We've covered much about the crossover workings of a subwoofer in these posts. We will cover more, such as phase, when we get to the setup portion of our efforts. For now let's move on to the second category of a subwoofer, the power amplifier. For those just catching up with this series, you can go here to start reading. Unlike the majority of loudspeakers on the market, which are passive devices requiring the user to supply the power to drive them, modern subwoofers are active with built in power amplifiers. These internal subwoofer power amplifiers vary in wattage, topology and design. Most active subs today use what we refer to in the industry as a 'plate amp'. This means the amplifier is mounted to a plate of metal and comes to the manufacturer as a pre-built unit. Look at the rear of most subwoofers and you'll see this plate I am speaking of. In the early days, these plates included a rather large heat sink to dissipate the heat generated by the more traditional Class AB power amplifier topology. Today, most subs require smaller heat sinks, while generating far greater power, because they are typically designed with high efficiency Class D amplifiers or a hybrid of a class D amplifier, called a tracking amp, whose brand name is BASH. Whatever their topology, power amplifiers in modern subwoofers can vary from 50 watts up to many thousands of watts. Here is a picture of the rear panel plate amplifier of my REL T9 subwoofer in Music Room Two. This is a great subwoofer using a more traditional class A/B amplifier design producing 300 watts. Many other designs, as mentioned earlier, can produce far greater watts with even smaller heat sinks. One thing about subs you may have already figured out. If you include a subwoofer in your system you are, by default, running a bi-amplified stereo system. The choice of how many watts are needed in a design depends greatly on the design choices made by the design team. Parameters include the size of the enclosure, ported or sealed, servo or EQd and overall performance expectations. I've been involved in a number of subwoofer designs: those of Genesis technologies and Martin Logan's Descent series come to mind. In both these design challenges the subwoofers were servo controlled and required enormous amounts of power; a topic we've yet to cover but will shortly. Enclosure size plays, perhaps, the second biggest card when it comes to required amplifier power. The general rule of thumb is the smaller the cabinet, the greater power you need to make it properly woof. A good example of this was Bob Carver's impressive work on his original subwoofer called the True Sub or the Cube. It featured a tiny eleven inch cubed enclosure with dual woofers, and was powered by a 2,700 watt amplifier. Talk about tiny! This less-than-a-cubic-foot of subwoofer rumbled down to 18Hz and, if you could keep it from walking across the room when it got going, produced prodigious bass. The point is, you really cannot make an adequate judgment on the quality of a subwoofer based on wattage alone. Properly designed subs should have the power needed to produce their rated results. What's the lowest it goes, what's its crossover features, size, etc.? They all matter. Tomorrow, let's look at connection. How do we connect a subwoofer to our systems.
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