HIFICRITIC Volume 8 No. 4
October / November / December 2014


Valve Audio Devices DAC-12 DSD
Andrew Harrison gets to grips with an idiosyncratic but undoubtedly interesting DAC from Valve Audio Devices.
Review By Andrew Harrison

  Given the brand name Valve Audio Devices (VAD), it's not at all surprising that the company thoroughly embraces valves in a three model line-up that currently includes the DAC-10 DSD, a DSD-only converter called the DSD Player, and the DAC-12 DSD which is the unit tested here. Equipping digital-to-analogue convertors (DACs) with valve circuitry is nothing new, but such devices are more commonly found only in the output stages, the final furlong within the DAC, by which point the audio signal is resolutely analogue and is therefore considered to be responsive to what's perceived to be the kinder, more warming treatment of vacuum-state electronics.

Designer Gregor Szymczyk is a Polish-born electronics engineer who has long been working with valves in the pursuit of musical excellence, and in the case of the DAC-12 we find no fewer than three pairs of valves ranged down the centre of the unit. Two 6SN7s are used to form the analogue output stage; two CV574 full-wave rectifiers are employed for the power supply of the input valves; and two E88CC double triodes are used on the S/PDIF inputs, buffering, impedance matching and creating a balanced output for the S/PDIF receiver.

In fact the DAC-12 DSD is two distinct digital converters in the one chassis. For PCM-based digital audio, whether from traditional S/PDIF or USB computer sources, the digital audio signal passes to two off-the-shelf hybrid-multi-bit stereo DAC chips, using each in dual-differential mode. The specific type is not disclosed by the designer, who feels that the specification is less important than the overall circuit application. More remarkable is the design philosophy of the DSD convertor. Instead of taking a DSD-capable DAC chip such as a delta-sigma device from ESS, TI or Cirrus Logic, VAD's designer has taken an unusual approach, based on simply filtering DSD's raw PDM bitstream to reconstruct the analogue waveform.

Either DSD64 (as used for Super Audio CD) or the more recent DSD128 variant can be accepted, through a separate USB input receiver (with its own USB port on the rear), using the DSD-over-PCM (DoP) protocol that's ignited computer audiophile's interest in DSD in recent years.

The PCM-encapsulated DSD bitstream is taken by another Combo384 Amanero USB input receiver, then fed via input buffer and shift register through an analogue FIR filter, followed by a 4th-order low-pass filter. This is fixed at around 50 kHz to remove the worst of the format's ultrasonic hash, and both filters are configured in dual-differential balanced mode. The resulting output is a reconstructed analogue signal that has not seen the usual digital signal processors (DSPs) we'd expect to find in a modern DAC.

New to the DAC-12 DSD is a three-way switch on the front panel, used for fine tuning the PCM inputs' digital interpolation filter: CD is a minimum phase 'apodising' filter; MIX is a minimum phase 'soft knee' filter; and HI is a linear phase 'soft knee' type.

Construction
The VAD DAC-12 DSD is a solidly constructed utilitarian unit, based around a heavyweight steel chassis and an aluminium fascia, in either natural silver, anodised black or piano gloss lacquer. Viewed from the front it has a somewhat V-shaped aspect either side of the two rows of valves in the centre; most of the digital and analogue signal electronics are housed in the right-hand block, with the power supplies in the left. Internal wiring is a combination of PCB with point-to-point across a number of power chokes, transformers, inductors and capacitors fixed to the chassis. Seventeen linear power supplies power various sections of the DAC-12 DSD, including a dedicated toroidal supply for the DSD section.

As befits a hand-assembled unit at a substantial price, the passive components include audiophile oriented paper-in-oil, custom silver mica and aluminium/Teflon capacitors, plus resistors from Dale, Fukushima and Arcol. Silver cable is used for the signal wiring.

The earlier DAC-10 DSD was widely criticized for having a user-hostile input switching arrangement at the back. This has now been addressed here by a rotary switch on the front that selects between OPT, BNC, COAX, PCM and DSD options. Switching between the PCM and DSD inputs requires a deep press on an unlatched LOCK button on the front panel, which resets the timing lock to the newly selected USB input. And to use the DAC as a digital-only pre-amp, the unit has a motorised volume feature that is controlled by an IR handset.

