When Class D amps first hit the sales floor, they were touted as the future of the industry. Problem was, they weren't quite as efficient as promised, couldn't drive a 2-ohm load, and weren't particularly reliable. That was then. Now, with a plethora of powerful, reliable units in the market, it has become pretty standard to choose a Class D amp to run your subs. Finally, the price for this technology seems to be coming down as noted in the $300 retail price of our subject. Let's take a closer look at Kenwood's KAC-9104D.
The Digs
The KAC-9104D is a Class D mono amplifier rated at 500 watts x 1 at 4 ohm, 900 into 2 ohms, and "> 900 watts at 1 ohm." I know what that last one means; I've just never seen a power spec printed like that before.
The preamp follows suit with other subwoofer amps in its class. The KAC-9104D comes with an adjustable 50-200Hz 18dB/octave lowpass crossover, a selectable 15Hz/25Hz/off infrasonic filter and 0-18dB bass boost at 40Hz. It also has speaker level inputs with signal-sensing turn-on.
The chassis is a good-looking cast unit, powdercoated black. A titanium-colored brushed aluminum plate covers a little over half of the top and hangs over the connector panel. The joint between the two pieces on the top glows blue when the amp is powered on. All of the connectors and controls are on a metal plate along one side of the chassis, which simplifies the installation. The speaker and power terminals all use Philips-head screws, so you don't have to keep track of two or three different hex wrenches while you're upside down in the trunk of your ride.
There's enough room in this amp to use the chassis for several different models-certainly one of the ways Kenwood has lowered the cost. For example, there are heatsink lands for about twice the number of hang-offs found in this model. The input and low-level processing take up the right end of the amplifier, the power supply uses the left end and the speaker output circuitry resides in the middle. Each section uses roughly a third of the real estate inside the chassis.
The circuit board is a bit of a paradox to my way of thinking. Kenwood uses the least expensive PCB, a single-sided phenolic board, as the basis for this amp. When I opened the cover, I could see the larger through-hole stuff like transformer, filter caps, inductors, and more, but also dozens of those jumper wires that make single-layer PCBs possible. The power supply and output "hang-offs" occupy the left end and back corner of the cast chassis. It honestly doesn't look a thing like a Class D amplifier, until you turn the PCB over and see all the surface-mount stuff on the underside, that is. This is where high-tech and low-cost meet-another cost saver.
The power comes in via gold-plated screw terminals, through a pair of 30-amp fuses and a low-impedance inductor before entering the laminate-core transformer. There are two 3,900F/16-volt capacitors for primary-side filtering. The transformer is switched at 27kHz by six 60-amp MOSFET devices driven by a TL494 PWM controller. The secondary side is rectified and stabilized by a pair of 3,300F/100-volt capacitors. Four large output MOSFETs, each good for more than 200 watts, switch the rails to the speaker outputs through a standard cap and coil filter.
On the other side, your signal enters the amp through a pair of RCA inputs. Use both of them and the amp sums them together for a mono (full) signal. The signal passes through a gain control, then the lowpass crossover, and finally the infrasonic filter before going to the output stage. All of the components in the signal side are surface mount with the exception of some electrolytic capacitors, which are inserted from the other side of the PCB.