Cary Audio Design CAD-300SEI integrated amplifier
To find out more about the single-ended experience, I asked Cary Audio Designs for a sample of their least expensive single-ended amplifier, the $3395 CAD-300SEI integrated model. I was prompted to try single-ended triode amplification by the Infinity Composition F-PR loudspeakers on hand in my listening room (see the review elsewhere in this issue). The Composition has a high sensitivity (JA measured 95.5dB/2.83V/1m) and a powered woofer, making them appear an ideal load for the 11Wpc Cary 300SEI.
After some critical listening evaluations—and many more hours of sheer musical magic—I've become convinced that single-ended tube amplifiers sound fabulous in spite of their distortion, not because of it.
The CAD-300SEI is essentially two of Cary's CAD-300SE monoblock power amplifiers ($3800/pair) combined in one chassis with a volume control and a class-A triode line stage. The compact (14" by 14") 300SEI is a visual work of art, with a chrome chassis, shapely 300B output triodes up front, and angled output transformers. The review sample had the optional ($500) 24k-gold faceplate and knobs, further heightening its beauty.
Three line-level inputs are provided on high-quality RCA jacks, with input selection via a front-panel rotary knob (footnote 2). A large volume control, smaller balance adjustment, and power rocker switch finish off the front panel. The rear panel holds the RCA input jacks, Edison-Price Music Posts for loudspeaker connection, a line fuse, and an IEC AC-line jack.
The front panel is also home to a ¼" headphone jack, which is activated by a small pushbutton on the top plate. This selects between the headphone output and the Music Post loudspeaker output terminals. When listening to the 300SEI through headphones, you're hearing the entire amplifier right off the output transformers—no op-amp circuits to drive the headphones here. Switching between loudspeakers and headphones is accomplished by a gold-plated 10A relay.
A ¼" 'phone jack on the chassis top provides access to the bias measurement point; simply insert a 'phone plug and measure the current flow with a DC ammeter. A trim pot next to the jack adjusts the bias. Not everyone has a two-conductor ¼" 'phone plug lying around; Cary should supply one with wires already attached.
The power supply is a full-wave center-tapped configuration filtered with a pi-L network. The rectifiers are bolted to the chassis top plate, which makes the entire amplifier warm to the touch when it's running. Two 1200µF and one 100µF electrolytic filter capacitors bypassed with polystyrene caps provide 230 joules of energy storage. Plate voltage on the 300B output tube is 430V. A "soft start" circuit powers the 300B output tubes' filaments first, then the high-voltage plate supply 60 seconds later. Warm-up time is said to be only three minutes.
The input/line amplifier stage consists of a single 6SN6 dual-triode tube, with one half of the tube used for the left channel and the other half for the right channel. This class-A voltage-gain circuit is the same as that used in the Cary SLP-90 preamplifier. This stage is followed by a pair of 6SN6 triodes (one per channel) in a constant-current, plate-loaded configuration, each of which drives a 300B triode output tube. No feedback—local or global—is used in the 300SEI. That's right, this isn't a low-feedback design—the 300SEI has zero feedback.
The 300B output tube is a classic triode design, with only three elements: the cathode, grid, and plate. Unlike nearly all other tubes in which a separate filament heats the cathode, the 300B's filament and cathode are one and the same, which gives the 300B the "direct-heated" designation. Seeing just four pins on the 300B made me realize just how simple a direct-heated triode is (footnote 3). The pair of 300Bs and three 6SN6 tubes are mounted in silver-contact sockets.
The output transformers, which are custom-designed for the 300SEI, are an E/I-laminate type wound with oxygen-free copper wire and feature an air gap to reduce the tendency of the transformer to behave as a big electromagnet. This is a danger with single-ended amplifiers because the bias current flows in only one direction through the primary winding. The two output transformers account for most of the amplifier's 42-lb weight. Indeed, the transformers look like they were designed to handle 100W each, not the 300SEI's 11W.
Footnote 1: See "The Listener's Manifesto"—a slightly revised version of my Audio Engineering Society paper "The Role of Critical Listening on Evaluating Audio Equipment Quality."
Footnote 2: A matching phono stage, the PH-301, sells for $1495. The unit is all-tubed and can accommodate MM or MC cartridges.
Footnote 3: The names associated with tube and solid-state electronics reveal much about their fundamental operation. The term "transistor" was coined in a casual hallway conversation among its inventors. If a vacuum tube is a transconductance device, they reasoned, the new solid-state amplifier was a transresistance device, or "transistor." Tube proponents like to point out that, in a tube, electrons flow through a vacuum, not through a solid semi-conducting material as they do in a transistor.