A loud and unwelcome double pop was heard on the device's output when first selecting DSD sources, or when skipping between DSD albums in computer playback. Szymczyk is aware of the issue, and is finalising a mute system with shunt contact discharger, fired directly from the FIR filter.

The DAC-12 was acoustically quiet in use, and only a barely audible buzz could heard with my ear close to the chassis. Used as a line source via a pre-amplifier, it passed no discernible hum to amp and speakers. However, in my system it didn't work as a digital pre-amp – a small but aggravating level of ground hum arose when connected directly in single-ended mode to my Chord Electronics SPM1200C; when connected in balanced mode with XLR cables, louder hum made the setup unusable. This DAC is fully grounded through its IEC earth pin, and other power amps may not suffer in the same way.

The casework runs barely warm, mainly on the side that houses the power supplies, and the entire unit draws around 65W mains power.

Sound Quality
The DAC-12 DSD's upfront presentation grabs the attention from the outset. Never tipping into the realm of strident, it tends to promote its midband, giving a lively and characterful sound. It would be tempting to simplify and say it goes for the metaphorical musical jugular in presentation, but that's only part of the story.

VAD's designer has taken pains to ensure that the DAC can pass muster with the vast catalogue of Red Book music that CD collectors still enjoy. Played over a BNC link from the dCS Verdi transport, the DAC-12, without recourse to upsampling, had plenty of front-to-back soundstage space, although it did fall somewhat short of other references here, notably the profound dCS Purcell/Delius combination. Likewise, while it did create some beyond-the-boundaries stage width, it was harder to forget CD's limitations while playing time- and dynamics-limited 16-bit content. The grain and grit delivered from a 16-bit disc was mercifully low, yet one couldn't accuse the DAC-12 of overly sweetening high treble like some tube-based syrup dispensers. Bass lines didn't always unroll with quite the easy freedom and unrestrained depth that some digital front-ends present. There was sufficient sense of slam and thwack in rocking pieces, but for this listener the low end didn't have quite the snap and timing to carry along the tune entirely effortlessly. Without quite going so far as to drag its heels, it did display a measured pace on its way to a song's end.

The midrange timbre could also be characterized by some hardening here, making one man's welcome focus on a favourite singer then become a little too stressed on guitars, rhythm and lead for another. A bizarre example: I'm very far from enjoying the work of Van Morrison, but listening to the 24/192 reissue of Astral Weeks I started to understand those that do, finding poetry in the louche rambles for the first time. Simply put, it reminded me of hearing an LP. However, notwithstanding a shift to 24-bit resolution from FLAC, WAV and AIFF files, I wasn't always transported to the desired liquid-flowing musical nirvana. Although there was a step up in overall performance – smoother, wider, more in-the-room tangibility – I also struggled to unravel the pulse to find the band's meter at times. Take the arguably shambolic Four Sticks from Led Zeppelin's recent 24-bit reissue. Yes it dodges between 5/8 and 6/8, and no hi-fi is going to unseat those signatures, but I still found that the overall timing didn't always hold my focus. It was helped by a direct-to-amp connection, which restored the missing foot-tap factor, but the ensuing ground hum made that inappropriate in my system.

Elsewhere, with similar archive treasures of less than perfect provenance, the DAC-12 could make a minor meal out of busy or heavy pieces. I rarely escaped the feeling that I was listening to an old tape, and rarely found myself transported to the being there moment of performance. Modern recordings could show a more wideband, low-noise experience, yet the tell-tale sounds of some metal percussion – such as hi-hat, for example – seemed more like CD than the approach to near reality that 24-bit/96kHz and higher PCM digital can bring.

Which brings me to DSD replay. After thinking that all the obvious time and love that's gone into the VAD design had not really deserved those rave notices up until now, I found that the brave decision to adopt chip-free Direct Stream Digital decoding could well vindicate the whole package. While the DAC-12 could sound somewhat brazen with PCM audio at times, its DSD playback was in turn the most licentious, audacious version of digital music making I've heard since Audio Note UK's knuckle-ride implementation of non-oversampling filterless DACs.

