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Will digital ever sound like analog? Development
of the Zen Triode NOS DAC
Aug 2009 Of course CD players have advantages and disadvantages that we're all well familiar with... Sometimes the disadvantages can overpower common sense resulting in total destruction of your player as shown below. Sometimes common sense can have the same result, sparing you from any further suffering, as was the case here.
Can digital sound like analog? If we use the CD player shown above (before I smashed it) and a hi-end vinyl rig (shown below) for the comparison then the answer biases itself heavily towards NO, CD's can't sound exactly like analog. But while CD's and digital are one in the same to most people, CD's are nothing more than plastic disks that are largely responsible for the huge gap in sound quality when compared to vinyl. This can easily lead an audiophile to hate all things digital, even conclude that the format itself is worthless. I have been guilty of this myself. The problem with this conclusion comes into focus when you listen to well recorded LP's on the turntable and the thought "analog kills digital" races through your mind followed by the realization that the LP you just listened to was mastered in 20 bit digital back in the 70's. IS IT ANALOG OR DIGITAL? Obviously it sounds pretty damn analog coming off the turntable, yet it was a digitally mastered recording. So how can the LP sound so much better than say the same recording dumped onto a CD? It's a combination of things. The tonearm and cartridge have their own colorations, usually desirable ones, the RIAA equalization curve has it's own interplay on the dynamics resulting in a certain signature which I'll get into in a bit. The high end vinyl rig is a superior playback device when combined with a hi-end phono stage even if there were no colorations of any kind because there is no Digital to Analog Conversion process happening. Also popularly known as the DAC, this process falls far short of having the same end result as when the process does not need to be used. IMPROVEMENTS ? So the focus has been on improving the DAC since it came out. BTW, remember that the Analog to Digital process (ADC) is not the same as the DAC process. This is part of the reason why the LP's that were digitally mastered didn't sound completely screwed up. For playback the "Redbook" format became the standard for consumer grade CD's and players that was 44.1KHZ and 16 bit, often written as 44/16, even though it was below the current technology of the day. Of course later we started to see 24/96 and various products that would upsample the data to 24 bit 192 KHZ. Each of these increases puts more points on the line making the top end sound smoother and more natural or phrased differently, less grainy sounding. Interestingly enough, being old enough to have been listening to music back when there was no such thing as a CD, it's been my experience that even these higher sampling rates do not make the CD sound like real music as was often the result with LP's. CD's always have that "CD" sound, where the music sounds like plastic. I'll admit that it has more than irritated me over the years that these increased sampling rates are not solving the problem and that I basically could easily live in a world without CD's. My biggest bitch with CD's is that it always seems to take something like a Wadia DAC and matching transport to get the CD's to sound acceptable. Yet you can set up an entry level vinyl rig for a grand or two and have potentially better sound. I don't remember being told when CD players came out, that it would require you to spend 10 times more money than you have in your turntable to get as good a sound, yet here we are some thirty years later and that is still the norm. TRANSPORT BLUES CD's require transports with lasers to read the data. This process is full of errors in most consumer grade products. Even in a good transport, the CD itself will have errors and scratches that cause more errors. This digital data stream from the transport goes into the DAC, where it's stored as read, errors and all, followed by the DAC telling the transport to read it again and again. This is called oversampling. Each stored package of data is then processed and combined together in an attempt to figure out what to put in place of the errors. So we've got two rather negative things happening in the playback process, jitter from the transport that creates errors and oversampling and processing that is done in the DAC. TOO EXPENSIVE This isn't to encouraging to a guy who's not made of money, because the only way to get less jitter is to buy a really good transport. Why even bother buying a really good DAC if you can't also spend the same amount of money on a transport? You'll just be feeding the DAC tons of errors, and the resulting sound from the DAC won't be that much better than the CD player you tried to use for a transport. NEW HOPE FOR THE LITTLE GUY Two things have happened in the last 10 years that when combined may bring some real hope to all of us who find something almost subliminally irritating about the sound from CD's. One is computers, and one is a man called Ryohei Kusunoki who wrote a paper in 1996 about a DAC concept where oversampling and fir filters are eliminated. My only regret is that I didn't run into his paper sooner. This started what is popularly known today as Non-Oversampling DACs, of which there are many available be they still preached against by most engineers. THE COMPUTER By using the computer hard drive to store digital