Cary Audio Design CAD-805 Monoblock Power Amplifier Roarin' '20s amp circuits naturally tended toward simplicity. A single power triode was allowed to handle the entire signal; ie, both negative and positive portions of the waveform. These amplifiers perforce operated in purest class-A: the signal was not shared between two tubes, as it is in a push-pull output stage. These minimalist designs typically used no global negative feedback, feedback not being widely used until the late '30s. This is one of the major reasons why a single-ended design recovers so gracefully from overload. In this age of 84dB - sensitivity loudspeakers, power amps clip routinely. "Redlining" an amp may push it into momentary gross distortion, or oscillatory behavior which persists for many milliseconds. That literally sounds bad. The single-ended amp, on the other hand, clips so smoothly and recovers so quickly that it's difficult to tell that it has clipped at all. In the '20s, a UX-171 triode reliably delivered a clean 0.7W, while a UX-210 power triode with a plate voltage of 425V could deliver 1540mW of undistorted power - a whopping 1.54W! Such power is actually adequate for home radio applications. Come to think of it, my daughter's new Sony boombox sports a specification of 15W at 5% THD. That's progress! In the '30s, Western Electric pushed the single-ended power amplifier to around 9W output using the 300A output tube. These 9W beasts were used extensively to power large horn-loaded speaker systems in movie theaters across the country. Later, RF transmitting tubes such as the 211 and the 845 were used to generate over 20W in single-ended fashion. What caused the near demise of the single-ended power amp in the '40s and '50s? The switch to push-pull was fueled in the '30s by the growing popularity of the pentode, which offered better load damping and even greater efficiency in push-pull. The proliferation of relatively inefficient direct-radiator loudspeakers demanded more amp power. The inefficient single-ended output stage finally gave way to push-pull designs that were more efficient by factors of two and three (less wasted plate dissipation). Ironically, the push-pull stage was already known in the '20s. Such a connection was recognized to balance out even harmonics - assuming tube-like characteristics - and therefore offered a possible means of reducing triode harmonic distortion, which is primarily even in nature. Unfortunately, the trend toward using power pentodes in a push-pull connection tends to emphasize their harsh odd-order harmonic distortion signature, which isn't canceled by balanced operation.2 Odd-order harmonics (third, fifth, etc.) are not consonant with the music's harmonic envelope and are therefore not easily masked, even at very low levels. Whether or not the distortion-reduction argument holds much water, undeniably the biggest practical advantage of push-pull operation is that direct currents in both halves of the output transformer's primary winding balance each other magnetically, minimizing the problem of core saturation and making a lighter, more compact, and wider-bandwidth design possible. Another factor behind the single-ended amplifier's demise had to do with the misguided drive for lower and lower harmonic distortion. The invention of the moving-coil dynamic loudspeaker in the '20s generated much excitement and a common perception that there was now a speaker that had no distortion detectable by the human ear. The next logical step in the development of perfect radio reception was to perfect a distortion-free amplifier; hence, the birth of the simplistic notion that lower distortion, at whatever cost, is hi-fi's ultimate goal. This idea has stuck with us through the years and was used to legitimize the supposed "superiority" of early solid-state gear. Tons of transistorized crap have been sold under the conceptual umbrella of "Less than 0.01% Total Harmonic Distortion!" Joe Public could sleep blissfully at night, secure in the comforting illusion that his receiver was true hi-fi. Over the years, the legacy of the single-ended triode amp and horn loudspeakers continued to find a receptive audience in Japan where, in the '70s, audiophiles discovered Western Electric gear. Japan is apparently one country where the dry, sterile sound of modern gear is frowned upon. Audiomart's Walt Bender tells the story of how he laughed at Japanese collectors who paid him big bucks for these audio antiques. He stopped laughing when he actually heard the stuff for himself. Like Walt, audiophiles whose sole exposure to tube gear has been via class-AB push-pull amps heavy on feedback go into shock the first time they hear single-ended magic; the sound's purity and musical intensity are that dramatic. The Japanese have combed the country-side for years for this classic sound; by now, practically all the available vintage gear has migrated East. The good news is that the growing