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CAD 805B
HI-FI NEWS & RECORD REVIEW, January 1995
By Ken Nessler
Are single ended triode amplifiers the gateway to audio nirvana, or just the flavour of the month?
The greatest liberties in this industry, after the scandalous pricing of most audiophile cables, have been taken by the makers of single ended triode amplifiers. Moreover, SET disciples spread their word with sicko religious zeal. Nonetheless, there are reasons why the more sane supporters of single-ended triode designs go all drooly and unnecessary at the thought of an '845 or a 300B a-glowing in the corner.
Quite simply, SET amplifiers used within their limitations have the potential for the sweetest mid band and the greatest transparency of any amp types available, and that's despite an intrinsic noisiness not uncommon in big ol' valves which were outmoded by the time Churchill met with Roosevelt and Stalin. It's because they're so damned simple, so uncluttered, so... virginal. If Uma Thurman turned into an amplifier, she'd be triode-driven.
Dennis Had, the President of Cary Audio Design, suggested that one should think of such amplifiers as huge, powerful pre-amplifiers with output transformers. And to understand that the signal arriving at the front end of a SET is amplified straight through to the output in an unaltered state. And that there are four critical advantages to SETs: linearity and gain stabilisation; virtual elimination of odd order harmonic distortion; the amplification of unaltered wave-forms within the amp (no phase inverters); the ability to operate with zero feedback (Note, though, that Cary gives you the option to apply small amounts of feedback if your system requires it.)
So, please, use Had's explanation to counterbalance the weirdness attached to SETs and not to other amp topologies: the romance, the unbridled anachrophilia, the contrariness for the sheer hell of it. What we're dealing with here is a monoblock amplifier selling for between £8050 and £8210 depending on the brand of 211, a 50-watter which has enjoyed that rarest of honours: rave reviews in the two most antipathetic US high-end magazines. The Cary CAD-805B garnering acclaim from these rivals is not unlike Labour and Tory agreeing on economic policy.
Measuring a floor-filling 10x12x24in(hwd) and weighing a healthy 75lb, a Cary CAD-805B is a thing to behold. The small frontal area is finished to jewellery standards; peeking out of the centre is a Magic Eye valve used as a power meter (as in the GRAAF GM200). At each of the front corners of the top plate are toggles, one switching the unit on and powering the heaters of all but the output tube, the other selecting between standby and operating modes and therefore sending the juice to the 211. And there's nothing more soul-stirring for a valve casualty than hearing that vacuum whoosh when the latter switch is operated. You can almost sense the big 211 awakening.
Behind each toggle is a grotesque rotary selector, topped by an emerald-coloured 'jewel' which was obviously chosen for markets where vulgarity sells. The one on the right selects the amount of feedback you might wish to apply, from 0dB to 10dB. Should you choose to apply feedback, the other rotary will select the output for 4, 8 or 16 ohm connection. Between these stand the valves, a series of devices consisting of a 6SL7 triode input tube feeding a 300B which in turn drives the final output triode, the 211. (This describes the CAD-805 with the 'B' suffix, and is a change from the earlier CAD-805; that amplifier used an EL-34 to goose the '211 up to its 50W, steroidal state.)
The replacement of the EL34 with the 300B adds so much trioded street cred that I'll bet there were swoons in every shop in the Akihabara when Cary announced the upgrade. In other words, you get your 211 and a 300B. The terrain then fills with the transformers, capacitors and, at the back edge, a socket and a potentiometer for the absurdly non-intuitive biasing arrangement. (Have these guys never seen a Lumley, a Beard, a Radford, a Sonic Frontiers?)
There's nothing more soul-stirring for a valve casualty than hearing that vacuum whoosh when the switch is operated. You can almost sense the big 211 awakening.
Finally, on the back panel, are copper Edison Price binding posts for the three impedances, a gold-plated RCA input socket, IBC mains input and easy-to-access fuses for the AC and the valves.
As you'd expect of an SET, this unit operates as a pure Class-A device, and it's specified as producing 50W out of that 211. The output transformer, rated at 100W, is air gapped and made with OFC copper wire, while the amp's innards – all hard-wired – features pure silver with a Teflon dielectric. The coupling caps are oil-filled, and the amp uses an expensive choke-coupled DC power supply. Er, that's about it, because this amp, like most SETs, is simplicity personified.
Now, proud possessors of the RCA Tube Manual, the VTL tube book or the Babani guides are thinking, 'Hey, the 211 is rated at around 10W. What gives? What's this 50W stuff?'
