max@trinity.uucp (Max Hauser) (05/23/88)
[Hope no one gets this twice -- original posting vanished into the aether] This was drafted 12/87 in response to a posting of Bill Mayhew's, and it became a mini-treatise, including a dogmatic-audiophile anecdote. Having stumbled on it again I belatedly post. Topics: merits of off-chip op amps; stereo separation issue (and anecdote); frequency-dependent nonlinearity. 1. Merits of off-chip op amps for audio circuits. From: wtm@neoucom.UUCP (Bill Mayhew) Subject: Re: NAK decks (degraded sound) Date: 5 Dec 87 17:06:35 GMT > ...One of the better things about the [Teac] C3RX is that its Dbx > encoder / decoder is built from "discrete" ICs using TL072s. > (Not exactly a terrible op-amp.) The tracking using a > non-monolithic Dbx circuit is much better. Background first. The (Texas Instruments) TL07X series is representative of the "mixed-process" monolithic op amps that emerged in the mid-1970s as low-price, relatively high-performance successors to all-bipolar chips like the (Fairchild) 741 and (National) LM 101. Texas Instruments (TL06X/TL07X/TL08X) and National Semiconductor (LFX55/LFX56/LFX57) used modified bipolar fabrication processes that allowed good-quality junction FETs also, while RCA (CA31X0 series) used an MOS/bipolar process. All aimed to deploy field-effect transistors where advantageous in the op amps, especially in input stages. Now for those interested, dbx gain-control subsystems contain as an essential component a bipolar-transistor variable-gain circuit. This particular circuit is the subject of the original Blackmer patent that underlies this whole product line. (Dave Blackmer is the db in dbx, or so I was told by one of his former associates. I wrote a thesis on variable-gain circuits and Blackmer's was one of many that I looked at in some detail.) This particular gain-control circuit uses one subcircuit for the positive excursion of the audio signal and one for the negative. A FAST op amp is needed in one particular spot (for each of the stereo channels) to guarantee that no glitch occurs on the transition as the signal crosses zero. Another requirement on op amps in dbx circuitry is low DC error (offset) in the gain-control path, for gain tracking in companding modes. Although I'm not sure exactly what Bill had in mind, from these considerations it does not follow that an outboard JFET/bipolar op amp like the TL072 is necessarily better than on-chip (presumably all-bipolar) op amps in the critical dbx core circuits. In practice it is much easier to get good performance if you design the op amp to fit the application --as a designer would do with an on-chip op amp -- rather than use an off-the-shelf one like TL072, good though it assuredly is in many applications. 2. The stereo separation issue I know that some audiophiles rail about how one cannot possibly get good stereo separation when both channels of circuitry share a common monolithic substrate. Before addressing this let me give an anecdote from my experience. A guy lectured me about stereo separation in a record store in Berkeley a couple of years ago and I'll never forget it. It was the kind of commentary that gives some audiophiles such a bad name: pseudotechnical, hip, and full of jargon. I bet it impresses the hell out of all of their friends. If these folks were content to be honest, and simply say that they *hear* less separation than they would like from a certain product (if indeed that's even true), they would have my respect. But when instead they feel bound to explain it in terms they are really not facile with, or (worse, the disciple syndrome -- this is how Fascism takes root) seizing on some fashionable and speculative explanation out of one of the (proudly) limited-circulation magazines that serve as mouthpieces not only for the insightful but also for the fatuous (simply being unpublishable in the major media is not always a sign of genius, I feel bound to remind), they are going too far. But back to my audiophile. Rather than tell me his impressions or experience, this guy was lecturing me that two-channel chips *never* have significant stereo separation since the single chip substrate is incapable of good electrical isolation between audio paths. Note the purely technical character of this assertion. Also, never say never. Actual electrical isolation depends on a gamut of things including basic physics, circuit, and chip layout, and designers have a lot of control over it. In contemporary work on oversampling data converters we are being surprised at just how much on-chip isolation we can obtain with straightforward measures (it's even more critical there since we put large analog and *digital* systems