music we now have the ability to store error free files thanks to programs like EAC (Exact Audio Copy) available free on the web. These programs will read the data from the disk and compare it against an international database of CD's to see if each track from the CD is now an error free copy on the hard drive. You'll be shocked at how many errors are on your CD's, but this type of program will read the disk millions of times if need be to get an error free read. USB PORT Disabling the Windows Kernal that handles the sound buss in a PC makes it possible to send a bit for bit output direct to the USB port on your computer. That means that if you get a USB DAC with a good reclocking scheme and super accurate clocks you can pass almost error free data to the DAC. This would be an ideal alternative to spending thousands of dollars on a hi-end transport. When you combine this with a DAC that's non-oversampling you've got a listenable source that no longer sounds like plastic so this is the approach I have taken. Since that time my focus has been on the NOS DAC technology itself to figure out what the best implementation of it is. And since USB has this potential, what chipset's and re-clocking schemes will give the best result. We already know that many early USB sound devices had issues that resulted in less than stellar error performance so this part obviously has to be right. THE ZEN TRIODE NOS DAC APPROACH Since the rest of this paper will focus on my DAC and output stage design and since it's a non-oversampling DAC I want you to understand why oversampling and fir filters found in almost all of today's DACs are poison to music playback. I've been asked when or if Decware would come out with a Zen DAC for the past 10 years and until I heard this poison removed from the playback I had no intension of pursuing any type of Decware DAC... basically just washed my hands of the whole thing. For me hearing my first couple NOS dacs left me realizing two things; A) There is twice as much information on a standard redbook format as I would have ever guessed, and while not an LP, we suddenly have usable playback. B) The timing and pitch and timbre are now correct, hence the plastic sound is reduced to the point of being no longer irritating, if even on a subconscious level. MAKE CD SOUND LIKE VINYL - NO It
doesn't have to sound like a record to be good, and
that's my contention. All I've ever wanted was
to get digital sound to be a usable source in my listening
room and it's not usable if there's a subliminal and
audible irritation factor in listening to it. Digital,
even poisoned, has always had certain things that could
be argued were better than LP's. Let's get it
to sound like real music with the proper weight and
interplay of dynamics throughout the frequency range
and you'll see the listening experience will become
enjoyable like it used to be when vinyl and tape was
all we had. This all starts with a non-oversampling,
non-filtered DAC design so I refer you to summary of
Kusunoki's excellent paper on the subject: Non-oversampling SOME ENGINEERS You'll conclude after reading his paper that during the development of DACs engineers came to a fork in the road where they had to choose between frequency domain or time domain. Since they could only have one of the two, they decided that screwing up the time domain was the lessor of two evils. However, the only thing wrong with the frequency domain was that it contained noise and garbage above the 20 kHz hearing limit of our ears, in some cases out to several hundred kilohertz. That would show up on any decent scope and on any frequency response test and since the whole push with CD's in the first place was less distortion they couldn't bear to have it there despite the fact that we can't hear it. We can hear changes to the time domain but either it went unnoticed or the politics of having clean specifications on paper left no alternative. EARS OVER RULE THE EYES I'm certainly not bothered by content above the 20kHz limit, because with my tube gear it's not going to irritate my amplifiers, my speakers or my ears. The amp will filter it away, as will the speakers that can't reproduce it even if it is there, and of course my ears which can't hear anything above 20kHz. A well designed analog filter could be used with minimal negatives making it compatible with any amplifier or preamp. TAKES MORE THAN JUST THE DAC But the DAC itself is only part of the equation. The analog output stage(s) and the impedance/gain relationships between them will determine how real the output sounds. This is one reason why a vinyl rig always sounds different than the average DAC. Most of the time, the output stage of any given DAC is designed to accomplish a 2 volt output with the lowest possible distortion, period. In the case of a vinyl rig, you have the tiny signal from the cartridge of just a couple millivolts that has no bass in it. The phono stage uses the industry standard RIAA equalization curve to put the bass back in and add enough gain to bring the output level up to a couple volts. This ends up being somewhere between 40 and 60dB of gain, more than any other part of your playback system by a huge amount. The phono stage then drives a preamp that in turn drives your amplifier. This gain from the phono stage is controlled by the preamp, in most cases reduced to achieve the desired listening level. If you could somehow get the sound to come out of the preamp without the phono stage driving it, it would not sound the same. You can think of gain as voltage or pressure. By creating a high pressure with the phono