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High-Performance Audio Reviews
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June / July 2009
North American Premiere
Any DAC will convert a digital stream to analog, and where they differ in functionality is how many different types of digital stream can be accommodated. This one can take standard 16-bit/44.1kHz inputs of course, but also high resolution PCM up to 24-bit/192kHz. There are two AES Balanced XLR inputs, two 75 Ohm SP/DIF BNC Coax inputs, two TosLink inputs and a USB (type B) input, in addition to a true CD quality wireless link using the A2DP option under Bluetooth. You need the dual inputs to accept the high resolution 24-bit/192kHz or 24-bit/176.4kHz digital signals. If you buy the matching Chord Blu Transport, it will have the corresponding dual outputs. Unlike most manufacturers, Chord doesn't use any of the readily available DAC chips. They prefer to roll their own, using Field Programmable Gate Arrays (FPGAs). The latest generation Xilinx Spartan 3 FPGA offers 1.25 million gates, allowing the designers to implement eighth order noise shaping, the highest of any manufacturer. The FPGA handles input switching, digital S/PDIF decoding, a digital Phase Locked Loop (PLL) to reduce jitter, a RAM buffer controller, WTA filtering as well as a fifth generation pulse delta/sigma DAC.
DAC Examination You may be wondering what WTA filtering is. WTA stands for Watts Transient Aligned, and the WTA filter is an algorithm developed over thirty years to preserve the all important transient information in the original digital signal while removing the spurious content of that signal. Chord's highly successful DAC64, the predecessor to this unit, used a tap length of 1024 taps to implement the filter, but the new FPGA allows Chord to extend this to an amazing 18432 taps using 18 separate DSP cores. Moving to 4096 taps allowed substantial changes to the algorithm, but moving higher still entailed extensive experiments and listening tests. Chord feels the effort has paid off in improved transient accuracy, and it is this that provides the most stable image, improved timing and pitch definition. All this results in a more accurate reproduction of musical instruments, particularly in the bass. For the computer nuts, Chord claims a DSP processing power of 2120 MIPS. The chief enemy to fatigue free listening for digital devices is jitter – timing errors. Many DACs employ an analog PLL to derive a master clock to run the DAC. This creates a clock in sync with the incoming S/PDIF signal. Unfortunately, such PLLs create random jitter, and also the more damaging data-related jitter. You can avoid this latter problem by introducing a RAM buffer and using a local quartz clock to provide timing. For this DAC, Chord has introduced a digital PLL which removes all data related jitter and reduces random jitter to a very low value. This digital PLL also reduces distortion inherent in the analog PLL designs. You have the option of using the RAM buffer (1 second or 4 second) or the digital PLL in the QBD76. Note that this is not the only way to remove jitter from the DAC. Some manufacturers have instead implemented an asynchronous mechanism, as used in a lot of telecommunications networks. So much for technical wizardry, and as to the cosmetics, you be the judge. What remains is to describe the user interface and then it's on to the listening test. Once you've made your connection, you have three top mounted push buttons to control digital source, RAM buffer (max, min and off) and phase. A small window in front of the buttons shows you what you have selected, and when a source is selected, it will show you the sampling frequency. No need for a remote control here. I listened to USB and Bluetooth audio and can confirm both work exactly as expected. It's great to have these connectivity options, but the meat and potatoes of a DAC like this is how it performs with your CD collection, and what it can do with a high resolution source. For these tests I used a very high performance drive unit, the EMM Labs TSD1. I connected the two units using a 1 meter length of Van den Hal Digital XLR halogen free cable, and I had access to a standard Redbook signal (from CDs) and a high resolution 24-bit/88.2kHz signal derived from SACDs. I also used my reference CDSA SE SACD player as a digital source but without a doubt, the TSD1 did a better job, so I'll concentrate on that one.
Amplification in these tests comes from the Perreaux R200i, speakers are Wilson Benesch Act 1 and also a pair of X1 Monitors from Quebec's Lafleur Audio, while cabling is Nordost Valhalla from end to end. One point to mention is the Chord has a relatively high output level of 3V (unbalanced) and 6V (balanced), but this should not cause any problem with most amplifiers. Let's get into the details. The first step is to determine the effect of the RAM buffer. Note using a RAM buffer is not a viable option if you've working off a video signal, since the delay introduced by the buffer will make the music out of sync with the visuals. I found the buffer made a considerable difference to the sound, although there is a very small difference between the min and max buffer sounds. You may prefer the minimum buffer since it makes the DAC more responsive, and you don't find yourself listening to track 3 while your transport tells you you're playing track 4. The four second difference can be disconcerting. As you introduce the RAM buffer the shrillness on trumpet is eliminated and the space around the instruments opens up, with greater detail available to each instrument. The larger buffer gives a darker background and a further relaxation to the listening experience, most noticeable in string tone.