When all is going well and a suitable recording is being played (typically a direct- or tape-to-DSD transcription, as opposed to something multi-bit mastered), there was that sense that nothing stands between you and the instruments in the mix. Being bombarded with incredible amounts of HF and ultrasonic detail, painting the performances between the speakers, has the potential to get quite raw, not to say wearing in long doses. One might question whether or not it's strictly accurate, but the razor-sharp percussion transients; the unique hammer impact of acoustic piano, where no two pianos sound alike; and the impulsive sparks that fly from strummed strings or bowed gut can be very persuasive.

The basslines that I sometimes struggled to follow from 24-bit sources were now timing on a sixpence, locked tightly to the band's beat. But all this extra musical revelation came at a price; one which may divide listeners: easy listening this is not, by virtue of the sheer attack of leading edge detail whistling from the speakers.

In a decidedly full-range 'revealing' system (and the diamond domes of my tweeters must earn the label, along with my choice of cables and amps) there was a serious excess of upper frequency musical content, with white noise hiss riding shotgun, to the point where one became unavoidably trepidatious. It depended on the album, but some DSD recordings seemed just too noisy to be enjoyable. In addition, behind the ssssss of tape-like hiss, some recordings had chuffing chirps to contend with: an example was Julia Fischer playing Russian Violin Concertos (on Pentatone Classics), probably the result of ultrasonic 'birdies' being demodulated down into the audio band.

However, settling for a more band-limited system makes it much easier to contend with unwanted high frequency hash. I tried speakers like the Eclipse TD508MK3, which majors on phase accuracy by virtue of a full-range driver and no crossover, and also tends to roll off the top end somewhat. This combination worked wonderfully well, combining the focused transient precision of the DSD DAC with the coherent impulse response of this single-driver speaker. Now that rainfall of white noise, softened by band-limited speakers, was turned into a far-off distant patter, which was much easier to tune out.

This time-domain focused DAC and speaker union made for a magically musical sound, even though it might not be considered a winner in traditional audiophile virtues like vanishingly low noise and coloration. Furthermore, the designer is currently working on tuning this noise issue, adjusting the DSD upper frequency response to more like -3dB at 25kHz via an optional switch. A frequency response limiter at 50kHz will also be user adjustable.

Conclusions
This DAC will not be for everyone. As a digital convertor it is much more of a musical performer than a device for providing a technical rundown on how a recording was put together, and the excessively exposed and bright sound from DSD may well divide opinions too. It may well sound rather too unrelenting for some tastes, particularly if used in a system balanced primarily for transparency, although this situation may become easier after the promised retuning.

Although the treble does sound rather too strong with DSD sources, this is currently receiving attention, and in other respects this DAC breathes valve sound from end to end, returning the sort of vinyl-like soundscape that I cannot achieve with my reference Mytek, dCS and Chord DACs. If that is what you seek from your digital music collection, the VAD deserves a personal audition. While certain elements leave one wondering whether it's rather too 'home made' for comfort, this designer clearly has too many good ideas to ignore.

The System
Listening tests used B&W 802D loudspeakers driven by a Chord Electronics SPM1200C power amplifier and a Music First transformer-based passive controller, using Nordost Valhalla interconnect and speaker cables. Digital sources included dCS Verdi CD drive and an Apple Mac mini (late 2012), playing PCM and DSD files through Audirvana Plus software. Reference DACs included Mytek Stereo-192 DSD, Chord Electronics QuteEX and dCS Delius.  

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Specifications
Type: Digital to analog converter
Digital Inputs: RCA S/PDIF coaxial, BNC S/PDIF coaxial, AES/EBU XLR and optical TosLink
Two USB 2.0 (one PCM and one DSD)
Analog Outputs: Unbalanced CA and balanced XLR
Features: 24-bit/192kHz DAC, DSP-free DSD-to-analogue conversion
Analogue volume control
IR Remote control
Dimensions: 300 x 4 4 x 1 5 0 mm
Weight: 14 kg.
Price £6995

Company Information
Valve Audio Devbices
Voice:  01604 922704
Website: www.ValveAudioDevices.co.uk