OP-AMP MEETS CD-- OIL & WATER OR MUSICAL SYNERGISM?
(also see MODIFYING YAMAHA'S CD2 and Operational Amps [1981])
If you are tuned in to the audio press, particularly the so-called 'high-end' magazines, you need not look far to find a thread of commentary generally opposed to the compact disc (CD) as a high quality medium.
Despite all its obvious good qualities (ease of handling, longer playing time, a lifetime of noise-free use, and tremendous dynamic range, to name only a few), some serious problems must exist if these reports are ac curate. Having explored the CD waters, however, I can report that I find them fine, with the exception of one general but important caveat.
Quite simply, if you expect to get high performance sound from a CD player, you should apply POOGE principles to the analog circuits (including associated power supplies, contacts, wiring, and so on). More than a few have failed to understand that these improvements using high quality components are applicable in general to amps, preamps, tuners, and yes, even in CD players. As we will see, especially in CD players.
I will not reiterate here all the de tails of selecting optimum passive components, augmenting power sup plies, and using good wiring principles. About these, I refer readers to previous articles' 1234. The basic concepts set forth in those articles remain valid, with the reminder that newer/ better parts are always appearing. I hope dialogue will continue in these pages regarding all the latest wrinkles. This article will concentrate on how op-amps and their related circuitry influence sound in CD players (as well as elsewhere, of course). If you do the “standard” POOGE updates, you will be much better equipped to appreciate the differences described below.
Over the past year, I have been working on a third edition of my book Audio IC Op Amp Applications (Howard W. Sams Co.), and have developed a number of new audio circuits using IC op amps, with both new and old devices. My involvement with CD circuitry has happened more recently, and has re minded me that many still seriously misunderstand just how good sound can be coaxed from IC op amps.
Of course, cost considerations will become an inevitable barrier, when all the electronics needed to realize a CD player are assembled. I cannot help noticing, however, that the analog portions of typical CD circuitry are quite minimal, while the associated digital system is sophisticated far beyond what we could have imagined just a few years ago.
In view of this, little wonder people are saying CD players sound fun ny when connected to a high resolution system. For anyone who cares to glance at a circuit and parts list, the homework necessary to achieve good sound has, in most cases, never been done or the analog circuits have been cost-reduced. But, worse yet, many critics knocking the CD medium are apparently not differentiating at all between the alleged pm defects of the medium, and the present aver age means of its realization.
Ask yourself these critical questions: Is what I'm hearing and don't like part of the process? (and thus doomed), or, 'Is what I'm hearing just poor execution in the playback?' (and thus fixable). Critics advance various “reasons” for the CD's malaise, from an insufficient sampling rate to excessive feedback and cheap op amps. Maybe this article, for everyone's benefit, can shed some useful light on these issues.
What can be done to fix' the analog CD player circuits? For openers, the standard POOGE one for-one substitution of higher quality components, replacing the original parts with the best available resistors, capacitors, wire and power supply improvement will certainly go a long, long way.
Is this enough? Unfortunately, the answer is no, not if you wish to hear all the recorded information on the disc. Topological changes in the circuit may be required as well.
This brings us to the topic of IC op amps, and making optimal selections for this type of circuitry. In this con text, I will treat the substitution of higher performance op amps as a topological change, rather than a one for-one component swap. This can be true, even if the change is a pin-for pin substitution. This is true because what is inside the feedback network is the focus here. The “too much feedback” criticism has valid elements, but is often an oversimplification.
Between 1977 to 1980, I was actively writing about op amp slew rate, because of its relevance to audio.
In our article 'An Overview of SID/ TIM' (Audio, Aug., 1979), which I co-authored with Mark Stephens and Craig Todd, we examined a very important concept closely related to the questions above. This concept is the amplifier's input dynamic range, or Vm, a design parameter peculiar to each op amp device, and (usually) not under control for a given application.
In other words, it is a specification which distinguishes op-amps, in the same way that maximum output swing does.
Later, in a related but more practical context, I wrote a brief insert within Jim Boak's “The Headphone Connection ” (TAA,: 3/82, p. 10), under the title 'Optimizing Op Amp Transfer Linearity.’ The first of these articles dealt with Vg in a general way as input overload related to SR.
In the second, I designed the circuit for Jim's amp which lets you manipulate Vg, external to the op amp in use. This relates also to Matti Otala's AES paper on feedback phase difficulties.
Using a mathematical equation, Vim can be related to the amplifier slew rate and gain-bandwidth as follows:
Va = SR/(cer) em)
In this relationship, SR is slew rate (in volts/uS), GBP is gain bandwidth product (in megahertz), and 2 x pi is 6.28. V_th will be in volts, and is b definition the input voltage which will cause the amplifier to slew or overload. If you work out the numbers for some of the IC op amps commonly used in CD players, such as the 4558, the 4560, the 5532 or the LM833, you will find that Vy is less than 0.1V. On the other hand, if you consider a device such as the LM318, the Vg is about ten times higher, or about 0.7V. The HA2525 or the similar 3507 have a Vy, of 1V, as do the single and dual FET types AD711 and AD712.
