Performance
During the testing of the amp, I was not surprised to see it meet all of the claimed performance specifications, exceeding them in more instances than not. With its regulated power supply design, the amp makes good power, whether at 14.4 volts or 12.5 volts, although as the voltage drops, the amount of current needed increases. Don't you love the laws of physics?
Distortion measured 0.06 percent at rated 4-ohm power at a battery voltage of 14.4 volts. The specs claimed a THD+N figure of 0.02 percent, which the amp did meet at a battery voltage of 12.5 volts. At any rate the difference is completely insignificant. The amplifier's efficiency was average for a Class A/B 4-channel amp: full power at 2 ohms per channel netted a 62 percent efficiency specification, which dropped to 55 percent at 1 ohm.
The frequency-response bandwidth of this amp was excellent: basically flat from under 10Hz to over 90kHz. This is a nice spec and it speaks to the quality of the parts used, but it really doesn't make an audible difference because the compact disc format uses very steep lowpass anti-aliasing filters at 20kHz that prevent any higher-frequency information (fundamentals or harmonics) from ever entering the amplifier to begin with.
Now is a good time to present a slightly different point of view on a commonly mentioned specification, slew rate. While still a relevant measurement on a bandwidth limited Class D amplifier, the fact is, slew rate is a more or less meaningless specification in a modern Class A/B amp.
Why? Back in the day when transistors were in their infancy, there was a need for audio devices to be able to reach their full switching voltage as quickly as possible. Remember, the ultimate goal of an amplifier is to mimic the input signal exactly, while adding gain. The term "slew rate" is used to define the maximum rate of change of an amplifier's output voltage with respect to its input voltage. In essence, slew rate is a measure of an amplifier's output section's ability to follow its input signal. The unit of measure is volts per microsecond.
Slew rate is usually measured by applying a high-frequency square wave input signal and raising the level to the clipping point of the amplifier. Since the perfect device doesn't exist yet, there will always be some tiny amount of time required for the voltage to rise from zero to maximum. To find the slew rate, you simply look at the output of the amp on an oscilloscope and measure the voltage difference and the amount of time it takes the voltage to rise. Then you divide the voltage difference by the time segment. This is why the slew rate spec is stated in volts per microsecond or V/s.
For many years the speed of the output transistors was the limiting factor in the high-frequency limitations of audio power amplifiers. Consequently, a lot of claims were made about the superiority of this or that amplifier based on a good slew rate. The fact is, slew rate has very, very little to do with the sonics of a modern Class A/B amplifier. These days, transistor technology has improved so dramatically that even relatively slow devices are more than adequate for audio bandwidth purposes. In fact, the slew rate of most A/B amplifiers is from three to as much as 50 times the required slew rate for linear amplifier operation! So, when examining the specs of a Class A/B amp, don't put much weight on the slew rate specification. Because of this, I'm not going to quote an actual slew rate spec, but as the folks at Rolls Royce say when asked horsepower numbers, "Rest assured, it is more than adequate".
There are a couple of other specifications that are over-weighted in terms of real world performance as well, but I'll leave that discussion for another time.
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