Greg Smith
January 1997
I'd really like to keep my audio system simple. But certain things resist simplification. Take the preamplifier. Years ago, I tried to get rid of my preamp by getting a CD player with a variable output on it. Conceptually, it was a superior approach; remove that whole unit from the system, and replace it with a simple circuit at the back-end of the player's analog output. That removes a whole component from the signal path, and a set of cables, too. Fact was, though, my system sucked when configured that way. The output on the CD player just didn't have the muscle to drive the power amplifier correctly. Putting the preamplifier back in the circuit made the player's job easier by presenting it with an easier load to drive, and it produced a considerably more stable output to feed the power amp with.
I find myself in a similar situation with outboard digital/analog converters (or DACs as the acronym laden like to call them). My gut reaction to the whole idea was that adding yet another piece of equipment was bound to be trouble. After all, my players already have good D/A systems in them, right? Why bother with the additional complexity and the potential problems it could introduce? Well, I've learned before that just because something doesn't seem totally sensible, that doesn't mean it isn't true. Adding an external DAC can be one of the most cost effective ways to upgrade an entry level system. This month, we'll look at one product worth considering, the CAL Gamma, and dispel some of the nonsense about these units that floats around.
Compact discs are recorded digitally; everybody knows that much. Your usual CD player produces music initially by bouncing light around to read the data on the disc and decoding it to a stream of bits. The result is 16 bits of digital data for each channel, 44,100 times a second. Typically, a couple of chips on the player covert this collection of bits into an analog output that gets updated every time a new data sample is read (a range of about two volts for this output is the norm). Some players let you circumvent this final stage by copying the data to a digital output somewhere. CD units with a digital out can be referred to as transports, with the distinction being that a transport doesn't necessarily need a regular analog output.
There are two standards commonly used for outputting digital data to another component. S/PDIF (Sony/Phillips Digital Interface Format) is the most popular for consumer equipment, while some AES/EBU (Audio Engineering Society/European Broadcast Union) interfaces have trickled down from professional units. These standards are implemented with a variety of connectors. The cheapest type connection is referred to as a Toslink. This is an optical connection, where the data is transmitted by light pulses traveling through a plastic cable; the connector is an odd six-sided thing, and units are shipped with a plastic plug in the jack you have to pull out (and immediately lose, in my experience) before you can utilize it. The most popular connection type is a S/PDIF coaxial one, where an electrical signal is transmitted with a regular looking cable using RCA connectors (it looks like an analog output jack, but don't go using a regular audio cable there). Occasionally you'll find coax using a BNC connector.
High-end equipment often implements the AES/EBU interface with an electrical XLR interface (that's the circular, 3-pin plug audiophiles normally see used for balanced audio connections). The other expensive system is the AT&T glass- fiber optical, which connects with the ST-type bayonet. Because they're complicated to implement, equipment with AES/EBU or AT&T style outputs typically are priced with at least a $200 premium each (adding these connectors is often optional). Accordingly, you're not likely to find either on entry level equipment, so I'm not going to discuss them extensively here. In the inexpensive realm, when you hear "optical", it means Toslink, not AT&T.
Why all the different types of connections? Well, each has its good and bad points. Some manufacturers aren't content with any of these four common systems, and implement their own setup. Theta Digital, for example, uses something they call "Laserlinque" as an option on their equipment, while Audio Alchemy has the I2S bus on their more upscale units.
Here's a quick ranking. Toslink is universally panned as the worst digital connection scheme, but it's cheap to implement. Coax offers solid performance at a reasonable cost. Both AES/EBU and AT&T are slightly superior to a good coax implementation, and which of those two is actually top dog is an endless debating point. You'll have to pay a bundle for either one of them, though. The proprietary schemes are always claimed to be superior to any of the four regular types, which may or may not be true; I certainly don't know. The generally accepted reason given for why all these schemes sound different is that they have different amounts of jitter.
