Swissonic's new high-quality preamp and A-D converter caters for 24‑bit/96kHz digital audio in AES-EBU and ADAT formats.
The origins of Swissonic can be traced back to a time when its owner was making inexpensive converters to upgrade MOTU and Emagic equipment. These devices — the original AD24 and DA24 converters — also carried the Sonorus badge for a while until the current Swissonic company was established. The original designs are still being manufactured, along with newer, high-sample-rate versions, but the topic of this review is the latest addition to the range, an eight-channel converter with comprehensive and high-quality preamplification built in.
The Swissonic AD8 is a well thought out, eight-channel preamp with onboard high-resolution converters and multi-channel digital interfacing, in a well-engineered, high-quality and very compact package. Each of the eight input channels is equipped identically, with buttons to select a high-impedance input mode, a 20dB input pad, polarity inversion, a 90Hz high-pass filter, and phantom power (with a bright blue LED indicator). The input gain is controlled by a small but nicely weighted knob, spanning a modest 40dB gain range.
An array of 64 LEDs provides bar-graph metering for each of the eight digital output channels, covering the dynamic range from 0 to -60dBFS in eight steps. To the right of the meter array are a further five buttons used to configure the unit (with associated blue LEDs grouped in pairs, and extremely bright). The first button activates output dithering (on all eight channels simultaneously), the threshold of which is determined by the second button to 16 or 20 bits. With dithering switched off, the output has 24-bit resolution. The next three buttons set the clock source (internal or external), the internal clock rate (44.1 or 48kHz) and either standard or double-rate operation (in other words 88.2 and 96kHz). Each of these modes has an associated LED, and a final pair indicate Mute and clock Lock modes. A mains power switch completes the front panel facilities.
The rear panel is just as businesslike, with a neat row of eight combi jack/XLRs to accept either XLR or TRS jack input connections. The high impedance input can only be accessed with an unbalanced TS jack plug. A separate pair of TRS sockets provide either an unbalanced monitor mix output (the odd-numbered channels are mixed to the left output and the even channels are mixed to the right), or unbalanced inserts for channels seven and eight. The required mode is selected via links on the internal circuit boards. The next section of the rear panel is taken up with the digital output interface card. Currently the available options are ADAT optical and AES-EBU interfaces, although others are apparently being developed. The ADAT board is equipped with a pair of ADAT lightpipe outputs, along with a pair of BNCs for word clock in and out. With standard sample rates, both lightpipes provide all eight channels, but in the double-rate mode the first port provides channels one to four, while the second outputs channels five to eight, both in S/MUX format (the Sonorus standard for sample multiplexing).
The AES board is equipped with four XLR connectors and a single BNC clock terminal. The XLRs carry either standard-rate or double-rate data, as required, and the word-clock terminal switches automatically between input and output depending on the clock configuration of the machine. Finally, the IEC mains inlet incorporates an integral fuse holder (an unusual design that sits proud of the rear panel) and there is an adjacent switch to select the mains input voltage. Internally, the digital electronics are carried on a motherboard with eight daughterboards for the input circuitry running front to back. These also carry the front-panel controls and rear-panel interfaces, and use top-quality surfacemount components — a single Burr Brown chip appears to handle the input stage, with Analog Devices chips elsewhere in the signal path.
Between each pair of input cards on the motherboard is a standard stereo Crystal A-D converter chip and associated support circuitry, all surfacemount components again. The digital outputs from each converter are routed to the right-hand end of the motherboard, where they are dithered as necessary before being passed through to the plug-in output card. It is on this card that the relevant signal formatting is performed. Swapping the output cards is not as trivial as it might at first appear. Removing the two screws on the rear interface panel does not allow the card to be withdrawn! The entire lid has to be removed (nine screws) and then three more screws released from the interface card itself before it can be unplugged and extracted. Good job this isn't the sort of thing you need to do very often.
The AD8 does exactly what it says on the box. It converts eight inputs and routes them to the appropriate digital output in the required form. The review machine was supplied with both ADAT and AES cards, both of which proved happy to communicate with other equipment without problems — although I only tested the AES card at the higher sample rates. The unit's internal clock appears to be stable and with sufficiently low jitter for all but the most demanding professional applications. Connecting the clock output from an Aardvark Aardsync master clock to the external clock input of the AD8 didn't appear to have a significant audible effect.
The handbook claims the AD8 has ultra low-noise mic preamps (better than -130dBu EIN) and, while this appears to be the case, the real world practicality is somewhat limited by the restricted gain range. Most mixers and preamps will provide at least 60dB and some of the more expensive designs as much as 70dB of gain. The AD8 offers 40dB at best. In practice, this means that the AD8 will be able to provide adequate amplification for high-output capacitor mics and many dynamic mics placed in close proximity to sound sources — and so will make an ideal high-quality multi-channel front end for digital workstations and recorders where this kind of recording technique is used — pretty much all rock and pop recording, then! However, where genteel sources are recorded using more distant mic placement, the AD8 may not be able to provide sufficient gain to optimise the conversion to the digital domain.
During the review period I found the gain range adequate for most purposes using high-output capacitor mics, and the noise floor was commendably low. Overall, the quality of the mic preamps was very good, with minimal coloration and a neutral character. On the odd occasion where I needed more gain, I simply connected the AD8 to the group outputs of my analogue mixer, and transferred signals at line level. The provision of high-pass filter and polarity inversion on every channel was also very welcome — would that all preamps and mixers offered the same facilities.
The ability to switch individual inputs to accept high-impedance sources — electric guitar magnetic pickups, piezo acoustic pickups and so forth, is also a very handy feature. With an input impedance of over 5MΩ, instrument tonality was preserved well, and there was sufficient gain to ensure such sources were converted with peaks close to 0dBFS if required. In practice, I tended to use the first yellow LED on the bar graphs as my aiming reference, which provided peaks between -6dBFS and -3dBFS.
With the channel gain control set to the minimum position, a 0dBu input will produce a 0dBFS digital output, and so peak digital levels can be achieved with signals down to around -40dBu — or even -50dBu if working with a nominal 10dB of digital headroom. With +4dBu line-level sources, the 20dB pad was a prerequisite, accommodating signal peaks to a maximum of +20dBu. Overload is unlikely to be an issue with all but the hottest of analogue sources.
The AD8 does exactly what it was designed to do, with a simple but comprehensive user interface and thoughtfully chosen specifications and facilities. This unit makes an ideal front end for a multi-channel digital workstation or recorder — a compact mobile system based around a laptop, perhaps. A TDIF interface card would be a useful addition, as would a Firewire interface, but I'm sure these things are on Swissonic's development plans already. The audio quality of the analogue front end and converters is top notch and hard to fault at the UK asking price. A built-in D-A with a headphone monitoring facility would extend the completeness of the package for mobile applications, but other than that this is a perfect solution to a very common problem. Recommended.