Have you ever purchased something, then regretted your decision a few days later? We recently ran into a situation regarding car audio subwoofer amplifiers where the answer to “Which is better?” became a complicated discussion.
We’re going to look at two amplifiers – one is a consumer-grade product that’s extremely popular, the other seems to offer too much bang for your buck but might be well-suited to applications where short play times are acceptable. We’ll be clear: If you’re building an SPL competition system, it’s likely that the majority of what we are going to discuss won’t apply to you. In those instances, power and efficiency are pretty much the only specs worth examining. For those who live with their stereos every day, choosing a reliable amplifier that adds minimal distortion or noise to their music is crucial.
A friend at a local car audio shop called and asked for an opinion about a new line of amplifiers being offered to his store. It’s worth a proper examination in every situation like this, rather than guessing about the product’s performance. Thankfully, we have the BestCarAudio.com Test Drive lab at our disposal. We dropped by the shop to pick up the sample amplifier and brought home the amp that’s usually in the store owner’s car.
For this comparison, the reference amplifier is an ARC Audio XDi 650.1. This amp is rated to produce 650 watts into a 1-ohm load and includes a wired remote level control. Physically, the amp has a footprint of roughly 9 by 6.25 inches and stands only 2.16 inches tall. For the power it produces, the size is compact and convenient.
In this case, the sample amplifier is a high-power subwoofer amplifier that is rated to produce over 2,000 watts into a 4-ohm load and over 3,000 watts into a 2-ohm load. The chassis is just under 9 by 7.5 inches in size, and the amp stands at just under 3 inches tall. That’s a pretty small package for something that claims to make that much power.
Physical Layout Affects Installation Options
OK, let’s talk about the layout of the connections and controls on these amplifiers.
The ARC Audio has power, signal, speaker connections, fuses and remote-level control jacks along the amp’s bottom edge. As such, your installer can place multiple amplifiers side by side in your vehicle to create an elaborate audio system. The crossover adjustments, infrasonic filter, bass boost and sensitivity control are accessible from the top of the amp. The ARC Audio badge in the top-center of the amp can be installed facing in any of four positions to increase installation options while keeping things looking great.
The challenger amplifier has power connections on one side and speaker and signal connections on the other. There’s no way to mount these amplifiers side by side. The signal processing controls and the jack for the optional remote level control are on the right side. Once installed, the tech will need to get very close to the amp to see the controls to make adjustments. There are no fuses on this amp, so an external fuse needs to be added, which will increase the installation cost. At almost 3 inches in height, it’s also unlikely this amp would fit under the seat of most cars on the road today.
Score: ARC Audio 1, Competitor 0
These are subwoofer amplifiers. As such, it’s fundamental that they can make generous amounts of power. We set the XDi up on the bench and wired it to our bank of load resistors configured to present 1 ohm of resistance. With 14.2 volts from our power supplies, the little XDi 650.1 cranked out a beefy 720 watts as measured using our D’Amore Engineering AMM-1 meter.
We wired up the competitor amp to see what this little monster could do. When loaded to 2 ohms, the amp produced an astonishing 1,799.8 watts of power. When connected to the 1-ohm load, the amp went into protection mode right at 1,200 watts of power. We realized afterward that this particular amp isn’t rated for 1-ohm loads. Nevertheless, the remote signal had to be removed for the amp to reset. That level of power is downright impressive from an amplifier so small.
Score: ARC Audio 1, Competitor 1
It isn’t easy to monitor input and output power simultaneously while trying to measure the maximum output capabilities of an amplifier. To simplify the task, we set the XDi up again and measured efficiency at its rated output level of 650 watts. The ARC was drawing 47 amps of current, and there were 14.2 volts at the terminals. That works out to an efficiency of 96.7% at a specific power output level of 645.3 watts, according to the AMM-1. For the competitor, it turns out it never made its rated 2200+ or 3300+ levels. So, to keep things stable, we measured efficiency at 1,100 watts into 2 ohms – an output level that will keep our power supplies and load resistors happy. We calculated the efficiency to be 85.1%, which is very, very good. This efficiency might have increased a percent or two at full power. The ARC measurement, however, is astonishing.