underground interest in single-ended tube gear has translated into a commercial rebirth of this class-A King. Cary Audio Design is the first modern US-based manufacturer to over several single-ended amp designs. CAD's designer, Dennis Had, says he's excited by and devoted to single-ended triode amplifiers: "It never ceases to amaze me how some of the golden classical designs of the 1930s were actually more advanced in their sonic presentation than some of the more current Hi-Fi designs," he says in Cary's promotional brochure. "Maybe we really don't even have a clue to what Hi-Fi of today vs live music is all about until we listen to some of the more simplistic, in terms of component count, audio circuits from brilliant minds of years ago in comparison to a live performance." In other words, unless we understand our audio roots, we will never find the path to musical fidelity. CARY'S 805 (AT LAST) With its ornate knobs and gold-on-black finish, the beautifully crafted Cary Audio Design 805 looks like a '30s period piece. A delightful feature is the "tuning eye" on the front panel. The eye, which monitors the AC output voltage, closes completely at the 50W output level. The open chassis is dominated by the large - almost 8" from head to toe - 211 output tube. This true three-element tube has been heavily used over the years in RF amplifiers. The filament, a directly heated thoriated-tungsten type, requires a current of 3.25 amps at 10VAC. The 211's plate dissipation in the 805's circuit is 94W (100mA at 940VDC), which means that, when you add the filament dissipation to that of the plate, the tube heats up like a 120W light bulb. This is a power triode! There was a time when General Electric built 211s, as they did just about everything; the Chinese are now the major supplier. (Although the 805's 211 is stamped "Philips," it is, in fact, Chinese.) I'm told that this is a tube they've been producing for over 20 years, so quality should be very good. The first question anyone is likely to ask is: "Why is this thing so darn heavy?" The main reason is the size of the 805's output transformer. Because the bias current for the output tube flows through the primary winding of the transformer, core saturation becomes a serious problem. An air-gap transformer has to be used, with its core laminations typically spaced 2 - 16 mils apart, in a precise manner. The air-gapped design gives more inductance for the same number of turns, but its inductance becomes proportional to the core cross-section. Maintaining a large primary inductive reactance relative to the load at very low frequencies therefore mandates the use of a very large, heavy core and lots of wire. A primary inductance of over 60 Henrys may also be required. Practically speaking, it's rare to see a single-ended transformer with a full-power bass response lower than 20Hz. Additionally, all that wire generates both ohmic losses and significant winding capacitance, which limits the HF bandwidth. Unless interleaved primary/secondary winding techniques are used to control winding capacitance, HF extension beyond 15kHz is not possible. The 805's output transformer uses an E-I core with large air gaps between laminations. Output taps are provided for 4, 8, and 16 ohm loads. Note that these taps are designed for efficient power transfer to the load. The actual output impedance of a single-ended amplifier is typically around 2 ohms, which gives rise to a poor damping factor. Defined as the ratio of the loudspeaker impedance to that of the amplifier output impedance, the damping factor is intended as a figure-of-merit to describe the amp's control or braking action over a loudspeaker cone. A solidstate design with high negative feedback may have an output impedance below a tenth of an ohm, generating a damping factor of well over 100. In contrast, a single-ended design can only muster single-digit damping factors, even into a 16 ohm load. However, there's some controversy over the need for, or significance of, higher (ie, greater than 10) damping factors. The following circuit description is based on information provided by Dennis Had. The input signal is DC-coupled to the grid of a 6SL7 dual-triode operated in parallel as a single voltage-gain amplifier. The 6SL7's plate is AC-coupled to the control grid of an EL34 pentode wired as a triode.3 The EL34 dissipates about 19W of class-A power and produces 4W of audio power to drive the grid of the 211 through an inter-stage transformer of air-gap design. The 211 is cathode-biased, and the output stage is operated in class-A1 (no grid current) up to 25W output. As the drive to the 211's grid is further increased, operation shifts to class-A2. The 211's grid goes positive (and starts to draw current) while the output power with increased efficiency reaches about 50W. The ability to drive a 211's grid positive is made possible by the use of the inter-stage transformer coupling between