Prism Sound Callia DAC & Headphone Amplifier

Full review and laboratory test of the Audio Analogue Maestro Anniversary Integrated Amplifier by Australian Hi-Fi Magazine. Free download.

The following equipment review consists of a full subjective evaluation of the Prism Sound Callia DAC & Headphone Amplifier written by Lesley Swan, but omits the technical analysis.

If you would like to read the complete review, together with a complete set of independent laboratory tests and graphs conducted by Newport Test Labs and a test report written by Steve Holding, click on the graphic at the right, which is a downloadable pdf that is an exact replica of the original pages on which the review appeared in the Esoterica section of Australian Hi-Fi Magazine, November/December 2016 issue (Volume 47 Number 6).

Prism Sound Callia DAC & Headphone Amplifier

UK digital specialist Prism Sound is a newcomer to the audiophile arena, but it’s an old hand in the professional audio sphere, having been founded back in 1987 by Graham Boswell and Ian Dennis, electronics engineers who’d just left Neve, where they’d developed the world's first commercial audio mixing console using DSP. According to Boswell, Prism Sound was founded ‘with the aim of making digital conversion good enough for the most demanding listener.’

They ended up making digital conversion so good that Prism Sound’s first DACs and ADCs rapidly became popular in recording studios around the world, in particular the ADC-1. And when Prism Sound developed the world’s first AES/EBU interface analyser (the DSA-1), it became the standard tool for broadcasters around the world, including the BBC, NHK, NBC, CBS, ABC and CCTV.

The Equipment

Callia is girl’s name that was once popular in Greece and elsewhere around the world. Some linguists say it translates as ‘beautiful voice’ while others claim ‘very beautiful’ is a better translation. It would apply either way in the case of Prism Sound’s Callia DAC, because it looks and sounds very beautiful. And if you’re wondering why a totally British company (all products are designed and manufactured in Cambridgeshire, England) has a Greek name, it may be because Graeme Boswell, Prism Sound’s founder and owner, has a penchant for it. Several of the company’s professional digital/analogue products also have Greek names: Orpheus, Lyra, Atlas, and Titan etc … though one is simply named ‘Dream’ … so not all Greek then.

The front panel is beautifully simple and beautifully laid-out. The large knob is the volume control for the line outputs, of which there are two: unbalanced (via gold-plated RCA terminals) and balanced (via gold-plated XLR terminals). It’s surrounded by a ring of blue LEDs that illuminate to give a visual representation of volume setting. The provision of a volume control means the Callia can drive a power amplifier directly, if required, eliminating the need for a pre-amplifier, but this firstly means all your source components would need to be digital, because the Callia has only digital inputs, and secondly that you’d be limited to three source components, because it has only three digital inputs: USB, coaxial digital and optical digital. I could not quite believe there’s no analogue input: IMHO, a definite oversight.

The USB input handles PCM at 44.1k, 48k, 88.2k, 96k, 176.4k, 192k, 352.8k and 384kHz at up to 32-bits as well as DSD64 and DSD128. The coaxial and optical digital inputs handle PCM at 44.1k, 48k, 88.2k, 96k, 176.4k and 192kHz at up to 24-bits as well as DSD64. Mac users using the USB input are good to go ‘out of the box’ but Windows users will need to install a UAC2 driver to send audio to Callia. This software is handily included on a USB stick that’s supplied with the Callia. The stick also contains the Callia’s User Manual (written by Ian Dennis himself!), as well as software that will enable you to perform firmware upgrades yourself as they become available. I liked this: So much better than providing an optical disc or forcing you to download an instruction manual from a website. (A printed ‘Quick Start’ guide is also included.)

The smaller rotary control to the right of the volume control is a volume control for the 6.35mm headphone jack on the front panel. Inserting a headphone plug into this socket mutes the analogue line inputs, which I didn’t like. It was only when I had to consult the manual to find out what the DIP switches on the rear panel did that I discovered you can defeat this muting using these switches. The DIP-3 and DIP-4 switches also allow you to optimise the Callia’s  low-impedance headphone amplifier to perfectly match your headphones, with one setting for phones with a nominal impedance of <32Ω, another for phones with nominal impedances of between 32 and 50Ω, and another for high-impedance (>50Ω) headphones.

When setting the DIP switches you do need to be careful not to move DIP-1 inadvertently, because it disables the front-panel volume control so the Callia delivers its maximum output voltage at the output terminals. DIP-2 switches DSD headroom between 0dB and +3.1dB (the latter position is used to prevent ‘hot’ DSD streams from clipping).

Input switching is managed automatically, with the Callia automatically detecting an active digital input and switching to it, after which it ‘locks’ into position and shows the format of the data stream on the front panel, using the LEDs immediately to the left of the volume control. If you don’t want to avail yourself of the convenience of auto-switching, you can switch the circuit off, after which manual switching is accomplished by brief presses of the power on/standby button at the far right of the front panel. (A longer press puts the Callia into standby mode.) This same button, used in conjunction with the main volume control, also allows you to vary the brightness of the front-panel LEDs.

