The subwoofer channel uses crossover points from 30 to 300Hz and includes a 5Hz to 55Hz subsonic filter to prevent woofer damage. An optional Lowpass Level control can be connected to adjust the volume of the subwoofer channels by remote control, which is a handy feature when your source unit doesn't have a subwoofer volume control.
Each set of channels also has a bass boost control that provides up to 18dB of boost at 45Hz. The "Q" of this boost is also fairly high, producing a narrow band of boosted frequencies centered on the 45Hz mark. This "narrow range," or "high Q" design, means it's much more useful (and better sounding) for bumping up the bass than some of the "Bass EQ" controls that seem to boost everything from 30Hz to 500Hz.
My only gripe with the feature set of this amp, and also with many other 4-channel designs these days, is that they lack a signal routing switch allowing the user to drive all the channels with a single pair of RCAs.
Design
One of the major hurdles that an engineer faces when trying to build an amp housing both fullrange and Class D topologies is noise. Any Class D amp generates a lot of noise from the switching output stage. When the amp is only connected to a woofer through a lowpass filter, noise isn't an issue but keeping this noise out of the fullrange channels in the same chassis is difficult at best. The Phoenix Gold Roadster 66 takes a different and clever approach that I haven't seen in a long time. Inside the single chassis are basically two completely separate amps. The PCBs are separated by several inches and each PCB has its own dedicated power supply. Having two separate power supplies in an amp like this is a good idea because with a single supply, when the bass hits hard, the amp will be "current starved" and generate distortion in the fullrange channels. By using totally independent supplies for each type of output, the Phoenix Gold engineers eliminated this problem. This method provides maximum noise isolation as well and betters thermal performance by spreading the heat from both amps over a wider area of the heatsink.
Inside, the amp is nicely laid out, and the high-current PCB traces are wide and well isolated from the low-level signal carrying sections. The PCB itself is a double-sided, plated through affair, eliminating jumpers and noise-inducing sky wiring. The parts inside are good stuff too, with thru-hole type 1 percent tolerance metal film resistors and poly caps for sonic excellence. The soldering and general build quality also looked good. The cabling, where signal is routed to a daughter board for signal processing, is short and distanced from the high-frequency switching noise of the power supply. Overall, nice attention to detail like this results in a better sounding and more reliable product. The Class D section uses the expected complement of high-current Mosfets and large, fast high-temp capacitors provide ample energy storage.
The fullrange side of things uses a tried-and-true Triple Darlington configuration that has been the backbone of many well-respected designs over the years, including the highly regarded Phoenix Gold amps of the early '90s. The design is stable and highly regarded as a good-sounding platform.