CD Redbook Audio:
16-bit/44.1kHz Holly Cole's "My Baby Just Cares for Me" from Girl Talk [Alert Z2 81016] quickly had me shuffling my feet and madly singing along, much to the distress of others in the vicinity. Transient response and pitch accuracy for the plucked bass are exemplary here while the image size is unusually intimate. This track has a sexy feel that makes you feel good about all the $$s you've invested in your hobby. Musicality in spades! Dr. Ray Kimber has produced some wonderful sounding SACDs using his IsoMike technology, and the Fry Street Quartet's recording of Beethoven String Quartet Op 18 No 2 is no exception. Here I'm listening to the CD layer of this Hybrid disc. The sound is quite sharp with rough edges to some of the more aggressive playing while the image is wide spread but not very three dimensional. In other words, it sounds like many other well recorded chamber music discs on CD – good but not great. There's no fatigue factor here and my slight dissatisfaction comes from knowing just how good the SACD layer of this disc can sound. For example, on the CDSA this is a recording of a very rare quality indeed, sweet, natural, focused, colorful, jaunty. It is fully open and listening to it is a relaxing experience. All those who insist SACD is not and cannot be an improvement on CD, please form a line on the left. For the rest of you, I urge you to attend one of Dr Kimber's demonstrations, where he reveals the full beauty SACD has captured on this disk using four identical speakers, two front-channel and two-rear channel. So in relative terms, the CD layer is disappointing, but in absolute terms, this is one helluva CD. We'll come back to this disc in a minute. My old favorite "Ansa Djallo" from Lilison Di Kinara's Bambatulu [MUS 2-1119] plays full scale, powerful, detailed and with a wonderful sense of timing. The layering is superb here and the bass definition is once again extraordinary. If you life African music, buy this disc, even if it was recorded in Montreal.
High Resolution Audio:
24-bit/88.2kHz Another top quality recording comes courtesy of Mark Levinson's Red Rose Music Vol1 [RRM 01]. Kenny Rankin's version of the Beatles classic "Blackbird" gains immeasurably in the transition from Redbook to high resolution. What was once somewhat slow, thick and loosely focused is now beautiful and flowing, more intimate and tuneful at once, giving away very little to the full SACD treatment within the CDSA SE.
My Recommendations I predict Chord's new QBD76 DAC will repeat the success of the DAC64 which it now leaves in the dust. The only question is, do you want the brilliant silver finish or the jet black?
Specifications
Company Information Voice: Tel +44 1622 721444
United States and Canadian Distributor Voice: (416) 638-8207
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Here is a company that marches
to the beat of a very different drummer. While some companies are renowned
for their unusual circuit design and others bring a distinctive flair to
the industrial design, Chord Electronics do both, and in spades. Here is
the Chord QBD76 DAC, a special unit to write home about and one with a
firm eye on the future of digital audio. The styling
of the Chord QBD76 DAC, while stunning, is a continuation of a theme the
company has been developing for years. The form factor is like no other
– compact, futuristic, with graceful curves, controls set in an arc and
a circular porthole so you can see the magic inside. I would call it
beautiful, were it not for the antenna that speaks to its wireless
capabilities. Without doubt the cosmetics and the stunning quality of fit
and finish must add to the price, but as we shall see, this doesn't stop
the QBD76 from representing strong value for money at $5995.
There's
no doubt about it, this is one superb DAC. It has such high resolving
power that it will expose the full strength and weakness of the material
you send through it. That is to say it's quite unforgiving of poor digital
recordings and demonstration class on the best material. So whether you
want this DAC in your system may well depend on what kind of collection
you have. Unlike some DACs it operates at full strength throughout the
frequency range, and does a remarkably good job with dynamics, both in
absolute terms and in the preservation of transients and low level details
which might easily be hidden in the mix. A Redbook recording of course has
its own limitations and there is no magic elixir hear or elsewhere to
overcome the limitations of the medium. However, it will go a long way
towards getting the best out of CDs, to a level that very few DACs can
match, and only, to my knowledge, that undercut the QBD76's price tag. The
challenger I refer to is the Berkeley Audio Design Alpha DAC which was
making excellent sounds in the TAD room at CES. It sells for $5000 but has
neither USB nor wireless inputs, so cannot be considered a direct
competitor.
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