the EL34 and 211. Unfortunately, substantial class-A2 power entails significant harmonic distortion, as the tube is eventually pushed into non-linear operation. The 805's power supply features a full-wave bridge rectifier using avalanche-protected diodes. The smoothing filter is a pi network using a series choke of air-gap design. Variable global feedback from 0 - 10dB (from the secondary of the output transformer back to the input stage) is offered via a pot and an impedance selector switch. The owner is thus given the chance to experiment with the amount of feedback - from none to moderate - within his or her own system. The On/Off switch powers up everything but the 211 tube. The Standby/Operate switch controls the filament current to the 211. The proper turn-on sequence is to power up the amplifier in Standby. After waiting a few minutes to give the input and driver stages a chance to stabilize, the 211 may be powered up by switching from Standby to Operate. (I mention this because there's no formal manual for the 805.) The amp sounds good right out of the box, but reaches prime time after about 25 hours' break-in. The chassis runs very hot to the touch because both the rectifier bridge and the 211's cathode resistors are heatsinked to the chassis. TUBE TERRITORY With about 20 clean watts at my disposal, the choice of partnering loudspeaker became a crucial issue. The two factors paramount to the selection process are sensitivity and impedance magnitude. I say "sensitivity" rather than "efficiency" because the former properly describes a speaker's bottom line in terms of how loudly it will play for a nominal watt input. Efficiency has to do with conversion of electrical into acoustical energy: how much of an electrical watt at the input terminals is converted into acoustical power. But if, for example, I were to use ten inefficient drivers instead of a single efficient one, I might find that the sensitivity of the multiple-driver speaker was higher. In a typically sized listening room with only a moderate amount of damping, a sensitivity of 90dB/W/m would allow a stereo pair of loudspeakers to reach 100dB peak sound pressure level at the listening seat with about 10 electrical watts. This represents an acceptable dynamic range for many listeners. Unfortunately, the great majority of audiophile loudspeakers are direct-radiator types with sensitivities of 88dB or lower, which makes them either marginal or simply unacceptable for use with single-ended amps. Ideally, the partnering loudspeaker should be blessed with a sensitivity of at least 95dB/W/m to make life easier for the amp... but that's horn-loaded speaker territory. Dennis Had reports good results with such average-sensitivity speakers as the Monitor Audio Studio Six and the ProAc Response One and Two. The key to using speakers with sensitivities of less than 90dB is a compatible (benign) impedance magnitude. The impedance magnitude should be fairly flat for efficient power delivery. The nominal impedance should be 8 ohms or higher, and ideally there should be no dips below 4 ohms - at least below 500Hz. Any tube amp is limited in its current delivery, and the lower the load impedance, the greater the current demand. As a simple example, compare two loudspeakers: one with an 8 ohm impedance vs one with a 4 ohm impedance. Assuming purely resistive loads, the current demand of the 4 ohm load is twice that of the 8 ohm load. I'm appalled at the recent trend toward vanishing-impedance loudspeakers. An impedance of 2 ohms or lower below 500Hz - the power-hungry musical range - is Krell country, not tube territory. There was a time when a 16 ohm nominal-impedance speaker was commonplace. More speaker designers should be considerate of the special needs of tube amplifiers. PRELIMINARIES If I told you that some of this galaxy's purest, most musical sound comes in a 10W or 20W package, would you (as some have done) shrug your shoulders and say, "What's it good for?" Or would you scramble around for a compatible load? Okay, so I can't use the Sound-Lab A-1 with the Cary 805. But if the ticket to Nirvana says "single-ended," I'd most definitely search Speakerland for a suitable mate. So I gathered several loaner speakers which (at least on paper) had a good chance of partnering the 805. These included the Audio Note 3/SPX-SE, the Reel-to-Reel Design Legacy Protege, the Solo Electronics H500, and the Audio Artistry Mozart. Speaker cable was primarily TARA Labs RSC. The front-end consisted of the Jadis JP 80MC preamp or the Air Tight ATC-2 line-level preamp used with Air Tight's matching ATE-1 phono preamp and the Ikeda step-up transformer; the Ovation turntable outfitted with a Graham 1.5t arm and an Audio-Technica ART-1 cartridge; and the Theta DS Pre Generation III used with both Micromega and California Audio Labs Delta CD transports. Interconnects were mostly Mapleshade's Omega Mikro and Hovland/Sonic Purity. SONIC