Have you noticed lately that a lot more audio manufacturers have been talking about efficiency? In a car stereo amplifier, we measure efficiency by comparing the amount of power consumed (in watts) from the battery, versus the amount of power (also in watts) delivered to the loudspeakers. The difference between the two numbers is the amount of power wasted in the generation of heat. Traditional Class AB amplifiers are notorious power wasters, particularly at fractional power levels, where we do the majority of our listening (ie: When a 200 watt amp is operating at 20 watts of output). At these levels, most Class AB amplifiers are less than 20% efficient, which means more than 80% of the current consumed is simply wasted as heat. Lots of heat generation requires large heatsinks, which translate to bigger amplifier sizes, and higher prices.
With the advent of high performance silicon integrated circuits and very fast transistors, it has become possible to build much more efficient Class D amplifiers that have full range audio capability. These designs benefit from dramatically improved power efficiency at all output levels, and as a result produce less heat and hence, can be made a lot smaller in physical size than a conventional amp that needs a comparatively larger heatsink. This review will take a closer look at Pioneer’s latest model of efficiency, the full range Class-D GM-D9500F.
The GM-D9500F is a four channel full range amplifier, rated at 75 watts per channel into 4 ohms, or 100 watts per channel into 2 ohms. It can also be bridged to create a pair of 200 watt channels, in either mono or stereo bridged configurations. Measuring only 10.7” x 7.9” x 2.2”, the amp looks way too tiny to be capable of producing 400 watts of power. And if not for its highly efficient Class-D topology, it probably would be. But with average operating efficiencies of better than 60%, not much of your power is wasted as heat with this one. The heatsink is smooth cast aluminum, and is finished in a semi-gloss black powder coat with a silver, brushed aluminum top plate. The result is a small, but classy looking piece of gear.
The GM-D9500F uses barrier-strip type terminals for all wire connections, and while pretty basic, the terminals are plenty beefy, enough to handle the required wire gauges. These terminals are found all along one end of the amp, and all the signal input and controls are located on the opposite end. Here you’ll find the front and rear RCA inputs, and a set of pass through RCA outputs. Controls are quite basic with a single gain pot for each pair of channels, and high pass, low pass and full range crossover switches. Crossover frequencies are variable from 40-500Hz, and use -12dB per octave slopes. A handy 2-Ch/4-Ch input signal switch allows you to drive all four channels with only a single pair of inputs if you prefer, and a pair of pigtail RCA’s are provided for high level input connections. Inside the amplifier a nicely manufactured PCB contains a mixture of through-hole and surface mount parts, and each one of the amplifier’s MOSFETs is secured to the heatsink with its own screw. No big multiple device clamps secured with only two or three screws found here! One 40A chassis mounted fuse provides over current protection.
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