Talking about jitter is a good way to ignite most audiophile gatherings. The concept is simple. A digital source (be it CD, laserdisc, DAT, etc.) produces its sound by outputting new samples from the digital data thousands of times per second. Inside the unit, there is a clock circuit that times this output, typically implemented with a clock crystal and some additional digital logic. You can think of it as being a sort of metronome, where every beat results in a new slice of the music being released. That crystal and associated circuitry isn't perfect. The new sample invariably is produced slightly before or after the theoretically correct timing. This variation between the perfect timing and the actual clock signal is referred to as jitter. Depending on who you believe, too much jitter can make your midrange harsh, destroy your imaging, and produce baldness (unless used with Joly speakers, which cancel out the effect).
Jitter is everywhere. The master clock that was used to cut the CDs you listen to introduced jitter into the pattern of pits that make up the surface of the disc. Your CD player clock is jittered. And if you use an outboard DAC, that whole digital interface adds some jitter to the whole works as well. The main thing you're getting from the more expensive digital interfaces is better timing mechanisms that control jitter and accordingly are more faithful to the original source. A number of companies sell little boxes that go between your digital source and your DAC to perform a function called re-clocking. This synchronizes the bits flowing through the system to a (hopefully) more accurate clock than the one your source uses. One inexpensive product I hear good things about is the Theta TLC, which for $200 will not only reduce jitter but let you convert Toslink to coax if the DAC you're considering only has a coax input. You can go crazy trying to reduce the jitter in your system, but you can never eliminate it.
Since there's no way for the typical consumer to know how much jitter they are suffering from, I'll choose to ignore further mention of it in this column. It's not worth worrying about for the normal entry level buyer. The main reason I brought it up was because the digital elitists like to make all sort of high-and-mighty proclamations about what's good and what's bad based on measurements of things like jitter. Read just about any discussion of budget DACs, and you'll hear that the Toslink interface is so inferior to the coax that it's not even worth using. Inferior I'll agree with, but I have to wonder if any of these people have actually tried it out. Based on my tests of budget equipment, the difference between the two is minimal in a modestly priced system. If you've got a less than wonderful CD player, using a good outboard DAC is potentially an incredible upgrade regardless of whether you have a Toslink or coax digital output. Sure, given a choice, I'd prefer to use coax, but the optical Toslink interface is certainly not useless. More on this as we go along.
Without further background, let's dive into discussing the tests I've been doing. First, a bit more about the DAC.
External digital/analog converter. 7x2x6 3/4". RCA Coaxial and Toslink inputs., RCA analog output. $295.
The CAL Gamma is a small unit that can easily fit on top of some other component in your system. Having read about the unit a bunch, I stopped by my dealer one day to find one sitting in their demo system. Since they happened to have the CD player I use most of the time in the store, I got them to rearrange things to use that player as a transport and compare the built-in player DAC with the outboard unit. I was impressed enough to want to buy one, but they had none left in stock. Not one to be so easily thwarted, I took home the store demo unit, which has been stuck in my system ever since.
I've got two CD players I commonly use. The Rotel RCC-940AX changer is the main unit that racks up play time; it's got a Toslink output. This one is fairly new, and has pretty good sound for a $500 unit. I've also acquired a Rotel RCD- 955AX, which sports a coax interface. This was also about $500 originally, but it's a much older unit which doesn't sound quite as good as the 940AX normally; I blame this on the older DAC in the player.
Using the changer with its Toslink output, I compared the outboard DAC with the regular analog output. The Gamma was a considerable step up from the stock player. The main difference was in treble resolution. There was dramatically more detail in the treble with the CAL unit. The thing that got me to buy the unit in the store was playing a recording that, come to find, had a soft bell ringing in the background. This was almost inaudible on my player normally, but using the better Gamma DAC brought it right out. At the same time, the output sounded quite a bit less harsh, less "digital". Typically, the biggest flaw found in cheap digital products is a grating treble that's far too forward. While my Rotel unit doesn't suffer from this nearly as bad as most less expensive units, there's still a trace of it. With the Gamma in place, it's only the most obnoxiously forward of recordings that still seem harsh. I've never heard any sub-$300 upgrade that is as effective at taming CD brightness while keeping an accurate treble as this CAL unit is. And this is with a Toslink connection, mind you, commonly derided as not being any good. I use a Monster Cable optical cable that goes for about $30; there aren't many other people making Toslink cables.
