Things change. And in technology a radical change seems to happen every year. With the popularity of subwoofers in general and dB Drag Racing (i.e. sound pressure competitions) in particular we've seen an increasing demand among enthusiasts for greater sources of amplification; and this need among consumers is naturally leading to the development of more powerful amps that can drive subwoofers higher and higher. For the most part the proliferation of new classes of amps that employ varying methods for greater power and efficiency was related mainly to a refinement of sound. Now, this most recent development in amps has less to do with sound than pure power, technology responding to the cultural shift. The change may not be revolutionary, but it is radical.
In this review we looked at four very high power amplifiers under special conditions. I'm referring to SPL competitions which require amplifiers that produce huge amounts of power, usually under heavily clipped (distorted) conditions. Often the competitors run these amplifiers at battery voltages as high as 18 volts. Efficiency is important, so we paid particular attention to these numbers. Our tests will reveal power output and distortion just below clipping into a resistive load.
Testing these behemoths required an equally gargantuan power supply. We used a Hewlett Packard 15,000 watt supply. This monster weighs in at 600 pounds! Of course some rather nasty loads were required. I noticed a new addition to the test setup: a fire extinguisher! That's right, Joe Sanchez, the tech running the tests, explained to me that he was just "more comfortable" with one by his side. Hmm...
Normally an amplifier plays into a reactive load (that load varies with frequency). This load remains fairly constant over time, at least while listening to music at moderate levels. But in SPL competitions the speakers are pushed to their limits and beyond. Thousands of watts are pumped into the voice coil, resulting in very high temperatures in just a few seconds as the coil becomes red hot. The load that the amplifier reacts to may start out at 2 ohms, but quickly rises to a much higher number as the voice coil heats up. As the voice coil continues to heat, the resistance of the coil climbs, sometimes burning up. Because of these particular conditions, we also measured power output in a vehicle, as bench testing under heavily clipped conditions does not always reveal the complete picture. A 2000-watt amplifier, driven hard into clipping could produce 3000 watts into a resistive dummy load. If this amplifier is 75% efficient, it would draw nearly 350 amps!
Testing these amplifiers on a speaker requires something with tremendous power handling. For this we chose an Audiobahn AW1508X dual 1-ohm voice coil subwoofer. This was then placed in our SPL enclosure in the back of a 1999 Honda Civic hatchback equipped with a high output alternator and four batteries. We chose to test each amplifier at 50Hz, for this was where we achieved the peak output of the subwoofer at the dash using dB Drag Racing microphone placement. Running an impedance curve on the subwoofer system in the car revealed that the impedance of the sub rises to 5.3 ohms at our test frequency. Each amplifier was then installed individually and the maximum AC voltage of each was measured. Then, using Ohm's Law, we derived the power output of each amplifier (V2/R). As you will see, the results varied greatly.
Remember, sound quality, distortion, and other normal audio specifications are not a part of the criteria for judging in this amplifier test. Rather, these monsters were judged on pure output power! We rated these amplifiers only on power output at 12.5 volts, 14.4 volts, and dollars per watt.