The first thing that you notice about the Planet Audio PA2500D is that it is a substantial power amplifier. The unit weighs in at around 14 lbs. The extruded heatsink appears to be powder coated with a metallic gray paint and sports the Planet Audio logo in the middle of the amplifier flanked by rows of heatsink fins. The PA2500D is not a conventional linear amplifier; it uses the class D topology with pulse width modulation. Class D amplifiers have become more popular lately for high power subwoofer applications where limited bandwidth is appropriate, together with the potential for increased amplifier efficiency. This amplifier is a monoblock and is rated for a maximum power of 2000 watts into a load impedance of 1 ohm. The power inputs, remote turn-on and remote level controls are located on one side of the amplifier while all signal inputs, outputs and configuration controls are on the opposite end. The power inputs are nice hefty gold plated barrel type connectors capable of accepting 4-gauge wire. There are two sets of RCA connectors, one pair used for the input signal and a pair used for line level outputs. The on-board crossover is flexible, having both a subsonic and a low pass section that are both variable. The subsonic is variable from 15 to 40 Hz, whereas the low pass is variable from 50 to 250 Hz, as claimed in the manual. These types of on-board filters are definite requirements on a subwoofer power amplifier. Another nice feature is that there is a phase invert switch, again a useful feature. Popping the bottom cover reveals the heart of the PA2500D. The design makes use of both through-hole and surface-mount components where appropriate. The overall layout of the circuit board is nicely done with a commendable absence of wires, a good thing with an amplifier of this power level. However there was evidence of a couple of small "kludges" on the PCB. Being a Class D amplifier there are a significant number of extra wound components taking care of output filtering. There appears to be two power supplies (the exact configuration is not clear) used to power the amplifier. Power supply control uses the standard 494 controller whereas the output stage uses the familiar Harris 4080 drive chip. There is also an encapsulated block box that probably contains the PWM generation circuitry. Another interesting feature is that the speaker outputs are relay-coupled to the amplifier -- obviously used to try to eliminate turn on/off transients. Overall the PA2500D uses decent quality components and is put together to a good standard. Other items included along with the PA2500D are mounting hardware with Allen wrench,;high quality fuse blocks; and a remote level control together with a 20' phone cable. The remote level control is housed in a clamshell sheet metal box. Also included are the manual that is nicely written (but does include anything about Class D technology, birth certificate -- always a nice touch -- and a large yellow flyer stating that the warranty period extends to five years if the unit is installed by an approved dealer.
Testing
Figures (1) through (3) show the filter function of both the lowpass and subsonic filters at the output of the amplifier. It appears that the Q of the crossover filters in conjunction with the output filter results in a slightly under damped response. This is evidenced by the peaking of the response around the filter cut off points. A little more attention needs to be paid to this area of the design. The subsonic filter has a range from approximately 15 to 55 Hz and the lowpass has a range from 55 to 250 Hz. These ranges are pretty close to the claimed ranges in the manual and are certainly useable in the given application. One problem is that the filter slopes do not appear to both 24 dB per octave as the manual states. The subsonic appears to be 24 dB per octave while the lowpass looks more like 18 dB.
Conclusion
The PA2500D is certainly a powerful subwoofer amplifier. Its feature set and output capabilities allow a flexible and powerful system to be built. The build quality is good and should provide long lasting service. The 1ohm ability of the amplifier is also a plus given the nature of the application of this unit.