Encouraged, I picked up an Audioquest Digital Two coax cable (about $75) and tried out my other player. The Rotel 955 is representative of a lot of older players that are still good transports, even if their D/A sections appear a big dated at this point. The Gamma proved to be a big upgrade for this unit as well, for most of the same reasons mentioned above.
As far as comparing the interfaces go, it was a mixed bag. The Toslink interfaced player did a better job at resolving the top-end. I could hear how the cymbals were being brushed in some recordings; the coax interfaced player didn't present that with nearly as much realism. On the other hand, the bass was more controlled and powerful with the coax linked player. This is especially interesting because its exactly the reverse of how the stock players sound--it's normally the RCC-940, with the Toslink output, that has better bass but worse treble detail. With these two players, I'd call the comparison a draw, with each having very slight advantages over the other. To put this in perspective, I'd say the difference in output quality between the transports was of approximately the same magnitude as a minor cable change, while the difference between the Gamma and the stock units was equal to a major cable or component upgrade. And the Toslink interface proved to be quite adequate in the context of my budget system.
The cable used for coax interfacing isn't all that complicated, as cables go. Transporting binary data, even at high speeds, is one of those things that you should be able to do inexpensively. The cable used for typical coax DAC applications is fairly standard 75 ohm video cable, with RCA plugs. Rooting around at Radio Shack, I found at least two ways to put together such a cable cheap. One option is to take a standard 75 ohm cable with F plugs on it (that's the connector usually seen for antenna and VCR connections), like #15-1533 at $2.69 (you probably have some of these around already), and get the F to phono plug adapter #278- 252 at $1.99 for each end. Total cost is at most $6.67 for this route. Another option is to get the A/V cable with RCA jacks on it already; this is #15-1538 for $2.99. I'm not totally certain that this one is actually a real 75 ohm cable, though. In any case, I tried the $75 Audioquest, the $6.67 contraption with the adapters, and the $2.99 video cable, and they all sounded exactly the same in my limited testing with this DAC. Further investigation is needed, but at the moment I'd recommend that the budget conscious buyer looking for coax digital hookups try shopping at the Shack before dropping real money on such a cable. The CAL has a useful feature where the front panel "California Audio Labs" lights up when the unit has locked onto a digital signal, which lets you test if you've got a good connection without even generating sound.
Now, I know that many of you don't necessarily have recent players drawn from the $500 price range. So I rounded up some other players to try out as well. Contestant number #1 is a Sony CDP-C515. This is a 5 disc changer with Toslink output, circa 1991, and about $250 at the time. Sony still makes players just like this one today, and although I'd like to think that they are at least slightly better now, I wouldn't lay money on it. The stock output is always harsh, and, in the wrong system (i.e. almost any inexpensive setup it would normally be mated with), can produce piercing sibilants. Plus, recordings lose their sense of space; everything sounds as if it were recorded in a closet. This is a typical mid-fi unit in just about every way, but the digital output on the back offers potential salvation. Hooking it up to my system, the sound was transformed by the Gamma. It easily was better, say, than the stock converters in either of my Rotel players. Unfortunately, the Sony had some limits even being used only as a transport. The playback never was quite as engaging as when I was using the Rotel units as digital source. It was weaker than either of those units in every way, from bass to treble to soundstage.
Let's do the math, though. $250 mid-fi Sony player plus $295 Gamma and $30 Toslink cable made a combination that easily whipped my $500 Rotel units. For someone currently living with such an inexpensive player with digital output, the Gamma would be an excellent move upward. You could always upgrade your player to something that would serve as a better transport in the future.
