U.S. Amps has been turning out quality car audio amplifiers since 1986. They're one of the few manufacturers that still build consumer electronics here in the United States (the number decreases every year). They hold claim to several firsts in the automotive amplifier industry, including the first Class D amp to be used at the IASCA finals.
This month we'll take a look at one of their new Class D amps, a Merlin Series II MD 3D. The first thing I noticed about the 3D is that it's relatively heavy for its size. Measuring about 24" long, by 9" wide and 2.5" tall, it tipped the scales at nearly 20 pounds. The amp has an extruded aluminum heatsink, nicely finished in matte silver, with black plastic end caps. The cosmetics are more purposeful than flashy, and for an amp rated at 2,000 watts, that's probably a good thing.
Controls and connections are all laid out along the long, chrome-plated front edge of the amp, and are what you'd expect: large-gauge connectors for the power and speaker cables, (1 gauge and 8 gauge respectively) and the usual control functions, including gain, a remote level control, crossover frequency adjustment, a subsonic filter and a bass EQ control. There are also output RCAs for running signal to a second amp, because if one of these isn't enough, you can strap two of them together to drive a single load!
Circuit DesignIn terms of actual circuit design, this amp is a bit unusual inside. When I removed the bottom cover for the amp - which, by the way, has no fewer than 14 screws holding it on - the first thing I noticed was that all of the power Mosfets and all of the output devices are mounted under the board. This allows the devices to use the entire top surface of the amplifier as a heatsink, rather than just the edges as most other amps do. The heat is spread over a wider area, and is dissipated through radiation more efficiently. Although this is not the first time I had seen this approach, it was the first time I'd seen it on a high-power Class D amp. Another unusual bit was located in the power supply. Where most amps of this nature use a large single toroid in this section, or occasionally a pair of them, this amp actually has four separate toroids and the associated rectification. Very interesting ...
But the fun had only just begun! If I wanted to see just what kind of devices were under that board, I was going to have to remove another 40 screws! After I got it apart, I found an abundance of current capability. The Merlin 3D uses no less than six Mosfets for each of the four toroids, for a total of 24 devices in the power supply. And when you consider that each one of these devices can switch about 60 amps of current ... giddy up!
The outputs are also high-speed Mosfets, as you'd expect, with a total of twelve IRF31N20D's. At well over 250 amps of output current capability, this amp should have no problem driving your woofers!
The power supply has 16,000F of capacitance, and the outputs use four 4,700F electrolytics, for a total of 18,800f of capacitance on the output stage. All of the parts used are of the through-hole variety, with high-quality 5 percent tolerance resistors and good sounding poly caps in the low-level signal path.