stage against the front door of the preamp, the signal enters the preamp with purpose. If you reduced the pressure from the phono stage by say half, the signal entering the preamp would have less pressure behind it and the preamp would have to add gain to increase the signal. Since the signal entering the front door of the preamp was somewhat whippy when the pressure was reduced by half, the preamp simply made the whippy sound louder. The end result is playback with a lack of dynamics, weight, and warmth. COMBINATIONS Playing with different cartridges and phono stage / preamp combinations made it possible for the audiophile to get this sense of liveliness and energy with the correct frequency balance and weight. To replace this with a CD player/DAC removes the cartidge and phono stage leaving the preamp to deal with a specified output from the DAC of 2 volts with no pre gain or post gain strategies to control the dynamic interplay between the source and the amplifier. So just having a DAC by itself will depend on the type of preamp and amplifier you have and if there is any synergy between those components. COMPONENT SYNERGY Synergy between components is partially dictated by impedances, such as the output impedance of one gain stage against the input impedance of the following stage. And of course the pressures developed in each stage. Left to chance it's a guessing game what component combinations will have a synergy similar to the phono stage/preamp combination. I'd like to involve pre and post gain strategies with the output of the DAC so that nothing is left to chance and the level of control is there to achieve the signature and presentation that best serves the music. After all, musicality is what brings long term satisfaction from good audio gear. I always enjoyed being able to adjust the signature of my vinyl rig via fine adjustments to the tonearm and by adjusting the output level from the phone stage against the input level of the preamp to get the right tone and presence. I also enjoyed using two or three different cartridge/tonearm combinations for different recordings. One was great for rock, one was unsurpassed on female vocals, and one was great on everything else. THREE BLOCK DESIGN It would be nice to maintain this kind of control if we're going to get a similar kind of enjoyment from digital, so the output stages in this design are absolutely paramount to how good the DAC itself is going to sound. To achieve this I have experimented with countless combinations and come up with a three block system, with four stages of gain. The first block will be the output from the DAC chips themselves which will be the equivelant of a single stage of gain with a 2 volt output and low impedance. The second block will be a de-coupled single triode gain stage with it's own tube regulated power supply. The third block will be a de-coupled solid state gain stage with two stages driven by a large self charging battery supply. This brings in the opportunity to manipulate the interstage pressures and impedances in front of and between the two final stages. The output from this final stage can be de-coupled or direct-coupled to drive the amp or preamp of your choice. This is where you'll be able to adjust the inner dynamics and presence of the music similar to the way small adjustments to your tonearm and cartridge changed the sound of your vinyl playback. Having this three block design makes it possible to change the analog output from the unit so that it comes directly off the DAC chip, or the tube stage, or the final stage. Both the tube stage and the final stage have external gain controls, aka volume knobs, that allow you to play one control against another when all three blocks are used. This will let you adjust for the proper weight and frequency balance when you get recordings that are too thin or too fat to be useful. Of course some might say you can do the same thing with a bass and treble control, but why kill the transparency when you can make it happen naturally by manipulating impedance. You can also adjust both volumes to the same output as the DAC chips and seamlessly go back and forth between the three while you listen. All three outputs sound very different from each other, to the same degree that phono cartridges sound different from each other. GOAL If the goal was to get the sound as close to vinyl as possible a reverse RIAA passive network would have to be added to the output of the DAC chips and then a 47dB gain stage with an RIAA network would have to be used in front of the second block, the tube stage. This would give the unit a vinyl signature, duplicating the bass and shimmer of LP's almost convincingly. But with two networks and two more gain stages or basically the equivalent of a phono stage with a passive reverse RIAA on it's input, we trade transparency for a less resolved but juicer sound. It would sound like a low grade cartridge lacking detail and air. My goal is not to make the DAC sound like a vinyl rig, but rather get the digital music free of time domain distortions that create that plastic sound AND make it usable by adding a considerable degree of control over all the subtleties, similar to what I can do with a good vinyl rig. PROS & CONS My implementation of all this has pros and cons. Pretty much all pros for the buyer, but serious cons during assembly. A single rack mount chassis houses the three main components and their individual power supplies all mechanically decoupled from the main chassis and hand wired to the mryiad of led's and switches. The build is brutle and after the first