Score: ARC Audio 2, Competitor 1
Amplifier Frequency Response
Sadly, many consumers think that subwoofer system frequency response isn’t a big deal as it’s “just bass.” For anyone who’s had an audio system upgraded with a digital signal processor, they’ll know that a statement like that couldn’t be further from the truth. Choosing source units, amplifiers, speakers and subwoofers that deliver flat frequency response is the first step in creating a high-quality audio system. We connected both amps to our digital interface and took frequency response measurements with the infrasonic filter set to their lowest frequency, the crossover at the highest frequency and the bass boost controls turned down.
In terms of the XDi amp, the overall bandwidth is on the narrow side. The -3 dB point is 25 Hz on the bottom and about 210 Hz on the top. I’d like to see the bottom end extended a little. Still, for most consumers using vented enclosures, this design will protect the woofers from potential damage due to over-excursion caused by the driver unloading below the tuning frequency. This would have been a conscious decision by the ARC Audio design team.
For our competitor, we see a lot of bandwidth. On the bottom, the -3 dB point is at 6.8 hertz, and we saw 500 Hz on the top. With that said, the dip at 60 Hz and a peak at 45 Hz are troubling. Yes, the amplitude varies by only a half-decibel, but amplifiers should never apply any equalization to the signal unless specifically requested.
We’ll call this a draw and give both amps one point. Score: ARC Audio 3, Competitor 2
Harmonic Distortion Test
Here’s where we step away from the typical measurements and get into the nitty-gritty of what makes one amplifier sound better than another when reproducing music. We set the amps up on the bench and measured the frequency output of each when a 1-volt, 60 Hz test tone was produced. The differences are dramatic!
Looking at the graph generated for the XDi, we see our test signal labeled with marker M0 at a level of 0.74 dB, which is 1.088 volts. The first harmonic at 120 Hz, marked with M1, is at a relative level of -70.33dB for a distortion rating of 0.0318%. That’s good for an efficient subwoofer amplifier.
For our competitor, the results weren’t as good. In fact, they were bad. Very bad. We can see the test stimulus signal at -0.07 dB (0.992 volts) labeled with M0 and the second harmonic (M1) of roughly 180 Hz at a level that’s only -40 dB quieter. That’s a THD specification of 0.995%. This is one of the worst distortion measurements we’ve ever seen on our bench.
Score: ARC Audio 4, Competitor 2
An amplifier’s signal-to-noise performance describes the amount of unwanted noise or hiss added to the output signal. In most cases, amplifiers that add noise at a level of -80 dB are considered pretty good. The great amps get up into the -90 dB range. For the XDi amp, the QuantAsylum software provides a measured SNR spec of -84.6 dB. You can see the hash (squiggly yellow lines) down at the bottom, sloping downward from about -100 dB to around -105 by 100 Hz. This is good performance.
Next, we have our competitor. Wow. Just wow. The software specs SNR at 30 dB, and you can see the noise floor sitting at just below -60 dB from 10 to 100 Hz. Once again, this is the worst we’ve ever seen on the bench.
While it’s hard to hear hiss through a subwoofer compared to a tweeter, this is frustratingly bad performance. Chances are, you can feel a subwoofer moving with that much noise. Can we take marks away? I suppose not.
ARC Audio 5, Competitor 2
Amplifier Controls and Adjustments
As we mentioned earlier, the XDi has all the controls on the top panel of the amp. This location makes it easy to adjust once the amp is installed in the vehicle. In contrast, the competitor has the controls on the side. Once installed, the tech would have to either feel around for the adjustment knob or physically get close to the amp to read the labels. We do like that the controls are raised so that they can be adjusted with bare fingers, though; that’s a nice touch.
We noticed with the competitor amp that with the sensitivity control at its lowest setting, the amplifier produced no output – no matter how much signal was applied. For installers who aren’t accustomed to this brand, this design flaw could easily waste a lot of time during the level-setting process.