on the same die). I have measured audio analog isolations of 110-120 dB, the limit of my HP spectrum analyzer when augmented and coddled, between amplifiers on a silicon substrate, with careful layout. But my deeper point is that my loquacious audiophile might even have been right and he STILL wouldn't have known it (although he assuredly would have "known" it), since he was talking about something he really had little clue about. [This recalls an even more than usually inane "analog-versus-digital" claim cited on the net months ago. It seems that no less an authority than _The New Republic_ had quoted "several audiophiles with $100,000 systems" as declaring solemnly that digital audio used discrete-time samples and was therefore "fundamentally incapable" of capturing continuous-waveform audio. One wants, as Paul Fussell wrote, to weep. Accordingly, I fully expect grave doubts to be expressed in similarly informed quarters against equipment containing two-channel chips (even digital ones!) because "it is fundamentally impossible for a single integrated-circuit substrate to keep separate etc. etc."] To finish the anecdote, I indicated polite skepticism about this stereo separation impossibililty, and he continued about a product he had bought where all the chips were ripped out and retrofitted with discrete components (taking the premise to its logical conclusion, I suppose). It was only when he started boasting of his new CD player with its "256:1 oversampling!" that I excused myself. I was with musical friends at the time. One of them said to me, "why do audiophiles get so serious about mere *sound*? I listen for the performance," a good point that I hear a lot from musicians. (I could answer, but it would be cynical, uncharitable, and maybe wrong besides.) 3. Second-order electrical behavior Engineers learn about things like gain, slew rate (the rate at which the output voltage can change; if you try to exceed it you can suffer, in audio, "transient intermodulation distortion"), frequency response, stability, and (if they go to a progressive school) noise. Op amps, being designed for negative-feedback applications, depend on lots of internal gain to make their closed-loop circuits behave the way the designers intended. Yet op amps are at heart DC devices, and above DC their gain quickly begins rolling off with frequency. It rolls off (in off-the-shelf unity-gain-stable op amps) normally in a single-pole response to unity at some frequency (typically 1 MHz for bipolar-input and 5 MHz for FET-input op amps). This means that at 20 kHz, the open-loop gain is respectively 50 or 250, not all that big. This open-loop gain is the negative-feedback correction factor that suppresses several nasties -- in particular, distortion in the op amp input stage. As soon as the AC voltage excursion across the differential inputs (which would always be negligible were the open-loop gain large) exceeds some number of millivolts, appreciable nonlinearity develops in the op amp's voltage-transfer curve and harmonic and IM distortion result (steadily, not just in transients). A former co-worker of mine at Signetics Analog, the venerable wideband-amplifier expert Ralph Lovelace (who also grows flavorful tomatoes), designed a "video" op amp with extremely high bandwidth, close to physical limits (can't find the data right now or I'd quote the part number). Not as fast but still enhanced in bandwidth, and tailored in other respects for audio (low input noise -- less than 0.6 uV in 20 kHz from low-impedance sources -- and high output current), is the Signetics 5534 family. Walter Jung (_Audio IC Op-Amp Applications_, Howard Sams paperback) sings praises of the 5534 family (only one reason, I'm sure, why the Signetics marketing folks sent free copies of the book to anyone they could). Other op amps have emerged more recently with the stringent audio applications specifically in mind. The upshot of this is that common off-the-shelf bipolar-input op amps like the 741 are virtually useless for handling large-signal audio (a few years ago I calculated just the *harmonic* distortion at audio frequencies with 741-class op amps (including LM 101 series; LM 124 series; RC 4136; etc.) and it was horrific. It can be expressed solely in terms of closed-loop output voltage for a given op amp, as I recall, even when frequency compensation is relaxed to enhance bandwidth.) Common FET-input op amps are better, since both their bandwidth tends to be greater and their linear range for differential input voltages is much larger than for normal bipolar-input op amps. It is not even unreasonable to use extra-wideband (or "video") op amps, especially if you are trying to reject