Cary Audio Design CAD-300SE LX20 monoblock power amplifier
More recently, Cary has used the powerful 572-3 triode from Svetlana in Russia to create a most worthy 20W monoblock—the CAD 572SE—at roughly half the cost of the 300SE designs (see Stereophile, October and December 1998). Meanwhile, Dr. Kron of Czechoslovakia has reached into the heart of the design and inner virtues of the 300B tube to create a higher-power version, the KR-300B LSX, that can deliver in the region of 20W in SE mode. This new tube carries an unprecedented two-year guarantee. Cary has chosen to use the KR-300B LSX in the 300SE, which he has renamed the 300SE LX20, priced at just under $5000/pair (footnote 1).
The amplifier's classic SE triode looks are unmistakable. The 300SE LX20 is housed in a narrow box chassis with brushed-alloy, black-anodized front panels. (Gold-plated panels are available for extra cost.) The chassis is of polished stainless steel, the audio output transformer is housed in a box with satin-black finish and gold label, and the power transformer's top cover is bright chrome—an incongruous note, I felt. The tube layout is symmetrical: two 6SL7s at the front, the KR-300B LSX next, and the rectifier tube one step back. On the front are two large chrome levers, On/Off and On/Standby, flanking the two blue LEDs that signify these functions. A user-replaceable power-line fuse is located on the crowded back panel, plus a phone jack for bias alignment, a preset control potentiometer for same, an IEC-type three-pin mains input terminal, an SE gold-plated phono socket, plus a pair of solid-copper binding posts in a natural finish for spade-terminated speaker cables.
An unmarked lever near the back on the top deck switches the output winding tap's nominal impedance from 8 to 4 ohms. Early tests suggested that this switch was effective, and delivers a firmer, more accurate output when set to 4 ohms for those more difficult 4 ohm speakers, providing that they are sensitive enough—for example, a 92dBW B&W Nautilus 801. Even with the Wilson WITT IIs, I often traded the 8 ohm setting's 1–1.5dB loss in absolute level for the extra grip and dynamics of 4 ohms.
With the accessory phone plug, you need only a simple meter to check and set output bias, either as precautionary maintenance or when the output tube requires replacement. The recommended setting is 110mA.
The output tube costs no more than $50, yet it's claimed to be engineered to unusually high standards, especially regarding the purities of the vacuum and the metal parts. This makes possible its two-year guarantee; in fact, under normal use, up to five years of operation can be anticipated. If the output tube is underbiased, full level and optimum linearity cannot be attained; if overbiased, the anode structure will run hotter than the design limit, and might even glow red. The tube's life is rapidly reduced under such conditions.
The 300SE LX20 is intended to be powered up long-term in Standby, in which mode the two channels draw a of total 100W of power. This rises to 210W in Operate mode—typical for a pure-class-A single-ended tube design. In Standby, the tube heaters are kept warm and undue thermal shock is avoided. The 300SE LX20 starts up immediately when the Standby switch is thrown, but for serious listening, I recommend putting the amplifier in Operate for a while. (The high-voltage supply capacitors improve after a little while as their leakage currents stabilize when the electrolyte films re-form to their optimum condition.)
The 300SE LX20 should be located out of reach of children. The tube envelopes are exposed, and contain dangerously high voltages. The output tube glass runs too hot to be touched. Still, accidents should be rare; output tubes are far less fragile than incandescent light bulbs.
Brainchild of the forceful Dr. Kron, the KR-300B LSX power triode is the driving force behind the new Cary 300SE project. The new tube can dissipate up to 90W at the anode without glowing cherry red, and looks good for some 20W of SE class-A operation. To ensure long operating life, Cary runs the tube at 63.8W standing power, operating with a 580V plate potential with a 110mA class-A bias. The output stage is of fixed-bias design (footnote 2) (ie, grounded output cathode), and bias current is monitored by inserting a ¼" jack with a meter connected and adjusted with an exterior preset locking potentiometer. Grid bias is applied after the 0.22µF oil-filled coupling capacitor from a separate, -140V negative bias supply.
Standby mode is engaged by lifting the center tap of the high-voltage winding from ground, thus preventing any rectifier current from flowing. Thus the full-wave rectifier tube, the 5U4, runs in idle in common with all the other tubes.
The signal path is very short, the input being direct-coupled to the input triode grid at an easy-to-drive 150k ohms impedance. The bigger output tube not only needs more supply power than a standard 300B tube, it also needs a bit more grid drive. The input section of the original Cary 300SE comprised a single double-triode 6SL7 operating in "cascode srpp"—half the tube is used as a single-ended grounded-cathode amplifier with the other half an anode load comprising a constant-current "follower"—to ensure low-impedance drive to the output tube grid.
For the LX20 version, two 6SL7s are used, each double triode strapped in parallel to increase the stage bandwidth and the available current drive at the output. In both versions, coupling to the KR-300B SXL is via a series 0.22µF oil-filled paper/foil capacitor rated at 600V. The single output tube drives the loudspeaker via the special output transformer.
With an amplifier of this caliber, the output transformer is a most significant component; you can't do without a really good one. Cary's well-trusted transformer recipe includes silicone-impregnated, mutually insulated iron-core laminations of grain-orientated Hypersil alloy, which is noted for its clean midrange quality. The generous quantity of iron used mean that the transformer operates well below saturation, while the essential air gap prevents the class-A standing current from driving the core into the magnetically nonlinear region. Conversely, there must be sufficient primary inductance to give low-bass extension. The winding is thus bifilar (trifilar for the 4 and 8 ohm tappings), and wound with oxygen-free copper in 16 interleaved sections.
Quite how a single-ended tube amplifier can sound so good with an transformer in circuit is something of a mystery. In addition to very good transformer design, with the use of close-wound conductors with top-quality acrylic insulation, it might have something to do with the SE topology resulting in a standing current flow through the primary. The transformer core does not pass, therefore, through the zero-magnetization zone of the hysteresis loop at the half-waveform crossing point or equivalent, and this could enhance transparency and clarity. Also, due to the low anode output impedance of the Kron tube (confirmed by the low anode voltage and relatively high bias current), the turns ratio, a significant factor in transformer design, can be a relatively low 8:1, aiding bandwidth and thus providing a better drive to the loudspeaker.
As in Cary's 300SE design, there is no loop negative feedback in the 300SE LX20. The cathode resistor of the input triode pair is fully decoupled, and the output transformer secondary feeds signal only to the loudspeaker terminals. Such a configuration generally infers unconditional stability with complex speaker loads, this achieved without recourse to any Zobel or equivalent frequency/phase shaping network.
Footnote 1: During my auditioning I also had access to a stereo version of the amp, the 300SEI LX20 ($4500 with remote control). A moderately higher-power version of the 300SE will also be available, offering perhaps 14Wpc for $4000/pair, though with less load drive ability.
Footnote 2: The CAD 572SE had a self-biasing output stage as compared with the 300SE LX20's fixed bias. In my experience, fixed bias can provide firmer, more dynamic-sounding bass.