Although extremely distracted by Alpine's (always) incredible show vehicle at the CES in January, the only other thing I noticed in the area was a stack of "Power Density" amplifiers in a glass case. I thought they were pretty cool. They look good, they're stackable, but mostly I wondered about the heat management in such a small package. Needless to say, I jumped at the chance to put one through my private wringer and find out how they do it.
Description
The PDX 4.150 is almost too obvious a model number for this 4 x 150-watt Class D amplifier. Therein lies most of the "how they did it;" being a Class D changes everything. Interesting that they're not really calling it Class D; rather, they refer to it as ICEPower, a supply design first brought to market by Bang & Olufsen (B&O), a high-tech, high-end home audio manufacturer. The amplifier sports two 12dB/octave highpass/lowpass/all-pass cross-overs: one for channels 1 and 2, the other for channels 3 and 4. The crossover range is stated at 30Hz to 300Hz.
Decked out in black and silver, the unit has the appearance of a small high-tech disc changer, or maybe a signal processor. There are no fins, vents or visible control panel to tell you that this 10" x 8" x 2.5" box is actually a 600-watt amplifier, except for the power and speaker connections tucked into the back side. Even so, the speaker connections look like oversized USB jacks--more on these later.
Getting this amplifier apart reminded me of those 3D wood puzzles, the ones where you have to slide the right piece just a little this way to release that other piece so you can get the first piece out. Not that you'd ever void the warranty by taking an amplifier apart, of course, but I've got a license to disassemble (now available at Radio Shack -ed.). There is a single hex-head screw to gain access to the control panel in the front side of the amplifier for the installer, but everything else is a little mysterious. The base and endplates of the amplifier are cast aluminum pieces, the top plate is extruded aluminum, and there are three major cast plastic pieces in the exterior framework. In addition, there are three separate circuit boards: the main PCB, a small board for the lighting in the top panel and a daughter board attached to the bottom plate that contains the channel 3 and 4 input jacks and crossover/gain controls. All this is held together by lots of different screws, making this perhaps one of the most labor-intensive amps to assemble/disassemble I have seen in a long time.
While the connector layout makes good sense, with the low level stuff at one end and the power at the other end and speakers in the middle, the layout is not exactly reflected inside the amplifier. Sure, the power supply begins near the power connectors, but it takes up fully half of the main board's real estate, extending diagonally to the middle of the board and then straight back. The double-sided, mostly surface-mount PCB is dominated by three square-core transformers, four output inductors and some very impressive looking capacitors. There's a little 4.5" x 1" aluminum heatsink bar at the back with the power supply switchers and rectifiers, along with two voltage regulators for the signal circuitry, but no output devices! That can't be right. There are two groups of four Mosfets standing up near the output inductors, but those are between the speaker connectors and the inductors, meaning that they are probably used to connect and disconnect the outputs. These would be used as muting devices, like the relays found in other designs. So where are the output devices?