will be a concern in the applications for which it is likely to be used.

Down The Garden Path

Since the dynamics section of the BG5 Channel Unit was based closely on the BG4, many of my earlier comments regarding the operational 'quirks' of the BG5's dynamics section apply equally here as well. In particular, the Gain Make Up control is always active, even when the compressor/limiter processing is switched out with the Limiters Off button. This makes it harder than it really should be to assess the appropriateness of the dynamics processing, since the unprocessed signal will be louder than the processed signal.

With its fixed 2:1 ratio and soft-knee characteristic, the compressor is both subtle and transparent, and ideally suited to the gentle control of complex instruments or mixes without it becoming heavy-handed. The simple three-position release-time control seems entirely adequate, with well-chosen options, although I suspect a degree of automatic programme-related adjustment is also going on here.

Personally, I'm not a big fan of using compression while recording. While it was a necessary evil in the analogue days to maximise the relatively poor signal-to-noise ratio of magnetic tape, modern 24-bit digital systems don't require such cosseting. To my way of thinking it makes far more sense these days — at least when multitracking — to record everything flat and unprocessed (other than correcting obviously unwanted LF rumbles and so forth), and then to determine the necessary dynamic control (if any) afterwards in the context of the complete mix. The BG4 is ideally suited to such post-production dynamic control, either on individual instrument tracks, across stereo submixes, or even across the entire mix.

However, if mixing straight to stereo on location, or if mixing a live concert with potentially unpredictable levels on some channels, the BG4's ease of use and inherently transparent nature could prove invaluable. Subtlety and gentleness is the name of the game here though — don't expect to find any rock & roll attitude here, no matter how hard you drive it!

The limiter affords a useful degree of peak control — again without trampling all over the sound character — but I detected some overshoot on loud, brief transients. Consequently, I would suggest taking the threshold calibration as an optimistic guide rather than as an absolute protection level. When used with an A-D calibrated for 0dBFS at +18dBu, I obtained reliable results from the limiter with the threshold set to 15 (a setting of 17 produced occasional digital overloads).

Although there is no external access to the dynamics side-chain, the unusual side-chain equaliser proved surprisingly effective in a variety of situations. In the wide-band mode, the dynamic processing reacts to all frequencies equally — as with most compressors. However, when necessary, the side-chain can be desensitised to extreme high or low frequencies by choosing the low-pass or high-pass filter modes, respectively, and tweaking the turnover frequency control as required. This feature can be used to help prevent the dynamics processing from reacting excessively to, for example, bass drums or cymbal clashes when these are present within a composite mix.

Overall, then, this a high-quality compressor/limiter ideally suited to delicate serious music applications and for use by the kind of engineer who still wears tweed jackets when mixing — that'll be me then... The BG4 doesn't draw attention to itself, either in terms of its sound character or its aesthetic design, but that is sometimes precisely what is needed.

Pros

  • The Decca sound character is evident.
  • Gentle and transparent dynamic control.
  • Simple switched controls enable easy operation and accurate resetting.
  • Flexible side-chain EQ facilities.

Cons

  • No independent channel bypass.
  • Gain Make Up control is always active, even when compressor is bypassed.
  • No external side-chain access.
  • No handbook supplied with review model — again!

Summary

A simple dual-channel compressor/limiter derived from bespoke Decca designs dating back to the 1970s and carefully optimised for serious acoustic music applications.

information

£1750 including VAT.

KMR Audio +44 (0)20 8445 2446.

sales@kmraudio.com

www.kmraudio.com

www.davelectronics.com

Published February 2005