For our next contestant, I went digging through the dust to locate a Magnavox CDB-650. This was one of the first digital players that people actually thought was usable from a sound quality basis, circa 1986. Look at the back, and sure enough there's a coax output there. The sound from the stock player is, at this point in time, hardly worth even commenting on; it's just boring. Perhaps it might still be salvageable as a transport, though. Hooking it up to the Gamma was disappointing. There was actually a hint of glare to the treble in spots that I'd never heard out of that converter before. Playing recordings with lots of space left me wandering where the room echo went. I really can't recommend this combination--if your player is this old, I'd have to recommend you give it up already and get a new one before you drop any more money in your system. Simply adding a outboard DAC is probably not sufficient to save it.
Stepping back from strictly looking at its audio performance, the Gamma is certainly not perfect. One thing that concerns me is that those of us who frequently switch between the coaxial and optical sources get an incredibly loud popping noise as a reward. This isn't an issue for most people, who only have one player they are hooking up, but it did bug me. More of an issue is that I found the Gamma to be noisy. When I use any of my CD players normally, I can crank the volume control all the way to the max and only hear a hint of background noise hiss. With the CAL attached, I could hear faint noise even with the volume at normal listening levels. It wasn't often noticeably audible with typical music playing, but it was still there messing around with the sound to some extent.
Since I bought my unit from the demo stock, I didn't actually get my manual until a few weeks later. This may have been a good thing, because if you read that manual you get the impression that the Gamma is the most fragile thing on the planet; I'd have been worried when I first hooked it up. There are endless warning about not changing anything around while the unit is on, along with warnings I normally only see on computer equipment about things like static electricity. I never had a hint of a problem with the unit operating properly, but you don't get the impression that this is a unit suitable for industrial use from the documentation.
While I'm discussing odd features, one nice on this unit is the inclusion of a polarity inversion switch on the front. This lets you experiment with absolute polarity without rewiring your whole system. I found this feature less than useful last time I tried it, but who knows how important it may become to me in the future. It's nice to have it available to try out. The inverter switch suffers from the same loud pop as the other switches on the unit.
As you have probably figured out, I like the CAL Gamma quite a bit. The CD player is often the weakest link in budget systems, and no improvements you can make in the rest of your system will let you recover pieces of the music that are lost by crummy D/A conversion. The Gamma is a cost- effective upgrade for many inexpensive CD setups with a digital output, although you will in most cases be limited in ultimate sound quality by what you're using as a transport. For most people, I'd recommend waiting until your CD player wears out before you replace it; that seems to happen all too frequently, anyway.
The more interesting question is what to do if you are buying a new configuration altogether. Is it better to buy, say, a $300 CD player and the $295 Gamma (or similar budget DAC units from other manufacturers like Audio Alchemy), or would your money be better spent buying a $600 player and avoiding the second box altogether? I couldn't answer that question without trying out a lot more players, but at the moment I'm leaning toward separate units myself. Lately I've been recommending that people trying to upgrade or construct entry level systems consider getting an inexpensive but usable CD player for use as a transport. Typically, you can find units from companies like Marantz (who have reintroduced the now discontinued Phillips units I used to recommend under their name), NAD, and Harmon-Kardon for about $300 with a coax digital output. Even though I don't think Toslink is nearly as despicable as most of the audio press does, I will still recommend that those getting a new player lean toward getting a unit with the coax output if possible.
Regardless of the nit-picking, the great thing here is that the level of digital performance from recent units like the CAL Gamma comes pretty close to what you'd expect from much more expensive high-end units for a very reasonable price tag. I'd recommend doing some more investigation into the whole DAC subject before you buy anything, though. CAL has a home page up for more information about their equipment. For another view of this particular component, with some more details about the innards, read Todd Warnke's CAL Gamma review. Other useful things from the Soundstage! archives include Doug Schneider's article on coax cables and his review of the chief competition for the Gamma, the Audio Alchemy DDE V1.2.
.....GS