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Cary Audio Design SLM-100 Revised Monoblock Power Amplifier
A REPRINT FROM STEREOPHILE, MAY 1996, Vol. 19 No.5
Equipment Report – Cary Audio Design SLM-100 Revised Monoblock Power Amplifier
by Robert J. Reina
Push-pull, class-A, monoblock tube amplifier. Output power: 113W into 8 ohms (20.5dBW), 100W into 4 ohms (17dBW). Frequency response: 9Hz – 30kHz, +0, – 0.75dB, at 1W; 15Hz – 23kHz, t0.5dB, ac full power. Input sensitivity: 750mV for full output. Input impedance: 150k ohms. Noise and hum: 80dB below rated output. Damping factor: >3, equivalent to an output impedance of < 2.7 ohms. Dimensions: 6" H by 10" W by 20" D.Weight 40 lbs. Price: $3495/pair; optional chrome chassis adds $500/pair. Approximate number of dealers: 65. Manufacturer: Cary Audio Design, 111-A Woodwinds Industrial Court, Cary, NC 27511. Tel: (919) 481-4494. Fax: (919) 460-3828.
Although I'll be spending most of my Stereophile hours reviewing affordable gear, I will from time to time examine so-called "trickle-down" designs from high-end designers who have made their mark in the upper-price echelons. More and more, such designers are taking what they've learned and applying it to less-expensive products in order to broaden their customer base. Cary Audio Design, for example, of single-ended triode fame, has entered the ring with the SLM-100 pentode monoblocks.
I should mention that I reacted to the single-ended triode phenomenon with great skepticism. My negative biases were twofold: 1) I have little interest in products with limited applicability to the marketplace – the low-powered nature of most of these beasts results in matching problems with many speakers; and 2) I have a natural bias against very expensive products. As I tend to compare amplifiers on a price-per-watt basis, single-ended triodes come out worst.
My first experience with this genre of "hornblowers" began with an invitation by neighbor Andy Benjamin, a long-time contributor to The Abso!ute Sound, to hear his new Cary 805 monoblocks. I trundled over and heard these sexy, glowing beasts perform incredible magic on his Martin-Logan CLS electrostatics. With a seductive midrange and uncanny stage presentation, the combo was one of the finest systems I'd ever heard.
My reaction to this experience was not to join in the single-ended triode frenzy, but to run in the opposite direction. "These Cary guys are pretty clever. I wonder if they make any normal amps," I thought. You know, push-pull stuff with 6550s or EL34s that can kick out at least 100 per.
A quick look at the Cary catalog, and Aha! I see the SLM-100 monoblock! A hundred push-pull watts into 4 ohms (113 into 8) for only $3495 retail! It would be a kick to compare a pair of Carys to my beloved reference amplifier of six years, the similarly priced (and tubed) Audio Research Classic 60.1
When the Carys arrived, I was stunned by their beauty (they sent me the $500-extra chrome-plated jobs). Moreover, I was impressed that the high standards of parts and construction quality featured in their more expensive wares seem to have not been compromised in the SLM-100. The amp is outfitted with 1% metal-film resistors, polypropylene coupling capacitors, and all wiring is point-to-point.
The design features a pair of 6SN7 tubes as the input, pre-driver, and phase-inverter stages, with the input stage buffered and direct-coupled to the phase-inverter. The amp uses Svetlana KT99A (6550) output tubes in a push-pull configuration, and employs a custom-designed Ultralinear output transformer. In the dark, the sight of the SLM-100's row of output-tube filaments gleaming off of the chrome was stunning. (A friend refers to them as "the apostles' tongues of fire.")
SYSTEM
To audition the Carys, I used my Goldmund Studio/Syrinx PU-3 Revised/Koetsu Urushi and CAL Delta/ Sigma II front-ends. Preamplification was via a Vendetta Research SCP-2C phono stage and Audible Illusions M3 and L1 line-stages. Speakers were the Infinity RS-1Bs above 100Hz, and the full-range Acarian Systems Alón V Mk.IIs. Wires included MIT MI-350 Twin CVTerminator and MIT '330 Shotgun interconnects, MIT Digital Reference interconnect, MIT MH-770 Twin CVTerminator, and (for the Alóns) Acarian Black Orpheus tri-wired speaker cables.