IMPRESSIONS Audio Note 3/SPX-SE: Because Audio Note UK distributes the fabulous single-ended amps of Audio Note Japan, you might have guessed that their speakers would be eminently compatible with 10W amps. And indeed that seemed to be the case - on paper. The Model 3-SE ($2995/pair) is said to possess a 6 ohm nominal impedance and is rated at 95.5dB at 1W at 3m. That's one hell of a sensitivity for a direct-radiator two-way design with a 7.5" woofer. While I believe the 6 ohm nominal impedance rating is reasonable, the sensitivity figure is straight out of the Twilight Zone. My measurements show a figure of 90dB/W/m to be much more appropriate. The 3 was stand-mounted using the MAF stands provided by Sounds Like Music, Audio Note's US distributor. I would describe the 3's low end as gelatinous, lacking definition and rhythmic strength. Nor is there much deep-bass extension. This may be due to the 3's enclosure construction: For its volume, the cabinet is distressingly lightweight. Although the lower treble is a bit too polite and laid-back, there's plenty of upper treble. But it's quite excitable in nature, which means that the sound is often raspy and raucous. (A time-domain measurement of the tweeter would probably not be a pretty sight.) The Model 3's only sonic virtue is its reproduction of the midrange's core with admirable tonality and decent sound-stage transparency. Ironically, a single-ended amp's weaknesses at the frequency extremes are blessings with this sort of speaker. The 805's inherent loss of bass extension wasn't noticeable, and with vinyl playback was actually welcome: it "filtered" out subsonic garbage which can overdrive a bass-reflex - loaded woofer. At the other extreme, the amp's closed-in presentation tended to tame treble nasties. The 805s thus catered to the needs of the Audio Note 3s. I found them to be much more listenable with these speakers than the Jadis JA 200s, even though the Jadises sounded faster and more transparent. The 805s highlighted the real glory of the mids with a suave, direct presentation that caressed a human voice as only divine intervention can. It wasn't so much that Marlene Dietrich (Mythos Marlene Dietrich, German EMI-Electrola 7 91287 1) sounded so good, but that her expressiveness was so well facilitated. In this, the 805s' role as communicators became rapidly clear as they recast harmonic textures in a more believable fashion. Whether this was a coloration or the gospel truth wasn't clear to me at this early stage, but I didn't really care - the music was just more enjoyable. Reel-to-Real Legacy: My sample of the Legacy was a high-sensitivity version of the standard Protege, made expressly for use with Coda Technologies' 25W model 2.5. Though I didn't measure it, I estimated the Legacy's sensitivity to be in the low 90s. The bad news was that the impedance magnitude dips as low as 2 to 3 ohms below 70Hz. The Legacy possesses undeniable flair, and, from my experience with the Coda 2.5, it certainly can boogie, even with only a few watts. Its inherent character is on the bright side of reality, with a lean midbass region. The soundstage, program material permitting, is quite panoramic, with excellent dimensionality. Bass extension is decent, with tight bass definition and a nice rhythmic drive. The midrange sounds sweet and cohesive. The treble balance, however, forced me to listen to the tweeters off-axis, with the speakers toed-in to the point where the tweeter axes crossed in front of the listeningseat. The Legacy was stand-mounted using factory-supplied stands. The lively nature of the Legacy's aluminum-dome tweeter was laid bare by Classe Audio's 700 monoblocks. When the tweeter was hit pretty hard, it started imitating a fire-breathing dragon: scorching heat cloaked with a thick metallic accent. This tendency to spit fire was subdued enough with the Coda 2.5 that my ears were no longer badly singed. It was, therefore, amazing to witness the degree to which the 805 tamed the Legacy's highs - a night-and-day difference. The extreme treble actually sounded closed-in, while the lower-treble brightness was almost totally tempered, with greatly reduced sibilance. The midrange was infused with a romantic blush. Harmonic textures were more liquid and flowing. A tropical warmth permeated the core of the music, like an electric blanket on a cold winter's night. Tommy Flanagan's graceful touch on the piano blends well with George Mraz's strong bass technique and Al Foster's dynamic drumming on Nights at the Vanguard (Uptown UP27.29). Rudy Van Gelder's live recording perfectly captures the mood and the feeling of being there, and the Vanguard's good vibes were superbly reproduced by the 805. Again, wonderful things were happening to human voice with the Cary. Rich and almost palpable, the individual voices of a chorus floated in space with remarkable resolution. The sensation of being able to pinpoint isolated singers in