Acurus A250 Amplifier
In only a few years, the Acurus brand of audio components from Mondial Designs, Ltd., designed and built in the U. S., has earned a reputation for high quality at reasonable prices. The Acurus A250 power amplifier exemplifies the design philosophy behind the whole line. The amplifier circuitry is fully discrete and completely symmetrical, with relatively little global negative feedback. Precision 1-percent metal-film resistors and metalized-polypropylene capacitors are used to achieve close circuit tolerances, and high-quality circuit boards contribute to a professional look and feel.
The Acurus A250 was designed to drive low-impedance loudspeaker loads safely at high continuous power levels. Its power supply has a 1,000-VA toroidal transformer capable of delivering the high currents required for driving some of today's loudspeakers. To dissipate the considerable heat generated in a powerful but relatively compact amplifier without resorting to a noisy fan, it is equipped with finned aluminum heat sinks on both sides of its chassis, amounting to more than 6 square feet of radiating surface.
The A250 is rated to deliver 250 watts per channel into 8 ohms, or 350 watts into 4 ohms, from 20 to 20,000 Hz with no more than 0.06 percent total harmonic distortion (THD). Its input impedance is 20,000 ohms, and its A-weighted signal-to-noise ratio (S/N) is specified as 110 dB.
The all-black front panel contains only a large rocker power switch, with a green light to indicate that the amplifier is operating. The rear panel has five-way binding-post speaker outputs, compatible with dual banana plugs as well as wires and lugs, and phono-jack input connectors. Since the input (and output) connectors for the left and right channels are 12 inches apart, some dual-signal cables may not be suitable. The amplifier weighs 35 pounds.
Although the Acurus A250 ran quite cool under normal listening conditions (never more than faintly warm), it became very hot during the mandated 1-hour preconditioning at one-third rated power into 8 ohms and during subsequent high-power testing. In fact, no part of the exterior could be touched for more than a second or two, and the heat sinks were hot enough to cause burns. That should not be a problem in any reasonable home listening situation, however. Although most amps run their hottest at one-third power, average power levels during even loud music are considerably lower.
We have never before tested an amplifier that became as hot as the A250 did without being damaged in some way, but the A250 never shut down, blew a fuse, or otherwise indicated its displeasure at our rather severe treatment. Clearly, it was designed to deal with low-impedance loads, and it did that with ease.
The A250's frequency response was very nearly a demonstration of the overworked term ruler-flat. From 20 to 20,000 Hz, the output varied a mere +/-0.02 dB, falling (if that is the word) to -0.5 dB at 110 kHz and -1.5dBat 200 kHz. Noise measurements also showed impressive performance.
Our power measurements highlighted the amplifier's conservative ratings. The clipping-level output at 1,000 Hz into 8 ohms was 300 watts per channel, and into 4 ohms it was 480 watts. With only one channel driven, those numbers increased to 350 and 600 watts, respectively. We did not make continuous power measurements into 2 ohms because of concern about possible damage to the amplifier (which became far too hot to touch on any part of its surface) and because it was not rated for use with 2-ohm loads. But the dynamic power output into 8 and 4 ohms was 400 and 700 watts, respectively (for dynamic head-rooms of 2 and 3 dB), and into 2 ohms it was 485 watts.
A spectrum analysis of the distortion at 250 watts into 8 ohms, covering all harmonics up to the twentienth but excluding noise, yielded a figure of 0.012 percent. At 350 watts into 4 ohms, distortion rose to 0.023 percent. The amplifier's single-channel maximum power at 0.1 percent THD plus noise, as a function of frequency, was about 320 watts into 8 ohms from 50 to 20,000 Hz, falling slightly to 300 watts at 20 Hz. Into 4 ohms, the maximum power output was 490 to 495 watts from 100 to 20,000 Hz, falling to 470 watts at 20 Hz.
The Acurus A450 is an enormously powerful amplifier. Even without considering its modest cost, its quality, no matter how defined, is equally impressive. Clearly, its electrical performance was topnotch, and there were no unexpected glitches or anomalous characteristics to diminish our highly favorable impression.
To gain some idea of the energy storage capacity of its power supply, we switched it off while playing music at a normal room level. For almost 30 seconds, the volume and quality of the sound did not diminish.
Considering the A250's power capabilities and current reserves, we would expect very few loudspeaker loads to present any serious challenge to its performance. And although it was one of the hottest-running amplifiers on the test bench that we have ever evaluated, it was also one of the coolest in normal operation.
Almost $900 for a power amplifier may not seem inexpensive, but one could pay many times that sum and not get any more performance than from the Acurus A250, if even as much. Not everyone needs to have several hundred watts on tap, but if you are one who does, this is an amplifier to look at, and listen to, closely. It is an outstanding product in terms of both performance and value.
