McCormack Power Drive DNA-1 power amplifier

For many audiophiles, choosing a power amplifier is a vexing problem. Just how much must one spend to get true high-end sound and a solid build quality? How much power is really needed? And what amplifiers are suitable for driving low-impedance loudspeakers?

These common questions reflect a need in the marketplace for an amplifier with high-end sonics, good build quality, and beefy enough design to drive even the most difficult loads. The ideal amplifier would approach the musicality of the best amplifiers available, have more than enough power to drive virtually any load, and be priced within the range of most music lovers. This utopian product would also put most of the build money into sonics rather than thick front panels and lavish cosmetics. In short, it would be a high-end amplifier for Everyman.

The amplifier under review, the $1995 McCormack Power Drive DNA-1, attempts to address this need. It aspires to high-end sonics, is a high-current powerhouse, and places an emphasis on most sonic performance for the money rather than ostentatious appearance. I've seen this product at shows and been eager to hear it had to offer. Does it fulfill the dream of nearly every audiophile—a truly musical muscle amplifier at an affordable price?

It's my job to find out. My name's Harley. I'm an audiophile.

Power Drive
The $1995 DNA-1 Power Drive amplifier is one of the first products to bear the "McCormack" name. Although McCormack may be unfamiliar to audiophiles, the other products from Steve McCormack certainly aren't; Steve is the designer of such highly regarded products from The Mod Squad as Tiptoes, the Line Drive, and the Phono Drive. In an interview elsewhere in this issue, Steve talks about the relationship between the musical experience and electronic design.

In deciding to design and build a power amplifier, Steve McCormack's goal was to make an amplifier that would drive virtually any loudspeaker, be intrinsically musical, and didn't cost an arm and a leg. He felt that the musicality of cost-no-object amplifiers could be brought to a more popular price. He did this by incorporating only cost-effective components, eschewing lavish cosmetics, and paying close attention to the product's execution and build.

The DNA-1 is attractively finished, with a ¼" grey faceplate and the McCormack logo recessed behind the front panel. An on/off rocker switch is accompanied by two LEDs to indicate power-on and if the protection circuit has been tripped. The rear panel holds a pair of gold-plated RCA input jacks, fuse holder, and IEC AC jack. Unusually, both five-way binding post and terminal strips are provided for loudspeaker connection. This gives the user the choice of screw terminals or posts, and also provides for easier bi-wiring. The screw terminals are the preferred connection method, both because they provide better contact with the spade lug and because they're connected more directly to the output stage.

The chassis is made from copper-plated steel, flanked by large heatsinks on either side of the unit. A "mechanical grounding" spike is provided that threads into the chassis bottom rear. In addition, DNA-1's Soft Shoes mounting feet can be fitted with Tiptoes for reportedly improved performance.

Inside, I was impressed by the DNA-1's clean layout and minimum of point-to-point wiring. The 900VA power transformer (made by Counterpoint) is mounted at the chassis front, with the left- and right-channel output boards on the chassis sides. The double-sided glass-epoxy input board—which also contains the protection circuitry and some power-supply components—is located at the rear.

The DNA-1 uses a unique power-supply configuration that gives it its name: Distributed Node Amplifier (DNA). Rather than have a few (usually four) very large reservoir capacitors next to each other (and to the bridge rectifiers), the DNA technique distributes many smaller reservoir capacitors among the output stage. Each output transistor (there are eight per channel) has its own 4000µF electrolytic capacitor mounted directly next to it. This gives the DNA-1 its unique internal appearance: 16 horizontally mounted caps attached to the output boards rather than a few vertical sodacan-sized caps on the chassis bottom. According to Steve McCormack, the DNA technique reduces the impedance of the high-current power supply, resulting in a greater sense of dynamic ease and clarity.

The power-supply regulation, mounted on the input and driver board, is a custom, all-discrete design. This circuit supplies ±60V to the input and driver stages. The only IC regulators supply ±15V for such housekeeping functions as the protection circuitry and DC servo. All power-supply electrolytic caps (including the 16 output-stage caps) are bypassed with Wima film caps.

The circuit is a direct-coupled design employing a DC servo. JFETs are used in the input stage, with a bipolar cascode stage and MOSFET drivers. The output section comprises four pairs per channel of complementary bipolar transistors. The design uses no local feedback, and only 6dB of global feedback. The protection circuit looks for DC at the output (appearing because of DC at the input or amplifier fault), shuts down the amplifier, and shorts the input to ground. The protection will also trip if one of the 8A rail fuses blows. The design of the driver stage is arranged to allow the amplifier to clip gently, rounding off the square edges of a clipped waveform. This prevents the hard snapping sound on transients that characterizes most solid-state amplifiers when driven to overload.

The DNA-1 can reportedly drive 2 ohm loads without problems. For those users with difficult-to-drive loudspeakers (less than 2 ohms), The Mod Squad will adjust the DNA-1's drive voltage to optimize 1 ohm operation (at no charge to the user except for shipping). Conversion to balanced mono operation will soon be available as an upgrade for $350 per amplifier. Retail price of a bridged pair is $4595.