So what does this mean, in practice? For a given feedback hookup, the device with the higher Vg will handle signals in a more linear fashion, since it has more input “head room” when the feedback is applied.
For input signals which are on the same relative level as Vg (as in CD players), this can make a big difference in perceived performance. And, when any appreciable out-of-band than ideal situation. The op amp is pressed further and further toward its dynamic range limits, and much more so by the high frequencies.
So, here we have a classical oil/ water mix-the more limited amplifier input range at high frequencies, and the excess of high frequency garbage riding along with the CD audio signal. Both of these operating conditions are present in the CD analog processing section, where the commonly used active filter and buffer are supposed to remove the undesirable high frequency sampling components from the reconstructed audio signal.
All available CD players now have some degree of analog filtering, and all use a current-to-voltage converter op amp (sometimes within the digital-to-analog (D/A) chip, but nevertheless Still in the audio signal path).
The active filter stage often uses a voltage follower op amp. This fact, combined with the signal present, is In many minds the question now is likely to be, 'Just how much of this can we hear, if any at all?' To put this in perspective, remember that some are saying the CD is the “perfect' “ medium! In no absolute sense can I say what a given listener will hear as a result of a change from a stock op amp to one of the more linear, higher Vg units. We have too little language to completely characterize our audio experiences for each other. Nevertheless, some may perk up when a listener says, 'mow it sounds like tubes!' For whatever it is worth, I pass along what I hear when I make such changes. I hear them on my own and on other systems, and on more than one CD player, as well as in other similar circuit locations. Fellow audiophiles Don Spangler and Walt D'Ascenzo corroborate these findings as well. (Also be sure to see Hampton Childress's article in this issue).
To my ears, the extended linearity, high Va op amps allow a much greater sense of natural imaging, dynamics, overall soundstage and detail within the reproduced signal. Martin Colloms in his classic essay, “The Sound of Amplifiers” (TAA, 3/85), uses the term “dimensionality” to describe this effect.
Tubelike? Maybe, if that pleases your fancy. I like to think of it as a sound which is more like real music, but I'll admit the standard descriptors in the litany of tube buffs, “open,” 'free,' “effortless,” can (and do) apply. This suggests, doesn't it, that such sound is not the exclusive province of the thermionic valve.
Naturally, this result also requires that the rest of your system be up to snuff. Just how much dimensionality your own system can reveal depends on its overall design (as distinct from the CD player portion). The 1979 article noted above also discussed at some length Vy, as a power amp design criterion. As the signal proceeds through the chain, SR and Va should, for best resolution, in crease commensurate with level. For example, amplifiers with tube front ends can easily have a Vy, of several volts, and would be likely to reveal more dimensionality than would a low Vy transistor amp (all else being equal).
Of course, regardless of your equipment, you will always benefit from the best in evaluation recording(s). I concur generally with Hampton's choices for what is useful CD material (as well as for some great music making!).
What hallmarks are most useful in assessing the recorded sound? I find recordings with the most natural sounding acoustic environment and miking to be most valuable. Some of Charles Dutoit's Montreal Sym phony recordings are simply out standing in terms of the degree of imaging and focus apparent on individual instruments, as well as a natural sounding front-back perspective. If, while listening you can visualize a standard orchestral layout in front of you, and perceive a realistic sense of depth between the first violins and the tympani, then this says a lot about the recording's merits (and, a lot about your equipment). I find my planar Magnepans (POOGE'd, that is) have excellent depth, to the extent that the wall behind them sonically disappears. I hear a similar phenomenon on Hampton's Quad ESL-63's.
One of the more elusive sonic performance factors of the traditional RIAA preamp is its ability to re produce the sense of space and air around individual instruments. This performance factor also serves as a yardstick for CD players. I am happy to report that rumors of the CD medium's inability to reproduce such subtleties are premature.
I never cease to be impressed in these upgrade exercises with the ability of a high resolution system to ex tract more and more detail from a given recording, when a single component within it is changed for a better one. For example, with the Solti/ Chicago Symphony recording of Mahler's Fifth Symphony ( London 414 321-2, and one I heartily recommend), more and more sonic details emerge with each upgrade. The amount of information contained in this particular recording is, at times, almost overwhelming when it is fully revealed. This detail is all the more fascinating considering how well a truly good CD player can allow you to follow the individual musical line of your choice. Of course, this is the kind of experience you can always enjoy in the concert hall with a live performance, but how many times can we realize it at home, due to equipment and/or recording limitations? Perhaps this is an appropriate place to put the message in another perspective. It is easy to be confused about the nature and sources of the “sounds” which emerge from a CD player (other than music, that is).
