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Reviewer: Stephen Marsh
Digital Source: Vecteur D-2 CD Transport, Audio Note DAC Kit 1.2 with upgrades (PS choke, tantalum resistors, Black Gate caps, copper grounding bars on digital chips wired to central ground, VTV silver foil/oil output coupling caps), Toshiba laptop with Empirical Audio Off-Ramp I²S and modified Benchmark DAC-1 [under review]
Analog Source: Nottingham Analogue Mentor turntable with 10" Anna tone arm, Roksan Shiraz cartridge
Preamp: Hovland HP-100 MC tube preamplifier, fully updated
Power Amp: Red Rose Model 2A Silver Signature tube amplifier (Mullard xf1 EL34s, Tung Sol black plate 12AT7s and RCA black plate 12AT7WAs)
Speakers: Audio Physic Anniversary (SLE) Step speakers, Audio Physic Luna active subwoofer
Cables: Music Metre Fidelis digital interconnect, Harmony Audio interconnect, Stealth CWS interconnect, homemade twisted pair of mil-spec silver cladded multi-strand speaker cables
Power Cords: Analysis Plus Power Oval (amp), PS Audio Mini Lab (preamp)
Equipment Rack: Michael Green Justarack
Sundry Accessories: steel shot loading in Audio Physic Step stands, Audio Point coupling discs under Step stands
Power Line Conditioning: PS Audio P300
Room Size: 29' long x 16' wide x 10' high (sunken living room with open floor plan, listening across width of room)
Review Component Retail: $2,645 - $3,895 depending on options
Are you feeling down because the megabuck digital front end you bought just last year is already looking a little hoary? Are you overwhelmed at the thought of trying to keep up with the advancements in digital audio playback? You're not alone. The array of new products and the concomitant depreciation on last year's digital components can be mind-numbing.
That is why I have elected to spend conservatively on my digital playback equipment while keeping an eye peeled for any really significant developments. My Vecteur D-2 transport and tweaked-out Audio Note DAC 1.2 have served me well and I am still enjoying their sound greatly. However, during my recent review of the Bastanis Prometheus Mk. II speakers, I was loaned an Empirical Audio-modified Perpetual Audio P-3A DAC. While I did not find the time to do a direct comparison with my Vecteur/AN rig in my downstairs system (the speakers were the focus), I was mightily impressed with the performance of this DAC in the Prometheus-based system.
Despite a penchant for tinkering with vintage audio, I also make every effort to keep abreast of developments in the modern audio marketplace. In particular, the rapid developments in the digital music server/computer audio sector have caught my eye. While none of my friends have gone this route yet, many are touting it as the inevitable future of audio. So, I decided it was time to take the plunge and find out what computer audio was all about.
Since this is my first venture into computer audio, those of you who are more advanced might find some of my explanations and revelations elementary. On the other hand, there is a large sector of the audiophile community who seem to have little knowledge, but much curiosity, about this relatively new method of music playback.
Audiophile-quality computer audio generally means using either a USB (or Firewire) DAC, which can be directly connected to the computer via a USB cable, or various combinations of a Wi-Fi wireless connection between the computer and a converter. Computer audio competitors include the high-end USB DACs from Wavelength Audio, Audio Note, Hagerman Technologies and Bel Canto, plus lower-priced options such as the Scott Nixon USB DAC, Trends Audio UD-10, MHdt Laboratory USB Constantine, Apogee Mini-DAC, APL HiFi NWO-DAC1 and others. You can also purchase an intermediate device such as the Waveterminal U24 to convert your computer's USB output to S/PDIF Coaxial & Optical digital I/O and use your computer with just about any DAC. Notable Wi-Fi wireless devices include Olive Opus and Musica, Slim Devices Squeezebox and the newer audiophile Transporter, Apple Airport Express and Sonos ZD80.
Steve Nugent, the proprietor of Empirical Audio (a one-man operation), is adding another dimension to this market, the I²S (I-squared-S) DAC. Steve, an emigrant from the Silicon Valley electronics industry (Intel), started his own audio business designing and producing a myriad of audio products. His special passion though is advancing the state of the art in computer audio. His prior expertise was in digital electronics so he is well suited to this task.
The assault on Mount Jitter
While Empirical Audio offers USB DACs and a vast array of options/upgrades for computer audio playback equipment, Steve Nugent's all-out assault on high performance computer audio playback is largely about the conversion of the computer's USB data output to a data transfer format called I²S.
