Cary Audio Design CAD-572SE monoblock power amplifier
He began building these when he was an active member of a high-school band in the 1960s. At that time he used the legendary 300B, a directly heated triode tube dating from the 1930s and already something of an antique. As a young musician, he observed that low- and zero-negative-feedback triode circuitry tended to generate the sound he preferred—to his ears, in fact, it sounded not so much "preferred" as "natural." Nevertheless, it took Had 30 years to return to these roots and to found his own business devoted to tubed amplification. It's no accident that low- and zero-feedback triode technology is now the mainstay of the Cary amplifier line.
Cary's new CAD-572SE tube monoblock is a relatively affordable design based on Svetlana's new 572 triode. Priced at $2495/pair, the 572SE is rated at a straight 20Wpc into 8 ohm loads. In practice, it should drive loads of up to 16 ohms equally well. Speakers with 4 ohm impedances are in contention—the 572SE has a rather low damping factor, and would need a 4 ohm loudspeaker with a combination of very high sensitivity (>93dB/W/m) and a smooth impedance characteristic (including through the bass region) for the result to be acceptable.
In the virtual absence of corrective negative feedback, a designer can mix and match a an amplifier's signature to get what he feels to be a musically palatable blend. The characteristic signatures of the amplifier's output tubes at their chosen operating points are thus laid bare.
The 572SE is built around the new Svetlana 572-series high-power triode tube, a mix of classic and modern design elements including bright-emitter (and they are bright), direct-coupled, thoriated-tungsten filaments. The oversized anodes are milled from solid graphite. With low secondary emission, the anodes run a pale cherry-red when fully warmed up, and hard-glass envelopes are employed to avoid thermal distortion. Of the four 572 types Had has tried, the 572-3 version is the one that has been optimized for SE use. General parameters for this four-pin triode show some resemblance to an uprated 300B in respect to a moderate transconductance (µ), this conferring favorable linearity in zero-feedback applications. This tube puts out a true 20W in class-A, promises high reliability, and facilitates the design of a competitively priced SE amplifier. In short, it boasts a relatively competitive price/performance ratio that Had has chosen to exploit in the 572SE.
The 572SE isn't a true zero-feedback design; some very moderate loop negative feedback is used to improve the matching impedance to the loudspeaker. But the amount of feedback—quoted at just 2dB—is so small that, to all intents and purposes, the amp can be regarded as a zero-feedback design.
Without negative feedback, errors and losses accumulate, so the inherent bandwidth and linearity of each stage must be optimized. The output transformer is a particularly vital item. On paper, this component looks simple enough, but for it to be able to operate over the audio band, it has in fact to be very complex electrically. Many aspects of the transformer's behavior can affect sound quality.
For an amplifier to provide a wide frequency response, the transformer's effective bandwidth needs to be even wider, and by a considerable margin. For example, an amplifier rated at 20Hz-20kHz (some 10 octaves) requires a transformer whose half-power points are perhaps 15 octaves apart. This transformer needs high primary inductance in order to function effectively at very low frequencies, yet very low mutual inductance and low stray capacitance are essential for good performance at high frequencies. These low- and high-frequency criteria are in conflict. The problem is exacerbated in single-ended designs, where heavy DC magnetization resulting from the standing class-A bias current that flows through the output tube, is present. This necessitates a flux-stabilizing, saturation-controlling air gap in the transformer core, which conflicts with the need for high primary inductance.
In other words, without a top-class transformer, the designer of a SE amplifier is wasting his time. In the case of the Cary 572SE, the air-gap transformer employs a costly bifilar winding method to tightly interleave the primary and secondary windings in a 16-section array. This technique minimizes shunt capacitance and leakage inductance. The transformer must also be physically large enough to support both the audio and magnetizing currents. The 572SE transformer conductors are of oxygen-free copper and its core laminations are of selected grain-orientated Hypersil, a low-loss steel alloy.
At the other end of the circuit, the input stage is an octal-based 6SN7 double-triode tube configured with one triode amplifying the signal with the second triode acting as a current source. This tube feeds the second stage, where both triodes of a 6SN97 operate in parallel as a low-impedance power drive for the 572-3 output tube grid.
The 572SE is hard-wired throughout—Cary doesn't use printed circuits—which potentially promises better sound through the avoidance of dielectric effects from the substrate. An unavoidable result of this, however, is higher build cost.
A full-wave semiconductor bridge feeds the main output power supply, this choke-smoothed via a Pi-network filter. A tube rectifier with good feed-forward noise isolation powers the anode supply for the more critical input and driver stages. All heaters are DC-powered. Polystyrene and film capacitors are used where relevant. The auto-biased output stage needs no adjustment when tubes are replaced.
Tube life is anticipated at 1000-1500 hours for the 572-3, and up to 5000 hours for the 6SN7 and 6SN97. An output-tube anode protection fuse is accessible on the back panel, while the front carries the Power On/Standby and Power Tube On switches.
The speaker output is via gold-plated three-way binding posts, at a nominal 8 ohms only, while the signal input is SE via gold-plated phono sockets of good quality. Specified as having a high input impedance, over 100k ohms, the 572SE also has a healthy sensitivity, just over 1V being required for full power. A wide frequency response of 15Hz-23kHz is claimed for just 0.75dB of loss relative to the midband reference level.
The CAD-572SE's build quality and appearance are almost as good as the 805C's, though I would have preferred black textured enamel instead of chrome for the mains transformer, which is located at the back of the chassis. The detailing is just too shiny to blend well in my listening room.
Clearly it is not sufficient to have low or zero feedback and/or SE and/or triode outputs; we also require good amplifier design. Some tube designers appear to have lost the plot regarding this fundamental aspect of sound reproduction.
Not so Cary, whose CAD805 and CAD805C have demonstrated a convincing combination of virtues despite the difficulties inherent in the technology and load interfacing. Following my experience of the reference 805C with an SE monoblock at a more affordable price required fine judgment. Out of the box, the Cary CAD-572SE needed almost 50 hours of break-in for its full bloom to develop. From first plug-in it sounded in character—very much a low-feedback triode sound—while continuing to gain in purity and clarity during the running-in period.