Digital Processor Reviews

Sure, Stereophile gets letters to the editor. We also get some colorful responses for our "Manufacturers' Comments" section. (Vince Bruzzese and Roy Hall are literary standouts among their component-making peers.) And, as one of the magazine's Contributing Editors (Audio), I get lots of personal mail from readers seeking my advice. I thought I might share some of these letters with you, and my responses.

Dear Erick:
I really want to get into computer audio, but the economy sucks and most of the stuff you review in Stereophile seems way overpriced. Is there anything that will play my audio files and CDs that won't send me further into debt?—Sue P. Cichen

Dear Ms. Cichen:
Yes, there is. In fact, I think we're entering new age of great-sounding, inexpensive digital-to-analog converters (DACs) that can do a lot for relatively little money. For instance, I recently spent some time with the CEntrance DACmini CX, which accepts S/PDIF, TosLink, USB, and analog inputs, and costs a very reasonable $795. You can buy the DACmini CX directly from CEntrance's own website.

The DACmini CX comes in a standard, silver-finished aluminum chassis that's designed to share the same footprint as Apple's original Mac mini. (Get it? Mac mini? DACmini?) The faceplate of the DACmini has knobs for selecting the input and setting the volume level, and a ¼" headphone jack that's driven by a class-A–biased headphone amplifier with no capacitors in its signal path.

The DACmini CX is also galvanically isolated: Signals cross the analog/digital divide via a sophisticated magnetic barrier without ever making actual electrical connection. The RCA connectors are isolated from the chassis with plastic washers, to avoid ground contact. Computers are notorious for creating grounding problems when hooked up to audio systems; galvanically isolating the DACmini's analog section means there's far less chance of creating grounding noise in your headphones or speakers.

The DACmini CX is powered by an external switching supply much like one you'd use for a laptop computer, and it has a typical IEC receptacle—you can use a fancy aftermarket AC cord, as I did. The line-level signal leaves the DACmini via a single pair of gold-plated RCA jacks.

The DAC chip in the DACmini (an AKM 4396) accepts signals with word lengths up to 24 bits and sample rates of up to 192kHz via the CEntrance's coaxial input, and up to 96kHz via USB. I think you'd agree, Ms. Cichen, that the DACmini CX offers a lot of features for a reasonable sum.

Erick:
It seems that lots of DACs these days have volume controls. I want to sell my preamp and plug a DAC straight into my amps. Any suggestions?—Noah Middlemann

I'm with you, Mr. Middlemann. I am a big fan of DACs that can also control a system's volume. The result is a much more direct signal path that lets folks save money they otherwise might spend on a costly preamp and extra cables. Companies are starting to figure out that customers want this kind of configuration and simplicity.

I was pleasantly surprised that, a few months after the release of the DACmini CX, CEntrance began offering modifications for it. Each mod costs $99.95; CEntrance will build your DACmini CX to order, or trick it out for you later. The available mods are: a black-anodized finish; Headphone Linearity, which lowers the headphone impedance from 10 ohms to 1 ohm for use with low-impedance headphones: Rock and Roll, which increases the gain for the headphone output (useful for high-impedance 'phones or listeners who want to go deaf); and Variable Output, which uses the DACmini's headphone volume control to adjust the level of the signal from the rear-panel RCA jacks.

Once the DACmini CX was available with Variable Output, I asked for a review sample. I found it very easy to hook up to my power amps and use it as a DAC, headphone amp, and preamp. Unlike some other DACs with volume controls, the DACmini attenuates volume in the analog domain.

Dear Mr. Lichte:
I wish Stereophile would stop covering computer audio. USB audio is stupid. So are you.
Eat my shorts.—Bea Ligerant

Dear Ms. Ligerant:
While I sympathize with your assessment of me, USB audio is not as bad as it used to be. CEntrance seems to understand the inherent pitfalls, and have made their own set of choices about how to handle them in their DACmini CX.

As most folks know, the big problem with using USB for audio is the timing errors it introduces in the streamed data. The USB format was never designed to provide a continuous stream of uninterrupted data. Instead, USB sends out data in packets. This hardly makes a difference when you click your mouse to tell your printer to print something, but it makes a big difference for digital audio, which requires perfect timing between source (a computer) and DAC. When timing errors result from this miscommunication, jitter-related spuriae appear in the reconstructed signal.

If audio data are accepted by the DAC whenever the computer's bus wants to send it, this is called a synchronous connection. Left untreated, such connections are typically not very good—the computer will send data only when it has a moment when it's free from the myriad other operations it's constantly handling.