To fully appreciate why Steve Nugent believes that I²S computer audio is the future of digital audio playback, readers should refer to his article in Positive Feedback, issue 22, Nov./Dec. 2005. Quoting from this article regarding I²S transmission: "...I²S or I-squared-S is an interface found on some transports, such as Audio Alchemy and on some DACs, such as the Perpetual Technologies P-3A and the Northstar 24/192, but is not a common interface. This interface replaces the "digital coax" or S/PDIF interface that is common for most CD and DVD digital audio. The I²S is a 4-signal or 4-wire interface, consisting of two clocks, a L/R channel select and a serial data signal. There are several advantages to this interface, one of which is that unlike S/PDIF, it includes a bit-clock, which eliminates jitter contributor 7 [referring to another part of the article which lists different sources of jitter - Steve Marsh]. Another advantage is that it is the "native" interface for most DAC chips. This means that no translation of the data stream need take place in order to drive a DAC chip. This inherently reduces the amount of hardware that the clock and data signals must pass through. As in most audio equipment, simpler is better and this is no exception. The clock jitter is minimized but not eliminated in systems that use the I²S interface. It is still possible to have significant jitter even with this interface if it is not well implemented. An interface that converts USB, Firewire or Wi-Fi from a computer to I²S driving a DAC chip directly has the potential to outperform all other current techniques if implemented well..."
Implementing it well is what Steve Nugent's background in digital electronics allows him to do. Even for manufacturers that include I²S input on their DACs, Steve says they have design issues that prevent them from performing optimally. If you are a bit rusty on the sources and evils of jitter, the 1993 archives of Stereophile are a good place to start. The linked article points out how the AES/EBU or S/PDIF interface between the typical CD transport and external DAC can cause jitter.
Steve Nugent's article gives a more current and comprehensive list of jitter sources, making his case for I²S even stronger. An email he sent me, commenting on the Stereophile article, said: "With I²S, the shorter the cable is, the lower the jitter. This is primarily because at very short cable lengths, it does not behave like a transmission-line but more like a lumped capacitance. Therefore, it is the HF roll-off that results in jitter due to the cable capacitance. However, it is not the HF roll-off of the cable alone that causes the jitter; it is the detection of the signal transitions. One thing that is not mentioned, which is very important and relevant to jitter, is the effect that power system noise and thermal noise has on the detection of signal transitions by the receiving chip. If the signal has a slow rise-time, then more jitter will be caused simply because the receiving chip cannot decide when the transition actually took place. There is more ambiguity due to the changing "switching threshold". This is the voltage at which the receiving chip decides that a signal transition has occurred. This is why both quiet power systems and fast rise times are essential to achieving low jitter."
Empirical Audio's computer audio solution to jitter converts the USB output signal from the computer into I²S format by means of an outboard box is called the Off-Ramp I²S. This is not a simple design exercise and care must be taken to interface the I²S properly with the DAC (more about this later). In practice, one end of the USB cable plugs into the computer and the other end into the input of the Off-Ramp I²S. Then, the I²S cable exits the back of the Off-Ramp and plugs into the DAC (modified to accept I²S).
The other important anti-jitter technology Steve incorporates in the Off-Ramp box is the Superclock (version 4 now). This is a circuit developed and produced by Audiocom in order to "minimize the effect of jitter". It is a master clock oscillator and is important for reclocking the data after conversion from USB to I²S format and transmission to the DAC chip. (The Superclock has been a somewhat popular mod that audiophile tweakers get installed into their CD transports.) As you can see, Steve Nugent is dead set on becoming the anti-jitter superhero. Or, using another metaphor, he is trying to climb the Mt. Everest of computer audio playback.
The tactical gear
Steve Nugent was until recently modifying Perpetual Technologies P-3A DACs (and others) for I²S input with computer audio but he has now switched his primary efforts to the latest Benchmark DAC-1 because he feels its new AD1853 DAC chip is superior. As a result, Steve sent me the following sizeable list of hardware and software for the review:
* purchased from M-Audio
The improvements wrought by the Superclock 4 over the Superclock 3 were not subtle. The version 4 provided this classical workhorse with greater dynamic authority throughout the frequency range, combined with more delicacy in the softer passages and a greater sense of presence. The improved bass power was particularly noteworthy. If you are currently using Superclock 3, do not hesitate for a minute to upgrade to Superclock 4. I think it ranks as one of the most cost-effective sonic improvements you can buy.
Camp One - Vecteur D-2 with Benchmark DAC-1 via S/PDIF
Not one to pass up new music being served to me on a silver platter - er, hard disk, I set out to listen to all 164 of the 16/44.1 selections in Steve Nugent's supplied playlist, with the SRC Resampler doing the upsampling. All listening from this point on was done with the level 2 mod Benchmark DAC-1.