The appearance of the Alón Vs represents the first time in 11 years that I've replaced the Infinity RS-1bs in my reference system. The Mk.II version of the Alón V is among the most dynamic, revealing, and transparent speakers I've heard.2 It can also put out a convincing 20Hz in the right room. But its greatest strength is in its tonal balance – it's the least-colored high-end product I've experienced. Moreover, the relatively benign impedance curve makes it a fairly easy load for an amplifier to drive. In effect, the Alón V acts as a magnifying glass for the electronics placed before it, making it an ideal tool for reviewing amplifiers.
SONICS
The first thing that struck me about this amplifier was its glorious tubelike midrange. Vocals, whether Janis Ian (Breaking Silence, Morgan Creek 2929-20083-2), Jose Carreras (Misa Criolla, Philips 420 555-2), or Mighty Sam McClain (AudioQuest AQ1015), the seductive reproduction of well-recorded voices melted me down into a puddle. Yes, there was a bit of euphonic tube sweetening (which also had a somewhat forgiving effect on massed strings on dryish orchestral recordings and added a touch of Log Cabin syrup to brass), but I didn't care. I just wanted to close my eyes, sway, and sing along.
Adding to the realism of the Carys' vocal reproduction was the dimensional body accorded to vocals and instruments in their wide and deep soundstage presentation. This, combined with excellent detail resolution, ambience retrieval, and image specificity, makes these amps well-suited for those who cherish well-recorded orchestral music. Simon Rattle's reading of Stravinsky's Pulcinella (EMI HMV ASD 3604, and one of the finest EMIs ever recorded), as well as Previn's recording of Messiaen's Turangalila Symphony (EMI SLS 5117), delivered a realistic replication of musicians performing in real space. And when I threw a highly dynamic classical blockbuster at the Carys, such as Crumb's Makrokosmos III (Nonesuch 71311), I found they could deliver staggering dynamic contrasts, from ppp to fff – the widest dynamic contrasts, in fact, I've heard in any of my systems.
The Cary's high-frequency reproduction was sweet and extended, but on certain recordings the transients could sound a bit slow. This occasionally made percussion sound a bit blunted, and highly modulated cymbal and gong crashes swished a bit. But this was a very subtle effect, and was less noticeable than with the original SLM-100, in production before the middle of 1994 (and which used a slightly different input stage and tube complement). The earlier model sounded noticeably sluggish on all material.
Bass reproduction could plunge convincingly into the nether regions, at least up to the limits of my speaker/room combination that, with the Alóns, is solidly to 25Hz, with somewhat reduced output at 20Hz. However, the greatest shortcoming of the Cary was apparent in the 80-100Hz region, where a ripe thickness could add an overly prominent halo to electric and acoustic basses.
In the early stages of my reviewing, I needed to send the amplifier I use to drive my Infinity woofers, a Perreaux 2150B, back to the manufacturer to be updated with the revised 3300 circuitry. As several weeks without music is untenable in the Reina household, I asked Cary's Dennis Had how he felt his monoblocks would do driving the Infinity woofers until I got my Perreaux back. (I would use my ARC Classic 60 to drive the midrange/treble panels.) Said Had: "Uh, sure, uh... I don't see why not. Yeah, go ahead, try it." Hmmm. In his voice was a definite tone of "I really can't say no to a reviewer, but hell, if he blows it up, I can always send him another unit." His hesitation, as Infinity fans would know, is derived from those pesky Infinity woofers, which, to really sing, need high-powered, high-current amplifiers with ultra-fast damping factors. (Solid-state arc welders, the line forms to the right.) However, the result was not what you'd expect from this modestly powered tube amplifier. To the surprise of both of us, the SLM-100 performed quite well. Although not as "fast" or as extended in the bass as the Perreaux, it put out consistently musical sound – and at loud volumes without strain!
AGAINST THE AR CLASSIC 60
I spent a great deal of time comparing this baby to the Audio Research Classic 60 on both sets of speakers. The results were fascinating: Both amplifiers were clearly impressive performers with totally different sonic presentations. They shared a wide and deep soundstage presentation, but it was difficult to determine which was more realistic. The ARC excelled in image specificity, detail resolution, and transparency, but the images on the stage with the Cary sounded more dimensional and lifelike. On dynamic contrasts, the Cary had the added edge on high-level contrasts (perhaps due to nearly double the power), but both amps excelled in reproducing music's microdynamics. The ARC's bass, however, was far superior through the bottom two octaves.