One way to combat jitter in a USB connection is to have the DAC and the computer shake hands before they begin exchanging data. In this secret handshake, the DAC is given permission to continuously monitor the computer's data-transfer rate, then adapts its own clock, on the fly, to match it. This is known as an adaptive isochronous USB connection.

In an increasingly popular way to use USB for audio, an asynchronous connection, the DAC's clock takes charge of the computer and tries to guarantee that the computer sends it data only at the rate the DAC needs. The DAC clock can therefore be of high precision, hence low jitter.

In the DACmini CX, CEntrance uses yet another way to minimize jitter via USB. As Michael Goodman, CEntrance's chief product architect, told me recently, "Any engineer skilled in digital and analog design knows that jitter is inevitable in digital communications. It results from the fact that computers always send data in irregular chunks, since they are doing several things at once. The crux of audiophile D/A conversion is how well you clean up the audio samples prior to their presentation to the DAC. Our products reduce jitter to well below audible levels. We employ a proprietary two-stage jitter-reduction algorithm, called JitterGuard." Essentially, JitterGuard is a buffer that sits between the USB receiver and the DAC chip. CEntrance's premise is that as long as the DAC chip sees buffered data that are correctly clocked, it doesn't make much difference if the USB connection is adaptive, synchronous, or asynchronous.

As always, Ms. Ligerant, the proof of any audio philosophy is in the listening. I hope this helps you hate USB, and me, a little less.

Dear Erick:
I want to use my computer as an audio source, but I hear that USB audio is really difficult to set up properly. I'm scared.
Trepidatiously—Freda Njüthing

There, there, Ms. Njüthing. I'm here for you. Many companies now make USB-based audio products that can achieve bit transparency without you needing a computer-science degree or having to constantly adjust your Mac or PC. In my opinion, the state of USB audio is vastly different from what it was just two years ago—it is now easier to accurately transfer audio data.

For instance, CEntrance's DACmini CX is able to work with any computer without the user having to install a driver program. You just plug it in and play. CEntrance also offers a free, downloadable, ASIO driver/control panel for Windows users. I listened to the DACmini with and without the ASIO driver and preferred the sound with it. I also liked the peace of mind of knowing that I was bypassing as much of Windows Vista's audio processing as I possibly could.

Erick Lichte:
I believe that computers are the Mark of the Beast. I don't want to EVER have a computer involved in my audio hobby. All I want is a nice DAC–preamp–headphone amplifier that will make my old CD player sound better.
Peace—Sev N. Tseel

Mr. Tseel:
I understand. I have a love/hate relationship with my laptop. Though I like some of the musical convenience of listening through it and I love the sound of high-resolution digital, I don't fancy having my computer as part of my listening experience.

I still believe that DACs need to play "Red Book" CDs well to be a viable part of my audio system. I recently hooked up the DACmini CX to my Bel Canto CD2 CD player, to get a sense of what the CEntrance sounded like when I played normal CDs like a normal audiophile. Listening to Four Tet's DJ-Kicks (CD, !K7 203), I was struck by the rather tubey and analog quality that the DACmini brought to the music. The DACmini gave the music on this album a full-sounding midbass, a generous midrange, and a treble that was delicately balanced but ever so slightly smeared. Transients didn't quite have the startling snap that I often hear through other DACs, but what the DACmini lacked in speed it made up for with a pleasing, almost decadent tonal balance. Listening to the DACmini, I would have thought it was some sort of tubed source component, not some under-$1000 digital player with a switch-mode power supply.

I decided to play to the DACmini CX's strengths by listening to the Latvian Radio Choir singing Peteris Vasks' imploring Dona Nobis Pacem, for choir and organ (CD, Bis CD-1145). The DACmini presented a generous picture of this music; the choir sounded rich and full, while the varied registrations of the organ came through with clarity. The choir's sibilants were beautifully balanced in level with the rest of the singing. But again, I heard a slight treble smear that made s sound a tad like sh. However, the treble also had a sense of delicacy and shimmer that made the DACmini's slightly smeared treble less of a problem.

In terms of soundstage, the DACmini CX threw a large one, especially in terms of width. Again, the CEntrance focused on the music's organic qualities. It tended to connect instruments and voices in a pleasing way, in a way it often does in concert, instead of concerning itself with the spatial dissection of instrumental images. In other words, the DACmini revealed the musical forest and let the individual trees take care of themselves. My only real complaint with this approach is that front-to-back layering of the soundstage was not as engaging as I've heard from the similarly priced Benchmark DAC1 or the more expensive Bel Canto DAC 3.5VB. I found the DACmini CX to be one of the more musical DACs in its price range, but not the most revealing.