After listening to all of the selections several times over, I was able to establish what could be considered the characteristic sound of the entire rig. It was one of remarkable clarity, unsurpassed levels of information, powerful bass, reference level dynamic performance and excellent transparency. The only criticism I had was that on lesser recordings, there could be some stridency in the upper midrange and treble. This was noticeable on the playlist's violin recording by Friedman, as well as the cymbal crashes in the Led Zeppelin cuts and others.
I decided to try to ferret out which of the components in the chain was responsible for this treble quality. Connecting my Vecteur D-2 transport to the BNC input (using an RCA/BNC adaptor), I listened to a variety of my workhorse CDs. It quickly became apparent that the Benchmark DAC-1 showed the same slightly tipped-up treble as it had when using the computer as the front end. Perusal of Audio Asylum turned up several comments agreeing with my observation while others disagreed. I would venture that this is a matter of taste analogous to the solid-state vs. tube camps. This gets into the whole sticky business of arguing whether the Benchmark is simply allowing you to accurately hear what is on the recording, or whether you feel it is the blame of the Benchmark and would prefer the tonal balance of another DAC.
I should add that playback via the Vecteur was definitely inferior to the computer and Off-Ramp I²S, with reduced information, dynamics and focus.
Sherpa guide needed
I would be remiss not to mention the most vexing annoyance that can be experienced with computer audio playback via the USB port. That is the problem of pops, ticks and hiccups during playback. Pops and ticks are not a problem with computer audio via wireless transmission but wireless can have other problems such as bandwidth limitations or interference from microwaves, wireless phones and other wireless networks. These pops and ticks sound like the music just disappears for a fraction of a second and that is exactly what is happening. I find it to be more annoying than a pop on a record where the noise is usually superimposed on the music. These seem to occur almost at random but can be as prominent as two ticks in an average length track. Conquering this problem can be a pain and the PC-Audio discussion forum on Audio Asylum is replete with testimony about how to solve this problem. It is certainly quite possible to eliminate the pops but it can require some patience and guidance.
Steve Nugent explained the reason for these pops to me. It has to do with the software (i.e. M-Audio Transit) driving the audio through the I/O (input/output) port. With large latency drivers (i.e. large buffering or storage capacity), there can be delays in delivering the information from the hard disc to the memory and out the I/O port. These delays cause the temporary dropouts. Steve is trying to get driver software with less latency.
|For now, there are a number of steps one can take in order to eliminate these pops:
1. Disable virus protection, instant messaging, screen saver and all non-essential software.
2. In the Foobar DSP Manager, Resampler SRC, switch to "slow mode".
3. In the Foobar Preferences, Output, ASIOdII, double or triple the buffer
4. In Foobar DSP Manager, Playback, try increasing the full file buffering from 0 to 2, 6, 10, etc. until successful.
I'm sure by now many of you are saying "sheesh, I don't want to go through all of that to play my music." I can relate and must state that this method of computer audio playback not only has a considerable learning curve, it can also require a lot of fiddling at the beginning. Steve Nugent assured me that the Toshiba was playing music fine without pops at his home before he sent it. While I greatly reduced the problem with Steve's help, I never did get rid of all of the pops and ticks and just lived with some. Even with the hassles of dealing with the pops and ticks, Steve summarizes the situation with USB output as follows:
"The advantage of USB is that it is a dedicated connection that will always have the bandwidth required. The problem with ticks on the USB interfaces is more of a computer resource issue and the efficiency of the USB drivers. With a fast-enough computer or a low-latency driver, you will never have ticks with USB, even with the current Synchronous-Adaptive protocols."
Apparently, the real solution to this problem is for engineers to develop a new type of driver software that feeds the data out of the USB bus in packets rather than a continuous flow. This new type of USB driver would be called asynchronous as opposed to the current synchronous protocol. (If you really want to dig deeply into this topic, go to the Audio Asylum PC-Audio forum and type in "synchronous" in the search window.) Steve's comment on the prospect of developing asynchronous USB protocol, excerpted from Audio Asylum is:
"If you read the blow-by-blow of the trouble that the TI (Texas Instruments) engineers had to even get the synchronous audio USB working properly, you will see that it is probably extremely hard. I understand from this article that the main problem with asynchronous is that it is not isochronous by definition. In other words, there is no bandwidth or channel non-interruption guarantee. I believe this is why they went with synchronous."
Venture capital, anyone?
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