Most of my friends know I’m an audio nut, and they’ve been exposed to my rather eclectic music collection. And most of them know I’m also seriously into motorcycles, and I have been for about 30 years. At any given time there are a half dozen or so bikes in my garage, and because one of my favorite kinds of riding is long distance touring, I have kept a touring bike of one kind or another in the stable for the last decade or so. The most recent touring mount in my garage is a Harley-Davidson Electra Glide Ultra. This is the first bike I’ve had with a full-on audio system, and I have to say, it’s a very pleasant way to make the miles go by. But, as usual with a factory audio system there is a ton of room for improvement. This fact has not gone unnoticed by the aftermarket, and in this review we’re going to take a closer look at one of the best value upgrades around, priced at just $469.00 in the USA. The Arc Audio MPAK4 system is designed to fit all Harley-Davidson touring models with a factory equipped radio.
The Kit
The Arc Audio MPAK4 kit is comprised of one of their KS 125.2 Mini Class G amplifiers, and pair of KS 6026 6.5-inch coaxial speaker systems, custom adapters to mount the large 
speakers in the OEM locations, and a custom wiring harness designed for the specific application. When you compare the cost of the entire MPAK4 system to some of the other aftermarket alternatives, the difference in value is obvious.
Also included in the kit are the adapter rings needed to fit the 6.5-inch speakers to the Harleys 5.25-inch openings, and an installation kit that includes a wiring harness to get the amp connected to the bikes battery. A complete set of installation and tuning instructions are also included, and even include photographs of an actual installation being done, which really makes them quite easy to follow.
The Amp
The KS 125.2 Mini is quite a diminutive amplifier, but packs power levels that belie it’s small stature. Regular readers will recall a previous test of the 4 channel version of this amplifier, the KS 125.4, which got high marks for sound quality and efficiency, and the 2 channel version will certainly share those attributes. With power ratings of 70 watts and 125 watts into 4 and 2 ohms respectively, the KS 125.2 can easily provide more than double the power of the Harley-Davidson (Harmon-Kardon) radio. Because the KS 125.2 uses higher efficiency Class G topology, the heatsink can be made smaller than you’d have with a typical Class AB design. Measuring only 8.25” x 4.85” x 1.7”, the amplifier is small enough to be mounted within the confines of the bikes fairing. This approach simplifies the wiring as well as preserves vital luggage space.
The KS 125.2 is a full featured amp with high, low and all pass crossover settings, and an adjustable frequency range from 55 to 550Hz. The instruction manual suggests a 100Hz high pass crossover setting for the Harley application.
The Speakers
The KS 6026 coaxial speakers in the kit use materials which are resistant to the typical sun, temperatures, and moisture encountered in a motorcycle application. These speakers undergo a rigorous 550 hour test process with direct salt water spray, ultraviolet light testing, and temperature extreme testing. The end result is a product that Arc Audio can confidently recommend for both marine, and motorcycle applications. The injection molded ABS adapter rings allow the large format speakers to mount in the factory locations with no modifications to the motorcycle, and even the tweeters remain covered by the OEM grille material. Weighing just under 30 ounces each, and using 80mm ferrite magnetic motors, the KS 6026’s make the OEM speakers look puny! The moving assembly is driven by a copper coil wound on a 25mm Kapton former. Connected to the stamped steel basket by a cotton/Nomex spider and a butyl rubber surround, the injection molded polypropylene cone is finished in a good looking “brushed-look” gloss black. The 1” tweeter has a silk dome, and is powered by a neodymium motor, and uses Ferrofluid cooling for improved reliability.
Read on for Results
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