Dedicated readers will recall a product review we did some time back on a rather unusual amplifier, the very large and heavy Arc Audio 4000SE. What made this big subwoofer amp unusual was the choice of topology. Rather than a common Class AB or Class D design, the 4000SE used the Class GH configuration, to improve overall efficiency, but without the usual sound quality issues associated with Class D. And if you recall, it worked very well.
Well, it seems that the product folks at Arc Audio have decided to continue the exploration of the efficiency benefits of Class G, and obviously realized that with improved efficiency comes the possibility of reduced heatsink size. So now instead of a monster of an amp, (and once again tapping the engineering wizardry of Robert Zeff) they’ve used the technology to create a series of “Mini” amplifiers. These Mini amps come in single, two and four channel designs, and the stereo models are full range and bridgeable. This time we’re going to have a close look at the Arc Audio KS 125.4, “Mini” amplifier, which has a manufacturers suggested list price of $519.00 in the USA, making it competitive with many of the popular more conventionally larger size amplifiers.
The amplifier is indeed small, considering the prodigious power ratings it boasts. Rated by Arc Audio at 75 watts x 4 channels into 4 ohms, and 125 watts x 4 into 2 ohms, it measures about half as big as a conventional competitive amp, at 11.625 x 5.125 x 1.75 inches. The Arc Audio Mini series is conventional in appearance, finished in a combination of semi-gloss powdercoat on the extruded aluminum heatsink, and using a black anodized, brushed aluminum top cover, with an oval, silver colored Arc Audio badge.
Continuing with the physical examination of the Mini four channel amp, I noted that the power and ground connections will accept 4 gauge cables, and the speaker connections can handle up to 8 gauge wire. The cables are held in place with nickel plated, Phillips head set screws. On the top of the amplifier is where you’ll find all the settings and adjustments. One of the first things I noticed were a pair of small slide switches. One was marked “Input Level”, and it had a Hi and Low setting. The other switch was labeled “Auto Sense” and simply On or Off. Here’s where reading the manual comes in handy… as it turns out, the Arc Audio amplifier can be turned on by a conventional +12V signal from a headunit, or it can be turned on by sensing when the amplified outputs of a factory headunit turn on. The amp does not sense signal, but instead works on a different type of signal commonly found on the amplified outputs of a head unit. Then in a stroke of brilliance, when the amplifier is used in auto turn-on mode, the turn on terminal of the amp becomes a source of +12V output, to turn on additional amplifiers conventionally. This clever feature also will prevent excessive turn on noise, because the consecutive amps will be delayed in turning on until the main amp is done. Sweet!