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Crane Song Avocet

Stereo Monitor Controller
Published November 2005
By Hugh Robjohns

Crane Song AvocetPhoto: Mark Ewing

This monitor controller sets new standards for operational flexibility and sonic fidelity.

This new high-end monitor controller has been designed by Crane Song's mastermind, Dave Hill — someone with a reputation for fastidious attention to detail and for demanding the highest technical and sonic standards. While it is a widely held belief that passive monitoring controllers must be best, in practice the inputs and outputs need buffering to remain reliable and consistent in the real world of unknown impedances. Furthermore, it is extremely hard to derive mono from a stereo source passively without affecting the sound, so some electronics are inevitable in a versatile high-quality monitoring controller. However, these electronics clearly define the ultimate performance of the monitoring chain, so the Avocet's audio path employs just three all-discrete Class-A buffer amplifiers in each channel.

Level control is another difficult area to address. Ganged potentiometers often track poorly and can produce image shifts, while active circuitry such as voltage-controlled amplifiers, although affording very flexible control options, often introduce unacceptable levels of distortion. The ideal solution is usually passively switched attenuators, often arranged around an expensive rotary switch. However, the Avocet manages all of its level-control functions (and I/O selections) with sealed, high-quality relays and resistor chains. Clever logic control of these relays also allows some quite sophisticated additional features — regaining much of the flexibility of a VCA design without sacrificing audio purity.

The bulk of the Avocet's electronics are housed in a 2U rackmounting case, which is controlled remotely from a desktop unit connected with a 25-foot cable. The front panel of the rack unit features only a stereo headphone socket and a large green vintage-style power lamp, while the rear panel is crammed full of XLRs.

Inside the mainframe is a large circuit board covering the whole of the available floor area, with a long row of relays for the stepped volume attenuator across the front. Mounted above this all-analogue board is a smaller digital board, the role of which is described in the 'Digital Interfacing' box.

The circuit boards feature more than a dozen jumper connections which allow the user to tweak the operation of the Avocet to their own personal tastes.The circuit boards feature more than a dozen jumper connections which allow the user to tweak the operation of the Avocet to their own personal tastes.Photo: Mark Ewing

The Avocet is equipped with three stereo analogue inputs referred to as Analog 1, Analog 2, and Aux — all nominal +4dBu balanced XLRs with multi-turn trimmers to fine-tune the levels over a ±8dB range — and there are also three digital inputs feeding a high-grade D-A stage. Apparently, an internal link in the current Revision 4 version of the unit (the review model had older Revision 2 circuit boards) optionally provides the Analog 2 input with 14dB of additional gain to accommodate a nominal -10dBV semi-pro input, which is a useful facility.

The rear panel also carries three sets of +4dBu balanced stereo analogue outputs on XLRs, a second stereo quarter-inch headphone socket, an IEC mains inlet (with integral fuse holder), and two D-Sub connectors. One of these (Remote) is used to hook up the remote control panel, while the other (Accessory) provides various auxiliary functions such as external switching of talkback, solo, dim, and mute, as well as generating buffered outputs for the talkback mic, headphone output, and unbalanced mono and stereo outputs (the last being intended for an external metering system). There is also provision here for an additional remote-control signal which will be used in future for linking Avocet units when monitoring in surround.

Although missing from the review model, the Avocet is normally supplied with adjustable in-line XLR attenuators for use between the Avocet's outputs and the inputs to power amplifiers or active loudspeakers. They are intended to address the perennial problem that most amplifiers are far too sensitive for direct connection to nominal +4dBu line-level outputs. Rather than compromise the signal-noise performance of the Avocet's electronics, Crane Song supply adjustable attenuators which can be placed at the inputs of active speakers or power amps to reduce the signal level by up to 30dB.

The Remote Controller

The remote controller is a chunky metal 'brick', roughly 45 x 200 x 190mm (hwd), the rear panel of which carries just a D-Sub connector, to link with the mainframe, and a female XLR connector. The latter is for a talkback mic (there being no phantom power available), and the mic preamp's gain is adjusted via a multi-turn trimmer adjacent to the XLR.