And so to Cary's Dennis Had to explain it (and with a nod to the wisdom of Tim de Paravicini, who told me instantly, 'They're talking Class-A2'):
'The final output tube is the 211 power triode utilising a thoriated-tungsten directly-heated cathode. The tube is designed to operate with a plate dissipation of 100W; in a Class-A single-ended audio amplifier, the 211 will yield up to 25W output in Class-Al. This figure is ideal and varies with the quality of the output transforrner, power supply and - most importantly - the driving source. In the case of the CAD-805B, the driving source is the 300B triode.
'To offer further insight, one needs to keep in mind that Class-A amplifiers yield only about 25% efficiency. In technical terms, in regard to a 211 tube, the following formula would apply: Input power equals the plate voltage multiplied by the plate current. (P=EI) The figure for the 805 is 930V DC on the plate and 100mA of plate current. P=930 x 100mA = 93W input power; 25% of the 93W on average will be usable, in this case 23W pure Class-A1.
'The big difference in the CAD-805B is the 300B drive circuit, driving the 211 through an "inter-stage transformer." Through the use of the interstage coupling transformer, the 211 can be driven positive on the grid, putting the CAD-805B in Class-A2 single-ended at an efficiency factor of nearly 50%. The 211 grid goes positive above the 23-25W output range and into Class-A2. The power above 25W is in Class-A2 mode for simplicity's sake.
'When a tube of the nature of the 211 is driven positive on the grid in respect to the cathode, the impedance at the grid falls to such a low value that a capacitively-coupled driver is unable to drive the tube any further.
'To put it another way, picture the CAD-805B as a pair of CAD- 300SE 12W monoblocks driving a linear amplifier with a 211 tube as the final output. In the CAD-300SE the output transformer transforms the impedance of the 300B tube to 4 or 8 ohms for the loudspeaker. In the CAD-805B, the interstage transformer transforms the high impedance of the 300B driving tube to 16k-ohms to drive the grid of the 211 output tube.' Given that the power meters, er, Magic Eyes barely blinked, I probably never even heard the Class-A2 wattage, but that doesn't interest me. What does is that the CAD-805B reveals its power limitations with speakers of medium sensitivity only through headbanging. With high sensitivity types like the WATT V/Puppy V, you can drive the Cary into clipping, but the levels border on the absurd (or the music is way above the norm, dynamically speaking). Still, I enjoyed the CAD-805Bs through the aforementioned Wilsons, Sonus Faber Minima Amators and Rogexs LS3/5As, rarely wishing I had more power on tap. And, yes, the midrange is, as they say, mouthwateringly delicious.
Liquid? I needed wellies and a bath towel. The CAD-805B makes midband magic, especially with female vocals; I had great fun with the new Aretha Franklin Best Of, shrinking into my chair with the admonition of 'Think'. If the conveying of emotion is what matters to you, you must sample Cary mid-band. And natural instruments? The 'unplugged' bliss of Ashley Hutchings, the acoustic of Water Lily recordings, Muddy Waters' Folk Singer; the sense of air and space is so convincing you'll wonder how you put up with anything less for all these years.
But the gorgeous midband and the three-dimensionality are insignificant compared to one group of qualities which seem to be underplayed or ignored when dealing with SETs and their minimum of active devices. The utter simplicity of SET topology gives top-to-bottom consistency and coherence, a combination of strengths which makes the music seem real and 'of a piece'. It's a seamlessness that, for example, is normally available only from transducers such as Stax's full-range headphone drivers and certain electrostatic speakers.
Certainly none of the other single-ended triode designs I tried had the midband transparency of the CAD-805B. That's because all of the others I've tried are softish at the frequency extremes, including the CAD-805B, regardless of the attention paid to ensuring that the amps aren't being overdriven.
But accepting soft frequency extremes is as much a part of SET life as the glorious midband or the low power. What this Cary does is minimise all of the trade-offs in SET ownership, bar one minor Achilles Heel. And that's the tendency toward microphony and occasional displays of temper (such as howling like a wolf or buzzing, like a bee) which Dennis Had suggests is the caused by the Cetron 300B valve instead of a GE – a small price to pay for its midband supremacy.
Maybe so, but it's disconcerting, even if the curative is nothing more than a couple of flicks of the stand-by switch.
I now understand, after a month with the '805B, how this amp united two warring factions. It is, within the confines of its power output, one of the most lush, musical and purely entertaining amplifiers I've ever heard. It is not, however, a 'universal' amp because of the very thing which makes it so wonderful: its SET status. So I can only say that it's the amp if you meet the criteria for SET ownership as regards your speakers' needs and your preferred playback levels and financial wherewithal and tolerance for heat and immunity to microphony. (Oh, and guilt by association with the sort of audio cultists you expect to commit suicide en masse in some hidden retreat.)
That aside, the Cary CAD-805B is so natural-sounding an amplifier that it scares me.
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