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Cranborne Audio 500R8

USB Audio Interface, Analogue Mixer & 500-series Host By Matt Houghton
Published September 2019

Cranborne Audio 500R8

This most hybrid of devices does far more than just put A‑D/D‑A conversion into a 500-series chassis. In fact, it could even make you reconsider how you use your studio...

Launched last year, Cranborne Audio's Camden 500 mic preamp was a wonderful debut product: boasting stellar technical performance and some novel processing options, it prompted our Technical Editor Hugh Robjohns to conclude ( that "this is how I'd design and build a quality preamp." Announced at the same time as the Camden 500 were the 500R8 (reviewed here, and from hereon 'R8') and the 500ADAT, and these actually piqued my interest more, since both do something I've long wanted to see: they add A‑D/D‑A conversion to a 500-series chassis. As its name implies, the 500ADAT's digital interfacing is via ADAT, but the R8 adds stereo S/PDIF and multi–channel USB audio, as well as a low-jitter (Cranborne claim <0.5 picoseconds) master clock, to create an ambitious audio interface. But the digital side of things is only the start, as Cranborne have managed to pack much more into these devices, including a high-quality analogue summing mixer, monitor control facilities, and some novel analogue routing functions.


The R8 is a 4U-high, 19-inch rackmountable 28–in/30–out USB 2.0 audio interface with a built-in eight-slot 500-series chassis. Other than two TRS jack headphone sockets and the 500-series slots, all the analogue and digital connectors are on the rear. The first eight interface channels are assigned to the 500-series slots, whose XLR inputs and outputs are accompanied by a TRS insert point that sits between the module slot and the A‑D converter in the signal path. (Without a 500-series module, the XLRs can function as regular analogue line-level I/O, courtesy of a module bypass switch on the backplane). Two more interface channels access the stereo AES3-compatible coaxial S/PDIF I/O, and 16 are mapped to the optical ADAT streams — two pairs of Toslink connectors can carry 16 channels each way at 44.1/48 kHz, or eight channels via S/MUX at 88.2 or 96 kHz (192kHz support for four channels is planned in a firmware update in the near future). The remaining two USB inputs receive the analogue mixer's output, and the remaining USB outputs transmit two stereo DAW playback busses. The clock is accessed via the usual BNC connectors.

The idea of all that ADAT I/O is that you're able not only to hook eight 500-series modules up to your DAW, but also that you can connect a 500ADAT to an R8 to increase both the analogue I/O count and the system's capacity as a 500-series host. If you're happy with 44.1 or 48 kHz, it would be possible to add two 500ADATs, giving your DAW access to 24 channels of 500-series preamps, effects or processors with only a USB 2.0 cable and four ADAT 'lightpipe' optical cables. Alternatively, you could hook up any ADAT-equipped mic preamps or line-level converters to increase the analogue I/O count.

The possibilities don't end there, though. If you really wished — and had the financial means to make it happen — you could attach further 500ADATs (or other eight-slot 500-series chassis) to the system via the insert points on the first 'set', and start to construct something more akin to an analogue console, but one with the preamp, EQ and compressor stages of your choice on each channel.

The build quality seems very good. The powder-coated metal case is strong and remains rigid even before you screw in any 500-series modules. The back-panel connectors feel secure, and all of the switches and pots on the front feel suitably robust. That you can change the orientation of the rack ears for use as sturdy carry-handles is a nice touch, too, and will increases the R8's appeal as part of a location recording rig.

The 24V 5A power supply is an external 'line lump' switch–mode type, capable of accepting AC mains between 100 and 240 Volts. Each 500-series slot can draw 250mA, and there's a little headroom on top. I know some aren't fond of external PSUs but it's the right decision in this case. I mean 'case' quite literally, since there's no room inside this one to accommodate a PSU — that would have required a 5U or a deeper 4U chassis, and would have added weight to what, when fully loaded with transformer-balanced modules, is already rather a heavy device. The PSU's four-pin connector locks securely in the socket on the chassis; you can be confident that it won't get pulled out accidentally. This PSU features linear regulation on the DC inlet, and there's linear regulation on each 500-series slot of the chassis too, to prevent power noise and intermodulation distortion making its way onto the audio rails.

CAST Away!

Hopefully, that's whetted your appetite... but I've only really scratched the surface. Cranborne have also invented a new system for sending mic/line-level analogue audio signals over network cable. Called CAST (Cat5 Analogue Snake Transport; a backronym, I'll wager!), it conveys four channels (two sends, two receives) up to 100m over CAT5e, CAT6 or CAT7 cable, all of which are less bulky than traditional audio cables, not to mention less expensive, and are readily available.

The 500R8 is laden with analogue and digital connectivity, the most interesting aspect of which is perhaps Cranborne's innovative CAST system for sending mic/line–level analogue signals over CAT5 cables.The 500R8 is laden with analogue and digital connectivity, the most interesting aspect of which is perhaps Cranborne's innovative CAST system for sending mic/line–level analogue signals over CAT5 cables.

To cater for this protocol, the R8 and the 500ADAT have five RJ45 connectors on the rear. The first four provide alternative inputs and outputs for channels 1-8, and the big idea is that Cranborne's forthcoming CAST 'expanders' will allow you to send a stereo monitor mix to an artist, and receive two channels back from them (for example, from a mic and DI, or a pair of mics). One of the two expanders already announced includes a headphone amp, but I believe a wider range of CAST expanders is likely to follow. I suppose you might think of it as a neat, low-hassle, and cost-effective alternative to networked digital audio such as Dante and AVB for smaller-scale installations, such as a small multi-room recording studio — and, being analogue, obviously there's absolutely no latency involved.

