Performance
Our measurements indicate that the 500D is an awfully strong “250-watt” amp. This rating is spec’ed into 4 ohms at 12.8 VDC with less than 0.5% THD+N. Under these conditions, we saw 456 watts at 1% THD+N. The biggest number that MTX is willing to come off of is a 775-watt rating at 14 VDC into a 2-ohm reactive load. We saw something more like a kilowatt. If MTX gets much more conservative with their power ratings, Pat Buchanan is liable to start calling for campaign contributions.

Although somewhat less critical in a subwoofer application, A-weighted signal-to-noise measured better than -121 dB. You’d pretty much expect an amp designed to play from 200 Hz down to do well on an A-weighted noise measurement. The A-weighting filter places the most emphasis on the frequencies that our ears are most sensitive to, between 2 kHz and 3 kHz. One would hope that a class D amplifier would have very little output of any kind in this frequency range. Most of them are very quiet indeed, it’s just that the 500D manages to better the rest by a good 20 dB. Obviously, at these ultra-low noise levels, that extra 20 dB isn’t going to cause an audible difference.

Many MTX amps have made their way through our lab, and when it comes to noise specs, they are practically always among the best in the industry. The amplifier’s 120 dB S/N isn’t at all uncommon, and we’ve seen turn on/off noises that measured in the single digits. The 500D, however, exhibits turn-on/off noises of 28.5 dB SPL and 14.4 dB SPL. This is an issue only if there’s a soundoff judge poking around in your car on a regular basis.

Efficiency is where the class D designs really shine, and the 500D is no exception. Efficiency measured 65 percent at 1/3 maximum power into 2 ohms. This is a few percentage points lower than many of the class D units that we’ve seen thus far, but still far better than the typical class A/B amp. About 35 percent of the 458 watts that the 500D draws from your charging system at 1/3 power is converted into heat rather than output signal.

Connections & Adjustments
Connections to the 500D include RCA inputs and outputs, a high level input connector (with supplied harness), and a telephone-style receptacle for the EBC’s wired remote. Power connections are 5/8-inch spade receptacles while speaker and remote turn-on offer 3/8-inch connections of the same type. Adjustments to the 500D, excluding the hard-wired EBC, are located on one end panel and include input sensitivity, low-pass cutoff frequency and bass boost adjustments.

Installation
Installing the 500D shouldn’t cause any undue stress; the amp has nifty little nylon iso-feet, a low 2-inch profile, and plenty of remote wire. The 500D is quite small considering its output potential, certainly small enough for under-seat mounting in most cases, although the 500Ds manual suggests using this location as a last resort. It’s vaguely possible that someone could get an unpleasant zap if they got a bare toe across the outputs of this thing, but, otherwise, about half-way between the battery and the speakers seems like a great place. Especially when the amp is physically small but has a substantial appetite for current. Of course the 500D isn’t the current hog that a class A/B amp in this power range would be, but if you do a little math, you’ll see that getting a kilowatt from a 12-volt system requires a boatload of amperage no matter how you stack the efficiency numbers.

Manual
In many of our previous amp reviews, I’ve been skipping the manual write-up for the simple reason that ‘If you don’t have anything nice to say...’ Most of the manuals that we see make me want to smack somebody upside the head with a physics textbook. If you don’t see a manual section in the review, it’s probably safe to assume that I read it and wouldn’t necessarily recommend that you do the same. That said, the manual for the 500D is a refreshing change.

While it’s not everything that I could ask for, it does have a few points that make it stand out. One nice touch involves noting the factory settings for the crossover and input sensitivity. Another is a complete lack of hocus-pocus recommendations. The troubleshooting section is primarily aimed at the amp and its functions rather than any noise problems that you might encounter along the way. A few points on troubleshooting noise problems might be a worthy addition.