Class D amplifiers have really become the standard for powering large subwoofer systems, and Crossfire was one of the first companies to embrace the new technology a little over a decade ago. Like many technologies, Class D amplification had its share of inherent hurdles to overcome, such as reliability at low-impedance loads (for every manufacturer), and the actual efficiency gains over traditional Class AB amplifiers.
Both the technology and design of Class D amplifiers have improved over the years as seen in our test subject, Crossfire's VRA1000D-the latest and most mature amplifier offered by the seasoned team. In this issue, we'll see how far Class D and Crossfire have advanced with the technology.
The Design
The VRA1000D is a single-channel amplifier rated at 250, 500, and 1,000 watts into a 4-ohm, 2-ohm, and 1-ohm load, respectively. The amp features a 20 to 50Hz subsonic filter and 50 to 250Hz lowpass filter, both 18dB/octave, as well as an optional remote level control-all pretty routine for a high-quality subwoofer amp. The extra bonus here is the ability to "strap" two VRA1000D amps across a single subwoofer or set of subwoofers and the pair will generate 2,000 watts into a 2-ohm load.
All black and sinister looking, the VRA series amps remind me of the monolith from the movie 2001: A Space Odyssey. (Remember the sinister black monolith surrounded by monkeys?) The unit is constructed in the upside-down extrusion style, with sheet aluminum bottom cover and stamped-steel endplates. While there are wide fins along the sides, the top is completely flat brushed and black anodized. The only break in the surface is the engraved black-on-black Crossfire logo in the center. Black molded plastic endcaps cover the connectors and controls on both ends. By cover, I mean that they form an extension of the extrusion about 5/8 inch beyond the endplates, visually covering the connectors while allowing for easy routing of cables and access to the controls.
The low-level stuff (RCA connectors, gain, and crossover controls) live on the left endplate of the amp, while the larger power and speaker connectors reside to the right. There are other ways to minimize interference and noise from the power supply and output stages into the signal processing stage, but I'm a big fan of this layout because it simply keeps all the noisy stuff physically separated from the low-level signals.
The B+ and ground terminals will accept up to 2-gauge cable, and the speaker and remote turn-on terminals are sized for 8-gauge wire. Power and protection indicator LEDs are located in between the speaker and power terminals, and they won't be seen if the amp is flush-mounted.
The internal layout is fairly standard, uh, sort of. It looks pretty much like what you'd expect from a two-channel amplifier. Of course, it's a mono amplifier, hence the "sort of."
The power supply transformer, capacitors, and switching MOSFETs occupy one side of the amp near the endplate with the power terminals. The transformer uses multiple fine strands of wire, sometimes referred to as "Litz-wound," around a good-sized toroid core. There are six 2,200f caps for primary filtering and eight switching MOSFETs running at a frequency of 22kHz. Crossfire uses the TL494 PWM controller in a surface-mount package as the brains of the power supply. The circuit board is stuffed with about a 50/50 mix of surface-mount and through-hole components.
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