production prototype I didn't want to ever build another one. The solution to this is of course re-design the chassis to accommodate all three main blocks and their power supplies, switches, jacks, connectors, and lights on a single circuit board. This would make building it take almost 30 minutes instead of the 30 hours it actually takes. But there would be no vibration control possible, and singles sound far better going through silver wire than they do through circuit board traces. I just can't see it sounding as good, not to mention the size of the board could invite damage from cracks during shipping. COSTS Besides the technical downfalls of doing the whole thing on one giant circuit board and wanting to get the best sound I can out of this thing, I know it's going to be expensive to hand produce and I'll likely have to make all of these myself personally. So with that in mind, this DAC is no help to the guys who can't afford it. My plan would be to follow this model with a two block system using the first two blocks of this design. Then eventually a one block system using the first block in this design. This would hit three substantially different price points, and it's possible to use only the first block of this system with a good tube preamplifier and have something good enough to bring big smiles. FEATURES AND SPECIFICS BLOCK ONE - THE NOS USB DAC DESIGN I've always enjoyed the transparency you get directly off a DAC chip before you feed it into analog filters and output stage. Problem is the output of the chip is too low to be of much use by itself. The output impedance is too high to drive anything with real authority other than an ideal load. Somehow I wanted to have the option to hear the raw output off the DAC chip but with the same output and low impedance you would normally get from the output stage. The best solution for this seems to be the Texas Instruments 1543 chip. By itself it will give about a volt with the ability to drive an amplifier directly with pleasing results. This is about a 20 year old chip design, still in production that represents the most straight forward DAC design I've been able to find so far. I don't want the DAC to have DSP capabilities or anything in it that is not being used. When you parallel these chips together the output impedance gets lower and the power output gets higher. You also reduce errors, because if one chip has an error and the other ones don't, there is no error. This increases the true bit rate over a single chip. I've spent some time listening to the chip by itself, and in groups of 4, 8 and 12 chips in parallel. Of course you can go much higher, but money doesn't grow on trees and the biggest performance change seems to come from comparing one chip to eight. So at this point, I'm using 8x1543 NOS with no output stage for block one. The Inputs are USB, coax, optical I2S to the 1543 chips. Both the USB and Coax/Optical are independently re-clocked with 1PPM TXCO clocks. Input frequency will be USB 32kHZ, 44.1kHz, 48kHz and coaxial & toslink 32kHZ, 44.1kHz, 48kHz, 96kHz. The paralleled 1543 chips will be at 2 volts with no fir filters or output stage. BLOCK TWO - THE TRIODE GAIN STAGE The triode gain stage will consist of a 12AU7a tube being fed by a tube regulated power supply with a starved AC filament. It's lower voltages will increase tube life and just accommodate the 2 volt voltage swings from the DAC. It will increase the voltage to around 10 volts with a variable output that lets you control the gain or loss from this stage to any level you want. The tube regulation in the power supply is done with an OC2 tube that is in series with the high voltage in place of a choke or dropping resistor. This has the effect of isolating the power supply from the audio circuit and filters off all the nasty harmonics off the power grid that came in through the power cord. BLOCK THREE - THE FINAL STAGE The final stage consists of a pair of Burr Brown OP amps being fed by an isolated DC battery supply for the ultimate in clean power. The battery offers some 400 hours of use on a single charge and re-charges itself when the unit is turned off. This block features an adjustable input impedance, and input gain control, defeatable crossfeed for headphone use, a Hi-Z direct coupled output or Lo-Z de-coupled output, and an inter-stage gain switch. It's noise spec is insanely low and it will drive world class headphones from 32 ohms to 600 ohms and a line level output into impedances up to 100K at the same time with world class sound quality. You can select one two or all three blocks at the same time on the fly while you listen. Using two or all three blocks gives you volume control and becomes your preamp and headphone amp all in one. You will also be able to select from two sources, the DAC and an AUX line input in place of the DAC. This makes it pretty easy to eliminate your preamp in most cases, reducing interconnect cables and further clutter in the signal path between the DAC and the amplifier. The reduced voltage tube stage stays on all the time to increase the tube life of both the 12Au7a and the OC2 regulator tubes. You can select between Coax or Toslink operation, or use USB which takes priority when hooked up. There is no remote to control volume because the further complexity and cost just aren't justified and likely wouldn't help the sound quality anyway. It is possible to remote control the volume and track selection on the computer if you use it's USB or digital output. Here's a shot of the unit on my bench. -Steve Deckert
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