out-of-band signals at even higher frequencies that are mixed in with the audio, a situation that occurs routinely in the active-filter stages of all CD and DAT players. As far as this frequency-dependent nonlinearity is concerned, what is directly critical in op amps is not the "unity-gain" bandwidth (at which open-loop gain drops to 1); nor the "power bandwidth" at which slew limiting is reached (although that is also an upper bound, it is rarely the main problem); nor the open-loop gain at DC; but rather the gain at the highest input frequency (whether audio or interference); the resulting "error voltage" across the op-amp's differential inputs; and the nonlinear distortion referred-to-input that this yields. Max Hauser / max@eros.berkeley.edu / ...{!decvax}!ucbvax!eros!max
wtm@neoucom.UUCP (Bill Mayhew) (05/24/88)
An integrated operational amplifier that would probaly perform quite well for audio (hifi?) applications might be the Analog devices AD318K. This chip can be characterised as a wideband, low drift, FET input opamp. Here is a synopsis of its characteristics: open loop gain: 100,000 output: +/-12v (+/-10 min) at Rl=1K 20 mA short circuit output current limit dynamic: unity gian, ss 5 MHz full power bw 500 KHz slew rate 30v/uS .1% settle 700 nS .01% 1.2 uS input bias current: 10pA avg, 50 pA max imput impedances: differential 1E12 ohms in parallel with 7 pF common mode 1E12 ohms in parallel with 7 pF input volrate range: differential +/- 20 v (maximum safe limit) common mode +/- 12 v (+/-10 min) CMR 80 dB minimum noise figures: 0.1 - 10KHz 2 uV peak-peak 10 Hz 70 nV/Hz^.5 100 Hz 45 nV/Hz^.5 1 KHz 30 nV/Hz^.5 10 KHz 25 nV/Hz^.5 case style: TO-99 This device is at least somewhat exemplary of the sort of device that was alluded to in the preceding article. Most notably, the device exhibits a wideband frequency response and high open loop gain concurrently. At the same time, excess noise is held to acceptably low levels. Analog devices does not publish a scheamatic representation of the AD381K's circuit topology, so I can not comment on that. Personally, I don't care about the topology inside the device as long as the terminal characteristics are what I want. I am not sure when the AD381K was conceived, that really doesn't matter either as long as it performs correctly. I would feel that the above amplifier would be acceptable for use in such things as CD palyer active filters, dbx de/encoders, etc,; certainly in mundane applications such as tonal shaping control circuits. I am not particularly impressed with the Signetics 5534 relative to anything equivalent from other fabricators; who cares who makes the part, given reasonable quality control is imposed. For a dual amplifier chip, The LF442 from National has many characteristics similar to the AD381K. National says that interamplifier coupling is no greater than -120 dB. That is certianly low enough for anything I do. National does not publish the full power bandwidth, but the unity gain point is 1 MHz. I really don't see any reason why multiamplifier chips should not be used. My test equipment can not measure to -120 dB limit; I assume that National's claims are correct. I doubt that I could decouple other parts of my designs much better than that anyway and still have them be easily assembled. Probably the desire to keep costs low and maintain easy board loading keep parts such as the AD381K out of mass produced consumer grade audio devices. (I'm gessing that it is more difficult for automated equipment to insert a TO-99 case device than a DIP or SMD). For snob-appeal audiophile equipment, there should be no reason not to use high quality parts, since the cost of the parts is a relatively minor portion of the cost of manufacturing the equipment. However ... it frequently would seem that logic and common sense are seemingly minor factors in design, manuafacturing and marketing (even purchasing!) of snob-appeal equipment. As far as fully integrated subsystems such as dolby or dbx en/decoders go... I don't design chips for a living, so I feel ill prepared ot comment on the praticality and merit of such things. However, I do suspect that some such devices are not optimally designed, which is not to say that is impossible or difficult. It seems likely that standards are often no better than is necessary to satisfy the public's desire. Most likely the manufacturer of equipment is to blame by surrounding a decent integrated circuit with suboptimal discrete components. No one fabricator of integrated circuits is superior above others. The speific devices that have been mentioned above are only chosen as exemplary of similar devices that are available from any number of suppliers.