The high-frequency comparison was most interesting. The Classic 60's biggest shortcoming is in its tight, etched low treble, which is the only flaw in what would otherwise be my ideal amplifier under $5000. The Cary's high-frequency reproduction was more natural, but the ARC didn't suffer from the Cary's slight slowing of transients. The comparison reminds me of Sarah Faust, a brilliant piano craftswoman in Irvington, New York. She rebuilds Steinways from scratch, using only Hamburg parts. The result is a grand piano better than what you can buy new, for about two-thirds the price. A few years ago a friend bought a Model B from Faust; for a while, each time I visited his house the piano's touch and sound had changed. He explained that Faust was in the process of "voicing" the hammers to his taste, softening them by pricking the felt with a sharp instrument to mellow out the attack of the notes. The Classic 60's high-frequency transients reminded me of my friend's Steinway before it had been voiced; the Cary reminded me of the softened, fully voiced hammers. Which is correct? It's a matter of taste.
– Robert J. Reina
MEASUREMENTS FROM TJN
Following its one-hour, 1/3-power preconditioning test, the Cary SLM-100 was typically hot for a tube amplifier. Its voltage gain into 8 ohms measured 26.7dB, the input impedance a high 123.6k ohms. DC offset was effectively zero. The SLM-100 was non-inverting, a positive going input emerging positive at the output. The unweighted S/N ratio (ref. 1W into 8 ohms) was 77.4dB over both 22Hz – 22kHz and 10Hz – 500kHz bandwidths. A-weighting improved the figure to 90dB.
The SLM-100's output impedance varied between 5.4 ohms and a very high 8.8 ohms, depending on the load impedance I used to assess the value. This very high output impedance will cause significant frequency-response deviations into real loads.
Fig.1 shows the frequency response of the Cary SLM-100 under the conditions indicated in the caption. With our simulated loudspeaker load, the expected deviations due to the amplifier's high output impedance are quite visible, though it's interesting that they appear to be no more severe than with tube amplifiers we've measured that have lower but still high output impedances.
The output of the SLM-100 in response to a 10kHz squarewave is shown in Fig.2. This is a good squarewave response for a tube amplifier: the risetime is short, there's no overshoot, and only a slight oscillation can be seen on the tops and bottoms of the waveform (these are too small to be easily visible here, but they're clear on a conventional 'scope trace). The 1kHz squarewave response (not shown) was also very good, with only a barely perceptible overshoot followed by a rapidly damped oscillation.
Fig.3 shows the Cary's THD+noise level plotted against frequency. These results, not atypical for a tube amplifier, are neither exceptional nor disappointing, though I'd like to see a little less distortion at low frequencies. High low-order harmonics in the bass will often add warmth – pleasant though unnatural – to the sound, while harmonic distortion above about 8kHz will very likely be inaudible (the harmonic distortion alone of an 8kHz signal will add no artifacts below 16kHz). The Cary is clearly not intended to drive 2 ohm loads, as is obvious from figs.3 and 7 (see below).
The 1kHz THD+noise waveform at low power into 8 ohms (fig.4) is primarily second-harmonic, with some higher-order components and noise apparent. The results into 4 ohms were similar, though with more visible higher harmonics. The main component with a 2 ohm load was third-harmonic.
Fig.5 shows the distortion spectrum resulting from a 50Hz input at 66.7W into 8 ohms (rated power). (Though we normally show this measurement into 4 ohms, the Cary is clearly optimized for 8 ohm loads.) The largest artifact is the third harmonic, at -42dB (0.8%). The distortion spectrum into a simulated speaker load (not shown) was similar in distribution though slightly different in magnitude (about 1.5% at the same third harmonic). This behavior is typical for a tube amplifier; ie, moderately high for a modern amplifier design.
The same can be said for the 19+20kHz intermodulation spectrum at 47.5W into 8 ohms (fig.6) – just below the level at which clipping is observed in the output waveform with this input signal. [The signal has a peak:mean ratio twice that of a sinewave meaning that amplifier will dip at about half their rated powers.– Ed.] The 1kHz difference-tone artifact is at – 49dB, or about 0.35%; the 18kHz component is -40.7dB, or just under 1%. The artifacts were slightly lower into 4 ohms (not shown), but the available power at clipping with this signal was slightly less with the lower impedance.