The control buttons are laid out simply and logically in two long columns, with colour coding to help navigation. In the centre of the panel is a large green rotary encoder with a detented action and 24 green LEDs around the periphery to indicate the current level setting. The knob feels great to use — just the right size and weight to allow accurate click-by-click adjustments or a quick spin up or down with a flick of the wrist, causing the relays to chatter away in the main frame. The level setting is very clear, and although marked in 2dB steps the LEDs resolve to 1dB level changes simply by lighting adjacent LEDs to show the midway positions.

At the top of the panel is a high-resolution stereo bar-graph meter, spanning a 46dB range and calibrated with 0dB at the top — although it seems a little odd to have a digital-style meter in an analogue controller. An internal link configures the meter to show either the level of the selected input (default), or the output level from the Avocet.

The right-hand column of buttons selects the input source, talkback function, and an odd 16-bit monitoring mode. Only one input can be selected at a time, and there is no summing or mixing ability. When the headphone control mode (of which more in a moment) is active, the bottom three buttons in this column serve to configure the headphone monitoring source. There are three options: a feed of the Aux analogue input (intended to pass through a cue mix from a console, for example); a fixed-level feed of whatever input is selected to feed the main monitors; or the same main-monitor feed but taken after the mono, phase, and level controls. The talkback signal is mixed in with the headphone outputs, regardless of what source mode is selected.

If the solo mode is activated via the Accessory connector, then the currently selected input is overridden by either the Analog 1 or DAW inputs, according to the position of another internal jumper link. The idea of this feature is to allow complete integration between the Avocet and a mixing console, allowing soloed channels to appear on the monitoring automatically.

Signal Selection & Conditioning

The left-hand column of buttons determines the output destination (only one of the three can be active at any time) and the input signal conditioning (phase, mono, dim, and mute), and activate the headphone control mode. With the headphone mode selected the rotary encoder sets the headphone volume, which is totally independent of the main monitoring volume.

The mono monitoring mode can be configured with another internal link to send the derived mono signal just to the left speaker, or to both (the default is both). The Mute and Dim buttons are conveniently located at the bottom of the column where they are easy to reach, and the dim level can be adjusted by turning the volume control while in dim mode. The setting is then remembered, even after powering the unit down. The monitoring level when the talkback button is pressed can also be set independently of the normal dim level, which is a superbly useful feature.

Every button has an associated LED to indicate the current status, and the buttons all have a very positive mechanical click action. There are no obvious pops or splats on the monitored signal when operating any buttons — in fact the unit even turns on and off gracefully, without any nasty thumps. When powering up, all the settings are remembered, but the Mute button is sensibly always enabled.

A small trimmer on the rear panel of the remote control allows the talkback mic's gain setting to be adjusted.A small trimmer on the rear panel of the remote control allows the talkback mic's gain setting to be adjusted.Photo: Mark Ewing

Input Gain Trim Options

To me, one of the nicest functions is that input levels can be temporarily trimmed from the remote controller, but without messing up the precision gain structure dialled in using the rear-panel trimmers. This is essentially done by manipulating the logic data that controls the attenuator relays, so that the audio quality is not affected. The only downside is that you can't adjust the monitoring level beyond the physical attenuator range. For example, if you dial in a +10dB offset, and then crank the level up full, you are effectively asking the attenuator to provide 10dB above the maximum level, which isn't possible. In practice, though, this is very unlikely to become a concern.

The gain trim mode is activated simply by pressing the selected input's button a second time, whereupon the gain can be trimmed by up to 10dB in either direction using the large green knob (the latest models provide 0.5dB increments, earlier units 1dB steps). Tap the source button a third time and the operation returns to normal, but with the new gain offset remembered — even if the power is turned off. This gain trim mode is excellent for allowing different sources to be compared accurately at the same perceived level — for example the original and compressed versions of a track.

Using The Avocet

Connecting the Avocet is very intuitive and simple, as is the basic operation. Selecting sources and destinations, adjusting the monitoring level, and checking mono and phase compatibility are all very straightforward. The ability to set, and more importantly reset, the monitoring level within 1dB is excellent, and essential in a mastering environment. Likewise, the ability to set the headphone and dim levels independently, using the same control and in such an intuitive way, makes life very easy. The facilities to enable complete integration with a console are well thought out too, but don't compromise the stand-alone operation, or indeed the sound quality, in any way.