This confusion could be what underlies the great variety of critical opinion about "CD sound." Many components which are inappropriate for high quality audio will be immediately evident to those tuned in to their particular non-musical signatures. For example, if we listen to a stock Magnavox model 2040 CD player in a revealing system we can hear the harsh sound of high-K ceramic caps. One look inside quickly reveals their presence. An out-of the-box 2040 playing a CD with ample high frequency energy (such as the Christopher Hogwood/Academy of Ancient Music rendering of Mozart's Eine kliene Nachtmusik, L'Oiseau-Lyre 411 720-2) can be troublesome. Indeed, it can be to an extent that makes it easy for me to understand why so many are upset with “CD sound.” Ah, but are we about to throw-out the-baby-with-the-bathwater, if we mistakenly write off the medium based on such experiences? A thorough cleanup of the player with standard POOGE techniques can eliminate virtually all traces of this CD's harshness making it sound great ( Hampton gives other examples, on this point).
Given this framework, a change to higher performance op am in a CD player should result in a fundamentally different sonic upgrade with an improved sound character. Of course, different people may hear slightly more or less of the “dimensionality ” characteristic, and may describe it with other choices of words (semantics being what they are). But, the main point I'd like to make is that the op amp change is not likely to be perceivable in a manner as are the other sonic functions. It will be more associated with imaging, dynamics, soundstage, and so on. Voila, dimensionality is enhanced, in a manner which I find to be consistently dramatic.
Exchanging the oft-used dual 8-pin op amps for singles will cost the amateur some extra work, since two of the suggested high Va units are both singles. A number of available FET dual op-amps are good performers, with high Va. You can make a choice for yourself here, between all out sonics at somewhat higher cost and trouble, and slightly lesser sonics with more convenience. The AD712 (dual) will drop right into many CD player layouts (i.e., Magnavox), giving a dramatic improvement for minimal effort.
Upgrading only your player's op amp to one of the above mentioned types may not make it 'perfect' (there being other factors as alluded to earlier). But, let there be no mis take, it is well worth the time and effort.
Listening is the final arbiter, even for the higher performance op amp units. In this article I purposely make no attempt to address another current CD controversy, namely phase correction for the A/D anti-aliasing filter. My idea is to concentrate on get ting the basic D/A circuitry cleaned up first.
Finally, a note on my little servo circuit Hampton used in his Yamaha player, which is a small preview from my forthcoming Audio IC Op Amp Applications. This is a non-inverting integrator, and must be used around an overall inverting gain stage(s). It can be adapted to other CD players, with results likely to be as good as with the Yamaha. I second Hampton's comments on its virtues, vis-a vis the best capacitors I've ever heard.
It is well the trouble to find room for, but do use nothing but the best parts in building it.
I will be interested to see how readers follow up on these suggestions, and report back on their own experiences. I hope we will be able to see more reports of CD enthusiasm, as opposed to the doldrums of the past. For now, I will be enjoying some new CDs, and hearing a lot of things I never heard before from vinyl records. Go forth and upgrade your CD machine with some POOGE quality parts, some musically synergistic op amps, and enjoy. And watch these pages for more on the fun story of “Op Amp Meets CD. ”
ABOUT THE AUTHOR: Walt Jung's articles are well-known to many TAA readers. Most noteworthy include the 1977 slewing distortion series, co-authored with Mark Stephens and Craig Todd; the PAT-5 mod series, co-authored with Dave White; and more recently, the POOGE-2 effort, co-authored with Dick Marsh. Walt is also the author of several books. His “IC Op Amp Cookbook” was recently updated to the third edition, while his “Audio IC Op Amp Applications ” will soon be reincarnated. Somewhat of an audio low pro file during the past few years, Walt recently had his aural sensitivities rejuvenated, thanks to the CD medium. This issue marks his return to TAA as a Contributing Editor.
TABLE 1
1. LM318. National Semiconductor Corp. 2900 Semiconductor Dr. Santa Clara, CA 95051
2. HA2525. Harris Semiconductor PO Box 883 Melbourne, FL 32901.
3. 3507. Burr-Brown Research Corp. 6730 S. Tucson Blvd. Tucson, AZ 85734
4. AD711, AD712. Analog Devices Rte. 1 Industrial Park Norwood, MA 02062
ACKNOWLEDGMENTS
My sincere thanks to fellow audiophiles and POOGE buddies Hampton Childress, Don Spangler, and Walt D'Ascenzo for their many helpful comments along the way toward realization of this article.
By Walt Jung.
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Also see:
A BROADBAND SIGNAL SNIFFER, By Benjamin L. Poehland