The SLM-100's THD+noise vs level curves (at 1kHz) are shown in fig.7. The Cary's measured discrete clipping points (defined here as 1% THD+noise at 1kHz) were 99W (20dBW) into 8 ohms, 24.8W (10.9dBW) into 4 ohms, and just 9W' (3.5dBW) into 2 ohms. (These measurements were all taken with a 116-117V AC line voltage.) You can see that the output power would have been significantly higher, especially into 2 or 4 ohm loads, had we chosen to define clipping as the 3% THD+ noise level. The clipping would then have been at about 110W (20.4dBW') into 8 ohms, 90W (16.5dBW) into 4 ohms, and 40W (10dBW') into 2 ohms.
The Cary SLM-100's test-bench performance was satisfactory for a tube amplifier, though its output impedance is a matter for concern. As with all amplifiers of high output impedance, the SLM-100's performance with any specific loudspeaker will not necessarily be representative of its performance with another.
– Thomas J. Norton
SUMMING UP
The Cary SLM-100 Revised is an excellent amplifier that is quite suitable for someone looking for musical 100W tube sound on a realistic budget. The high qualities of sound, parts, and construction make it a bargain. (Definitely go the extra bucks for the chrome!)
No tube amplifier that I've heard in this price range is free from tradeoffs. I'd like an amplifier that combines the detail, transparency, and bass of the Audio Research Classic 60 with the dimensionality, high-level dynamics, and high frequencies of the Cary. (While we're at it, let's throw in the midrange of the EAR 509.) I'm still looking for my dream tube amplifier costing less than $5000. Until then, I'll be happy settling for the Classic 60. Your tastes may be different. Check out the Cary.
– Robert J. Reina
©Stereophile – Vol.19 No.5
MANUFACTURER'S COMMENTS
CARY SLM-100
Editor:
Once again I am delighted and honored with the opportunity to read and reply to the latest reviewer of a Cary Audio Design product in your magazine. A review of the SLM-100 push-pull design monoblock amplifiers. Yes... as Mr. Reina points out in the opening paragraphs... "normal amps." Now the public is aware that Cary is normal and not a bunch of "hornblowers"! I've not met many hornblowers lately, so I am so pleased with the confirmed fact that "push-pullers" are normal.
In all seriousness, Mr. Reina certainly has given us a credible review comparing the SLM-100s with his reference system amplifier of six years. In fact, a most formidable competitor at that. When Robert called me that day to inquire if the SLM-100s would be capable of driving the Infinity woofer columns, I did in fact hesitate. I mean generally... well... tube amps generally are not called upon to "arc weld." I am pleased that the Cary SLM-100s passed the test of the "BIG WOOF."
Mr. Reina has put a smile on my face as he describes one of my push-pull amplifier designs as an "excellent amplifier." In my opinion, I do believe if Robert had not already owned his present amplifier for six years he would be settling in with the Carys. Thank you, Robert Reina and Stereophile, for your continued ethical reviewing practices and conduct as a professional publication. Oh...one last comment...yes, yes, Cary audio does in fact build a complete line of push-pull amplifiers.
DENNIS J. HAD
President, Cary Audio Design
Fig.l Cary SLM-100,frequency response (from top to bottom at 20kHz): at 2W into 4 ohms, 1W into 8 ohms, and 2W into simulated speaker load (0.5dB/vertical div.).
Fig.2 Cary SLM-100, small-signal YACHTS squarewave into 8 ohms.
Fig.3 Cary SLM-100,THD+noise vs frequency at (from top to bottom at 20kHz): 4W into 2 ohms, 2W into 4 ohms, 2.83V into simulated speaker load, and 1W into 8 ohms.
Fig.4 Cary SLM-100, IkHz waveform at 1W into 8 ohms (top); distortion and noise wave-form with fundamental notched out (bottom, not to scale).
Fig.5 Cary SLM-100, spectrum of 50Hz sinewave, DC – IkHz, at 66.7W into 8 ohms (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level, at – 42dB (0.8%).
Fig.6 Cary SLM-100, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 47.5W into 8 ohms (linear frequency scale).
Fig.7 Cary SLM-100, distortion (%) vs output power into (from bottom to top): 8 ohms, 4 ohms, and 2 ohms.
1 Reviewed by JA in September 1990 (Vol. 13. No. 9).
2 Reviewed by Russell Novak in July 1996 (Vol. 19 No. 7). - JA
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