There are only a couple of niggles. The inability to select mono to one or both speakers from the remote panel is disappointing. Clearly, David Hill recognises the important distinction between these two monitoring modes, because the option is available via an internal link. Also disappointing is the lack of individual speaker muting. Often a problem can only be identified by listening to each channel separately. Bearing in mind the absence of separate speaker mutes, it is interesting to read in the Operator's Manual that there are plans afoot to enable the linking of multiple Avocets to form a surround monitoring controller. Being able to solo and mute individual channels is of paramount importance when working in surround, so it will be fascinating to see how this key functionality is provided.

The most important thing about a monitoring controller, though, is how it sounds, and in this there is no doubt whatever. The Avocet sounds totally transparent and I could not detect any quality changes whatsoever when inserting the unit into my monitoring chain. The analogue signal path is exemplary in every way, demonstrated by the fact that the tonality and stereo imaging don't change at all, regardless of the volume setting. Auditioning digital sources was equally impressive, and it quickly became clear that the internal D-A stage easily matched the quality of my favourite new reference, the Benchmark DAC1. The performance of the Benchmark is truly stunning (once it has warmed up), so for the Avocet to match its resolution is praise indeed, and particularly noteworthy given that both D-As employ a very similar technological approach.

The Avocet's headphone drivers use high-quality op amps and powerful LM3886 integrated power-amp chips rather than discrete Class-A circuitry, but even so I can't fault the performance at all. There is more than enough volume available, even for the most demanding of artists, and the design provides absolute clarity and neutrality, conveying accurate transients and powerful deep bass.

The Avocet is as near to the perfect monitor controller as I have yet found. Yes, it lacks individual speaker muting, the mono check mode is not as versatile as I would like, and the truncated 16-bit mode is, to be blunt, utterly pointless. But the plus points far outweigh these irritations. Excellent ergonomics, stunning audio quality, precision level adjustments, console integration flexibility, and the potential of an upgrade path for surround work all make a very convincing case indeed for the Avocet.

Digital Interfacing

The Avocet is equipped with three 24-bit digital inputs. Two are straightforward AES inputs able to accommodate a full range of sample rates from 32kHz to 192kHz, and labelled Digi 2 and Digi 3. The third is referred to as the DAW input, and this has three input modes selected by a switch on the back panel. One accepts a normal 'single-wire' AES input via an XLR, while another accommodates a conventional coaxial S/PDIF input via a phono socket. The third mode invokes a second XLR socket to partner the first so that dual-wire high-sample-rate AES signals can be connected.

The digital inputs do not need to be synchronous — the internal DAC simply locks to the selected source — and changing digital sources is fast, without any nasty bangs or crashes. Interestingly, the Avocet's D-A design employs a similar strategy to that used in the Benchmark DAC1 (which I reviewed back in SOS July 2005), oversampling the selected input to 192kHz and passing it through a sample-rate converter to help remove jitter before D-A conversion clocked by a local crystal. Internal links allow the oversampling to be disabled or limited to 96kHz, if required, although the designer claims the 192kHz option sounds best.

The selected digital input is received through a pair of Cirrus Logic CS8416 chips, before being routed to a Burr Brown SRC4492 sample rate converter, and then on to a Cirrus Logic CS4398 — the company's latest 192kHz flagship device. A local crystal is used to clock the output of the SRC and the D-A chip independently of the incoming digital signal's embedded clock. Among the impressive specifications and features of the D-A chip is the choice of two oversampling interpolation filter designs. This function has been made available through another link on the digital board, with the default being a 'slow roll-off' setting. A 'fast roll-off ' mode is also available, and may be more appropriate if source oversampling is not employed.

One rather odd function of the D-A board is operated by a button on the Avocet remote control labelled 16-bit. This truncates the selected digital input to 16 bits and is apparently 'for checking what 16 bits sounds like'. I can't imagine any situation where one might want to do this — at least not without applying proper dithering as part of the process. Not a button that will see much use, I suspect!

Published November 2005