To be able to build high power and physically small amps that don’t overheat, requires a great deal of attention to the overall efficiency of the amplifier. That means the goal is to waste as little power as possible to the generation of heat, and put that power where it belongs, at the outputs used to drive your speakers.
Making full use of the design and engineering brilliance of Robert Zeff, the folks at Arc Audio are quickly gaining a reputation for building very good sounding, small footprint amplifiers. Some months back we looked at an Arc Audio mini amplifier that used Class G/H topology to achieve these goals, and it was an excellent performer. This time we’re going to look at another Class G/H model from Arc Audio, the KS 300.2, which is a two channel, bridgeable design rated at a serious 350 watts per channel into 2 ohms. However, the amp is roughly half the size of what you’d expect from a chassis capable of producing over 700 watts of power. How do they do it? A Class G/H amplifier has variable “rail” voltages, which are modulated with rise and fall with the input signal. Thus the amplifier increases efficiency by reducing the wasted power at the output transistors at all power levels. Many people don’t realize that a typical Class AB amp that has a full-power efficiency of 75% is only about 20% efficient at low power levels (1/3 full power) where we do most of our listening. That means that for every 100 watts of power consumed, 80 watts is wasted as heat!
Obviously, when you increase efficiency, you build an amp that generates less heat per watt of power consumed, and you can use a smaller heatsink. Generally speaking, Class G/H amplifiers are significantly more efficient than class AB, but a bit less efficient when compared to Class D. However, Class G/H designs are not saddled with the negative EMI effects and output filtering requirements of Class D designs. Some will argue that this makes the Class G/H amplifier an inherently better sounding design. I have heard good and bad examples of both, and the Arc Audio example is definitely one of the good ones.
The Arc Audio KS 300.2 has power ratings of 180 watts per channel at 4 ohms, and a whopping 350 watts per channel at 2 ohms. Yet the KS 300.2 measures only 2.375”H x 8”W x 11.25”D which is small for a 700 watt amplifier.
Cosmetically, the amplifier is definitely more substance than flash, with a simple stamped- steel smooth cover finished in matte black powdercoat. A silver oval badge is attached via adhesive to the cover, resulting in very simple and understated cosmetics, almost bordering on plain. The stamped cover has perforations running down each side providing air supply to the internal fan, which exhausts the hot air out each end of the amp.
Connections for power and speakers are found on one end of the chassis, and signal input and control functions are on the opposite end. The wire terminals are beefy and solid, with 4 gauge power terminals and 10 gauge speaker connections. Over-current protection comes from a trio of 30A ATC fuses, found adjacent to the power connections.
Signal input and control features are what you’d expect, with good quality potentiometers for gain, bass boost, and frequency selection. The crossover functions can be set for high-pass, low-pass or full range, with a variable frequency range of 55Hz to 5500Hz, a very wide range of adjustment thanks to the incorporation of a x10 multiplier switch. The crossover slopes are -12dB/Oct.
Another handy feature is the amplifiers ability to sum the L+R channels, which is very useful in providing a true summed mono output to a subwoofer connected to the bridged outputs. And if you should ever have a problem, the Arc Audio KS 300.2 also includes a handy diagnostic tool to help you find the problem and get it rectified without having to call for help. The normally green power LED will turn red and flash a coded signal if the built in microprocessor detects a short, over voltage, or an overheating condition exists. By counting the number of flashes and referring to the codes in the owner’s manual, you can quickly tell what the issue may be, and get it corrected.
Internally, the KS 300.2 uses a top quality PCB, and extensive use of surface mount components. There is plenty of onboard capacitance, and all of the transistors are the large case TO-247 style for maximum reliability and current transfer.
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