AudioSource Amp One Power Amplifier Review

The compact, low-price AudioSource Amp One stereo power amplifier, although conservatively rated at a modest 60 watts per channel, can meet the requirements of most home audio systems. It is housed in a lightweight (a little more than 14 pounds), low-contour package that measures 16-1/2 inches wide, 11-5/8 inches deep, and only 2-3/8 inches high. A distinctive feature of the front panel is a pair of round, illuminated level meters about 1-1/2 inches in diameter.

Individual level-control knobs for the two channels are at the right end of the panel; several small pushbuttons and a headphone jack occupy the left. Each button displays a small red light when it is activated. The power button is red, but the others are black, like the amplifier’s panel and case. Two of the buttons activate the two pairs of speaker-output terminals on the rear apron, another increases the level meters’ sensitivity by 10 dB, and the fourth turns on the soft-clipping circuit, which rounds off the peaks of a signal that might otherwise drive the amplifier into hard clipping. By reducing the level of high-order harmonics in the amplifier output, soft clipping lowers the risk of damaging a tweeter.

The level meters have two scales, calibrated in watts (into 8 ohms) and decibels relative to the 60-watt maximum power rating. The meter movements respond rapidly to level increases, with a slower decay. The power-scale calibrations extend down to 0.2 watt (200 milliwatts), or 20 milliwatts in the expanded mode, providing useful readings over the full listening range.

On the Amp One’s rear apron are two sets of insulated binding-post speaker terminals, which accept single or dual banana plugs as well as stripped wire ends. A slide switch connects the two channels for bridged (mono) operation, increasing their maximum output to about 170 watts into 8 ohms. The amp has a single unswitched AC outlet and a user-accessible fuse holder (an increasingly rare convenience). Unconventionally, there are two pairs of input jacks, marked for line and CD sources. They are not selectable and differ only in their sensitivities and impedances, which are 30,000 and 50,000 ohms, respectively.

Through the ventilating slots on the top cover can be seen a husky toroidal power transformer as well as the amplifier circuit board and its two rows of black heat-sink fins. The specifications include a power rating (with both channels driven) of 60 watts per channel into 8 ohms with less than 0.04 percent total harmonic distortion (THD), frequency response of 20 to 20,000 Hz ±0.5 dB, 110 dB signal-to-noise ratio, and 2 dB headroom (presumably dynamic). The rated sensitivity is 0.8 volt for the line input and 1.3 volts for the CD input. Price: $300.

Lab Tests

The 1-hour preconditioning at one-third rated power from both channels left the top of the Amp One comfortably warm. During the following high-power tests it became noticeably hotter, but in normal operation it became only faintly warm. With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 80 watts, for a clip-ping-headroom rating of 1.25 dB. The output into 4 ohms (for which the amplifier is not rated) was 100 watts per channel. We made 2-ohm measurements on only one channel, with the other connected to a 4-ohm load, since we have found that many amplifiers not specifically rated for 2 ohms will blow an internal fuse or become otherwise disabled when driving 2-ohm loads with both channels. The 2-ohm output at clipping was 84 watts.

Dynamic power tests produced an output of 110 watts into 8 ohms (for a dynamic headroom of 2.6 dB), 170 watts into 4 ohms, and an impressive 225 watts into 2 ohms. The last figure is actually a more important performance indicator than the continuous 2-ohm output power, demonstrating that the Amp One can deliver high-level program peaks, without clipping, to almost any speaker.

The amplifier’s distortion varied only slightly with power and frequency. From 6 to 60 watts into 8 ohms, at frequencies from 20 to 20,000 Hz, distortion remained between 0.032 and 0.036 percent. With 4-ohm loads, the range was 0.062 to 0.07 percent from 1 to 100 watts, and into 2 ohms it was 0.11 to 0.14 percent from 1 to 80 watts.

The soft-clipping circuit commenced rounding off the waveform peaks well below the maximum unclipped power available without it. For example, at a constant 0.2 percent distortion, the Amp One delivered 90 to 94 watts into 8 ohms from 20 to 20,000 Hz when soft clipping was turned off. With soft clipping switched on, the power was 51 to 55 watts over the same range. More power was available only at the expense of more distortion than in the normal operating mode.

The amplifier’s frequency response was flat from 10 to 2,000 Hz, rolling off at higher frequencies to — 0.6 dB at 10,000 Hz and —1.6 dB at 20,000 Hz. This was the only measurement we made that did not match the manufacturer’s rating.

The slew factor was 3 because of a visible waveform distortion that occurred above 60,000 Hz.

Input sensitivity at maximum level settings, for a reference output of 1 watt, was 95 millivolts (mv) at the line inputs and 160 mv at the CD inputs. The respective A-weighted noise levels, with EIA standard gain settings, were —91 and —91.6 dB, relative to 1 watt. Referred to the amplifier’s 60-watt rating, these figures correspond closely to the rated 110-dB signal-to-noise ratio.

The meters proved to be as accurate as could be expected from such small scales and certainly more than adequate for their intended purpose. At rated power, they read about 10 percent low and showed an increasing error at lower levels, down to about 50 percent low at the 60-milliwatt calibration, where the actual output into 8 ohms was 88 milliwatts. These errors were comparable to the width of the slender meter pointer, which is highly visible against the white scale.

Comments

The AudioSource Amp One is a good value in a compact and attractive amplifier, and, although it won’t appeal to “high-end” enthusiasts, it can do a first-rate job in most installations. It sounded as good as most of the more powerful and better-known amplifiers we have used, with more than enough power for the kind of listening most people enjoy—that is, not at the levels experienced in a concert hall by the conductor, the orchestra, and the first few rows of listeners.

The Amp One’s size, weight, and cool operation make it a practical, stylish companion for most of today’s source components. We operated it on top of a CD player without fear that its weight would deform the player’s top or that its external hum field would degrade the player’s signal-to-noise performance (the amplifier’s toroidal power transformer generates a very low external magnetic field). The Amp One produced no turn-on or turnoff transients. Indeed, it was as civilized an audio component as we have used in some time. And, not least of all, its price is right.

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