an ensemble was very believable. There was something about the 805 that made human voices sing more soulfully. The ability to convey human emotions transcended the realm of mere reproduction. Even off the 4-ohm taps, the bass lacked impact and a convincing foundation. Taj Mahal's "John Henry's Fiddle" (string bass) on "Texas Woman Blues" (from Recycling the Blues & Other Related Stuff, Columbia 31605), while sounding full enough through the upper bass, sounded murky, lacking definition in the midbass. That impedance magnitude below 70Hz is a killer for a single-ended amp. The Legacy's current-drive demands proved too much for the 805. I did experiment with the feedback control, which turned out to be rather instructive Increasing feedback improved the resolution of transient detail, which I felt was being obscured, as if a thick glaze was overlaying the soundstage. The sense of speed and directness also improved, at the cost of some lushness. About 5dB proved right to my ears. Both the magnitude of harmonic distortion and the output impedance are reduced with increased feedback. If you desire a softer, less distinct presentation, then no feedback is what the doctor ordered. On the other hand, increased purity and directness with a small level of feedback appeals to me as a more realistic balance between music's hard and soft textural qualities. Driving the Legacy with the much more expensive and higher-powered Jadis JA 200 monoblocks drove home the 805's limitations. The Legacy's upper octaves opened up significantly, while treble transients sounded faster. Bass control improved dramatically; bass lines were tighter and much easier to resolve. Soundstage transparency also improved, all the hall's recesses lighting up. What was missing, however, was that beguiling lushness, that romantic touch that allowed the 805 to speak more effectively to the heart. It's a given that any small power amp will be routinely driven into clipping. The 805's ability to clip gracefully and recover quickly, without harshness, was very evident with this speaker. There was none of the pronounced ringing and oscillation that often plague push-pull designs. The 805's ability to move from soft to loud was quite impressive. Only when attempting to scale the dynamic range from loud to very loud did it run out of steam, especially with speakers whose sensitivities were at the lower range of acceptability. After all, there's a limit to what 25W can do. Compression was subtle, not gross and harsh like a rubber band stretched beyond its elastic limit. Solo Electronics H500: This high-sensitivity (96dB/W/m), $2480/pair loudspeaker features a 2" titanium-diaphragm compression driver loaded by a wooden exponential horn. The horn driver is crossed over at 1kHz to a pair of 8" poly-coned woofers. A titanium metal-dome tweeter is used to 611 in the extreme treble above 12kHz. This visually attractive three-way caught my attention at the 1993 Winter CES, when it sounded very listenable indeed. The nominal impedance is 6 ohms, which, together with its sensitivity spec, makes the Solo H500 quite attractive for single-ended applications. Matching Sound Anchors stands, which elevate the enclosure to the proper listening height, are available. Critical listening in the reference room showed the H500 to be grossly colored in the midrange. (I experienced a similar frustration with the PAS Studio Monitor that I reviewed a year ago.4) Considering that the speaker was designed by audiophiles, I'd hoped for more. My measurements noted problems in the integration of the horn and woofers. The horn's cutoff frequency simply isn't low enough, resulting in a discontinuity at the crossover point. The upper mids feature far too many resonances to make this loudspeaker a serious tool for evaluating associated equipment. Dynamic headroom was great, but the voicing was too far off-base for me to pursue the Solo experience. Audio Artistry Mozart: This $2495/ pair two-way design features three Vifa drivers: two woofers flanking an aluminum-dome tweeter, a la Joe d'Appolito. A product of computer-aided design, the Mozart lived up to its namesake in the sense of evincing a classic tonal balance. Although bass extension is flat to only about 50Hz in-room, the lower mids and upper bass are full, and the midbass is well-defined, lending a convincing foundation to jazz and classical music. The Mozart's ability to preserve the rhythmic drive and pacing of live music is outstanding. The treble is equalized to be flat on-axis - a refreshing change from the rising high end of which so many designers are enamored. I took an instant liking to this speaker. The Mozart also measures very well, which implies that its drivers integrate smoothly. Designer Marshall Kay took a calculated risk by crossing the tweeter over at 1.8kHz - rather low by metal-dome standards. His tests have shown that this particular tweeter can handle a lot of power around