Internally, Prism Sound does most of its processing using in-house algorithms, via a Spartan-6 FPGA and 32-Bit ARM Cortex microcontroller along with Prism Sound’s own clocking circuitry, which it calls a ‘CleverClox’. This is a hybrid phase-locked loop that locks the Callia’s clock to the selected SPDIF source with better than ±50ppm local clock accuracy, resulting in ultra-low jitter, claimed to deliver >60dB/decade above 100Hz jitter rejection. Although some Cirrus Logic CS4398 DACs are on the PCB, Prism Sound reportedly uses only their final switched-capacitor stage.

The Callia is physically quite small, measuring just 285×242×50mm (including feet) and correspondingly light, at 2.1kg.

Prism Sound Callia DAC & Headphone Amplifier

Once I had installed the Callia and start to use it, my first thought was that either I would to have develop stronger fingers, or that the Callia’s volume control would loosen-up after continued use, because out-of-the-box, it’s quite stiff to turn. The headphone volume control, on the other hand, was quite easy to turn, but had a slightly ‘raspy’ action. Both are potentially teething issues that could disappear after some use, or simply may have been confined to my review sample. A remote control with the ability to adjust volume would have solved both issues but alas, the Callia does not come with a remote control. Prism Sound is likely expecting it to be used as a ‘desktop’ unit, in which case the user would always be within arm’s reach.

The very first album I played through the Callia proved to be jaw-dropping on two fronts. For a start, it was made immediately obvious to me that the Callia is ‘telling it like it is’ and providing super-precise digital-to-analogue conversion, yet it wasn’t a ‘digital’ sound, which made me immediately suspect that whatever filtering Prism Sound is using, it isn’t a standard ‘brick-wall’ filter. The result was a ‘cleanness’ to the sound that was as revealing as it was involving. The second jaw-dropping event was the sound I was hearing from the disc, a 1958 live recording of Harry Belafonte in Carnegie Hall. It would appear that the engineer (Bob Simpson) just set up a few microphones, and didn’t mess with levels or equalisation. Amazingly, it then seems that no-one ‘mastered’ the tape before its transfer to CD. The result is a recording that is amongst the best and most realistic I have ever heard in my life. I am not exactly a Belafonte fan, but I could listen to this album over and over just for the sheer pleasure of hearing how live recordings should be made—and no matter what I think of the music itself, the actual musicianship is stunningly good, both from Belafonte himself and his backing musicians.

After the smoothness of Belafonte, the grim grit of The Peep Tempel’s latest CD, ‘Joy’, was a jolt back to modern reality, but what a jolt. There’s story-telling here (via spoken word), and more story-telling (yep, spoken word again), but it’s also a musical bacchanalia. Beautifully recorded, too, as just a few seconds listening to Neuroplasticity will prove (and will show where the album title originated). And if you’re looking for a track to show off your subwoofer’s prowess, you could do worse than spin Joy’s lead track, Kalgoorlie, which is bass and grunge pierced by ear-shredding, stabbing guitar. All of which was exactly revealed by the Callia, whose own performance sinks you deep into the trio’s performance, but at the same time almost contradictorily reveals the precision of Anna Laverty’s superb engineering. But not an album for the faint-hearted, so listen at your own risk…

Trialled with even-more testing fare, a 70-piece symphony orchestra, the Callia once more showed its ability to reveal the ‘weave’ of the music while at the same time uniting the threads into a glorious tapestry of sound. No, not classical, but ‘Live in Columbia’ by The Alan Parsons Project. The only problem is that when you hear, say, I Wouldn’t Want to Be Like You, you’re going to want to hear the whole of the album that gave it life (I, Robot), and the same would be true of the tracks Turn of a Friendly Card and Eye in the Sky. (Though hearing Friendly Card made me wonder why Parsons wasn’t enjoined in the recent ‘Stairway to Heaven’ lawsuit.)

No matter what type of digital music I played through the Callia, from CD quality up to DSD, I always perceived the backgrounds as being totally silent, but thankfully, it was always a truly ‘musical’ silence and not the ‘digital black’ that some DACs deliver when there’s no music playing or, worse, ‘between the notes’. When I was listening to the Callia, any silences—however short or long—merely served as pauses in the cause of the music, either to separate individual notes, musical phrases or tracks. And when listening to solo instruments played at a live venue, I could hear the ‘acoustic’ of the venue itself… though also, sometimes—alas!—traffic noises from outside that venue, such is the revealing nature of Prism Sound’s Callia.

I could not conclude this review without a mention of the Callia’s headphone output, which is awesome. It drove all the headphones I had to hand to their maximum level without any audible distortion while also delivering outstandingly transparent sound. I particularly liked the dedicated volume control, because it meant I could leave the headphone volume at my preferred level while still adjusting the main volume control to suit the listening situation (with the same scenario operating vice versa, of course!).


The inexplicable omission of an analogue input or two aside, not to mention the lack of a remote for the purpose of volume control and input switching, Prism Sound’s first foray into the consumer audio market is a tour de force, a genuine state-of-the-art DAC and headphone amplifier at a genuinely entry-level price. # Lesley Swan

Prism Sound Callia DAC & Headphone Amplifier
RRP: $2,695
Warranty: Three Years
Distributor: CDA Pro Audio

Superb sound
Headphone amp
Dual volume controls

Analogue inputs
Remote control

A full technical appraisal of the performance of thePrism Sound Callia DAC & Headphone Amplifier with test results, graphs and an analysis of the technical performance is contained in  the LABORATORY REPORT which is in the pdf version of this review. (Click the RED box above).