A fifth such connector is labelled 'CAST Link', and is reserved for two-way communication between the 500R8 and a single 500ADAT unit. It routes the 500ADAT mixer's stereo bus signal to the 500R8, where it is merged with the 500R8 mixer's summing bus at unity gain — thus creating a 16-channel summing mixer. (The mix bus, whether the standard eight-channel one or, augmented by a 500ADAT, a 16-channel one, is available to interface channels 9-10). It also routes the aux monitor signal — which can draw on that 16-channel mix and the DAW playback — from the 500R8 to the 500ADAT. So for an end user, this single cable connection should result in a more seamless way of combining the two devices than using ADAT alone. Not having a 500ADAT at my disposal, I wasn't able to test this properly, but it appears a very elegant solution.

That you can change the orientation of the rack ears for use as sturdy carry-handles is a nice touch, and will increase the R8's appeal as part of a location recording rig.

Mixing & Monitoring

The controls for the 500R8's analogue mixer occupy the space beneath the 500-series module slots. Each channel has a source selector — a chunky metal toggle switch which conveniently protrudes beyond the front panels of any modules you've installed. The available sources are the channel's CAST input, the 500-series slot, and the corresponding USB output from the DAW. Between each channel's controls is a smaller toggle switch, labelled Chain, which allows you to pass the signal from one module to the adjacent one — to create a channel strip, for example, without requiring any manual repatching.

Beneath the switches are two pots, one for the channel level and the other for pan. The latter is a continuous rotary pot, with a centre detent, which I prefer to the LCR switches found on many stand–alone summing mixers. There seems to me to be bags of headroom available on the mixer, which sounds lovely and clean — just as it should. And if you do want character, then you have the 500-series slots to play with.

Over on the right-hand side of the front panel, you'll find the monitoring controls. In the lower half of that panel are four knobs, the lower two of which determine what you hear on the main monitor outputs. The smaller knob on the left blends between the stereo out from the DAW and the analogue mix-bus signal, while the larger knob sets the monitor level. A small toggle switch determines which of the two USB DAW signals is heard, and the idea is that 'DAW 2' could be used to set up a click track or cue mix in your DAW. But there's another twist — a pair of analogue line-level inputs on TRS jacks intended for use with external DAWs/converters (but which could be used to monitor any line-level source). This is labelled DAW 2 and is summed with the USB DAW 2 signal in the monitor mix when DAW 2 is selected as the source. Another pair of knobs performs a similar function for the aux bus, the primary intended purpose of which is to deliver a cue mix to the performer.

A number of different physical outputs are associated with these four knobs. On the rear are two pairs of monitor outputs, one on XLRs, the other on TRS jacks. The stereo aux output is on another pair of TRS jacks. Also, the monitor and aux signals are delivered to the corresponding front-panel quarter-inch headphone jacks. The headphone amps provide more than enough 'welly' for my Sennheiser HD650s. Much more in fact — I doubt you'd have problems driving any headphones with them but, more importantly, they sound good. There are also dual quarter-inch jack outputs for the stereo mix-bus signal, and a talkback mic input, for which 48V phantom power is permanently on (which will be fine for moving-coil dynamics and capacitor mics... I'm not aware of anyone who likes to use a vintage ribbon for talkback!).

The 500-series module slots occupy most of the front-panel real-estate, but Cranborne have also managed to incorporate a  summing mixer along the bottom and monitor control facilities on the right.The 500-series module slots occupy most of the front-panel real-estate, but Cranborne have also managed to incorporate a summing mixer along the bottom and monitor control facilities on the right.

Returning to the front panel's right-hand section, the upper half sports more controls. At the top is a stereo LED peak meter, with 14 LEDs per channel. This is configured such that signal presence is indicated by the leftmost LEDs, with the rest showing increasing levels up to 0dBFS: LEDs are green until the final three, two of which light amber (at -3 and -1 dB) and the third red (at 0dB). Yes, I did say dBFS — while this is an analogue meter, it's calibrated to show the level that will hit 0dBFS on the converters and thus in your DAW. There are no indicators for the individual channels; you'd have to rely on the 500-series modules or your DAW if you want that facility, but I didn't miss it.

Immediately below the meter are various buttons. Currently, these are all single-action but, intriguingly, I gather that it would be possible via a firmware update for Cranborne to make these perform different functions for momentary or longer-held presses. We'll have to wait to see if anything happens there, but in the meantime we have: a speaker A/B selector (pushing switches, pushing again switches back); a mono button (dual mono, in both speakers, and again push once to switch, push again to switch back); mute and dim buttons (again, these toggle the status, and the dim button attenuates the monitor level by 15dB); and a talkback button, which routes the talkback signal to the aux bus and the CAST outputs. The talkback level is set by an adjacent knob (the talkback preamp gain is fixed at +30dB), and this button automatically engages dim, while ducking the rest of the aux bus by 9dB. The only other control on the front panel is the power button, which glows blue when the R8 is on. I'm pleased to see this on the front panel, as that makes it much more convenient to power down and reboot when installing/uninstalling 500-series modules than the rear-panel switch found on some other rack units.