the crossover point with little distortion. As a result, the Mozart is blessed with excellent dispersion through the critical midrange. Its ability to suspend a wide, spacious soundstage in the front third of my Reference Room is quite remarkable. The sensitivity is about 91dB/W/m and the nominal impedance is around 6 ohms, without any impedance dips below 4 ohms. I'm told that CAD's single-ended amps were used as part of the design process to refine the speaker's voicing. This I could readily believe after my first listen to the Mozart with the 805. The Mozart and the 805 formed a synergistic duo. Objectively, the Mozart's upper registers are lively, with a notable metallic flavor characteristic of all the Scandinavian aluminum-dome tweeters I've heard. With the Classe M-700 solid-state monoblock, the metallic aroma had no place to hide. The 805, on the other hand, civilized the Vifa tweeter to the point of respectability. Again, I found 5dB of feedback to be an ideal balance between lushness and intimacy. The Mozart simply brought out the 805's best, resulting in a fully 3-D illusion of space. Not only were instrumental outlines fully fleshed out, but the space around each instrument was convincingly resolved. There was a wonderful transparency to the soundstage; the sensation of being able to reach out and touch someone was strong. The extreme treble sounded rolled off and shut in, while the lower treble's bite was somewhat reduced so as to emphasize the midrange. This, together with the delectable lushness and textural liquidity added by the 805, served to change the "stage lighting" from sunny to patently yellow. This duo's harmonic compass, while at times softer and less bright than the real thing, could conjure up a whole new spectrum of soulful moods. Violinist Arturo Delmoni's Music for Violin and Guitar (with David Burgess, Sonora SACC-102) features about as sweet a violin tone as you're likely to hear anywhere. I've heard this disc through a variety of push-pull tube amps, but in this instance the 805's clarity of expression and palpable imagery clearly surpassed those of the push-pull crowd. Harmonic nuances were fleshed out with startling vividness. Gene Ammons's tenor sax (on The Gene Ammons Story: Gentle Jug, Prestige 24079-2) never sounded sexier, the slow, elastic tempo serving to accentuate the mood. The pacing sounded oh-so-right. While the deep bass was typically indistinct, the mid-and upper bass regions were well defined and tonally authoritative. Nor was double bass slighted - Gary Karr's instrument sang out with believable body and heft on Adagio d'Albinoni (King K33Y236). JA once pointed out to me that it seemed as if I listened to nothing but Belshazzar's Feast (EMI SAN-324). Well, hardly a week goes by without my taking Walton's epic work out for a spin. The large chorus and orchestra work well together to present any audio system with certain basic problems. First is that of adequate dynamic range. The Mozart/805 duo ran out of steam only on very loud passages, when the 805 clipped but seemed to recover instantly. The stage came alive, every rhythmic nuance nicely resolved. The surging imagery of a large chorus was allowed to bloom nicely, with full emotional impact and outstanding spatial resolution. As far as fleshing out an orchestral foundation, the deep bass lacked full extension and impact. But the lower mids and upper bass regions were quite convincing. Again, female voice was superbly expressive. Whether it was Jennifer Warnes, Joni Mitchell, or my wife Lesley on ViTaL, the brilliance of the upper registers was slightly subdued, though the music's drive and energy came through in spades. The emphasis was typically on the middle registers, which were reproduced with plenty of heart.
FINAL THOUGHTS FROM DO 1 Lee de Forest insisted that the triode's behavior was due to the presence of gas in the primitive audion's soft vacuum. It was left mainly to Armstrong to provide a strong theoretical footing for the triode's behavior and to develop circuits that made use of its properties. Those incerested in the early days of sound reproduction and broadcasting should read Tom Lewis's Empire of the Air: the Men Who Made Radio (HarperPerennial Books, 1991). Written in conjunction with an excellent PBS program (which is also available as a videotape), this is the best account I have yet read on the legal disputes between de Forest and Armstrong and between Armstrong and Sarnoff. - JA 2 The necessary phase splitter or inter-stage transformer was thought, even 70 years ago, to introduce some distortion itself, thus negating at least some of the push-pull stage's inherent distortion neutralization. 3 As this review went to press, we were informed that the EL34 driver tube was going to be replaced by a 300B triode. DO comments on the change this makes to the CAD-805's sound in the February 1994 Stereophile. - JA 4 Vol.16 No.1, January 1993, p.183. - JA
|