ARC Audio 6, Competitor 2
The XDi has four RCA jacks along the lower edge panel. Two jacks are inputs, and the others are input and output for use when using the amp in a master or slave configuration. The competitor amp has only one RCA input jack, despite there being two terminals on the side of the amp with the standard red and white labeling. The lower terminal is an output to drive another amp. It’s not hard to imagine someone connecting a second input to the lower terminal. We checked to ensure that both amplifiers used a differential input design to keep noise out of the system. The XDi does; the competitor doesn’t. This isn’t 1988. Speechless.
ARC Audio 7, Competitor 2
Both amplifiers have good quality terminal blocks for power and ground connections; both will accept four-AWG cables. For the XDi amp, it draws about 60 amps of current when pushed hard. That’s a reasonable amount of current for four-AWG cable. The competitor, on the other hand, can draw well over 150 amps of current. That’s a lot more than is recommended for a four-AWG cable. The amp really should have 0-AWG terminals, or you should invest in those little billet adapters that let your installer connect 0-AWG to four-AWG terminals. One hundred fifty amps of current, drawn through 30 feet of high-quality, all-copper four-AWG power cable, will result in a voltage drop of about 1.3 volts. The XDi would only lose about 0.5 volts over the same run in a cable of that size.
ARC Audio 8, Competitor 2
Cooling Design Determines Play Time
Last and certainly not least, we examined the physical design of the amplifiers to evaluate their cooling capabilities. When you drive a subwoofer at high output levels, an amplifier, even as efficient as these, needs to dissipate the heat it generates. The ARC amp uses a small cooling fan that is mounted on the back edge of the amplifier’s vertical heatsink. The fan draws air in from the sides of the amp, then pushes it out the back. There’s a plate behind the finned heatsink that causes the air to flow along the heatsink and out of the amp’s ends to maximize the moving air’s cooling effect.
The competitor amp also has a fan on the side of the amp. It draws air into the case, and it exits out the other side. The airflow path is near the output switching devices but isn’t routed through any part of the heatsink.
We let each amp play at its maximum output level for a few minutes. The ARC reached 140 degrees Fahrenheit (60 degrees Celsius), and the competitor was a mind-blowing 204.8 degrees Fahrenheit (96 degrees Celsius) in less than three minutes. The chances of this high-power amp overheating due to the heatsink’s small size is a serious concern.
A look inside the XDi amp showed temperature sensors near the output switching devices and the power supply transformer. We couldn’t find any temperature sensing in the competitor amplifier.
A microprocessor controls the fan in the ARC Audio amp. The fan goes to full speed when the amp gets hot and continues to run until it is cool. On the competitor amp, the fan speed seems related to the power being produced by the amp. If you get the amp hot then turn the signal down, the fan slows down. That doesn’t make any sense.
ARC Audio 9, Competitor 2
Factory Stereo Integration Features
If you’re hoping to add one of these amplifiers to the factory-installed radio in your car or truck, then the ARC will be the best choice. The XDi has automatic turn-on detection so that the amp will turn itself on when you turn on the radio. Likewise, it includes a pair of RCA adapter pigtails that can be connected to the speaker wires from the radio or amp to provide a signal. The competitor amp will need an external line-level converter that includes a remote turn-on output.
ARC Audio 10, Competitor 2
Remote Level Control
The XDi 650.1 includes a remote level control. The other amp doesn’t, and we didn’t see a part number listed in the owner’s manual or on the product page on their website.
ARC Audio 11, Competitor 2
Upgrade Your Stereo with a Quality Subwoofer
As we said at the beginning, the right choice of amplifiers for you depends on your application. If all that matters is a three-second burp at an SPL contest, then the choice is crystal clear. If you’re shopping for an amplifier that will sound good and continue to play for hours and hours, the information provided above should make your choice obvious.
It was purely chance that we pitted these two amplifiers against each other. In terms of power, no, these weren’t equal. With that said, ARC has two larger amplifiers in the X2 family (which replaced XDi). There’s the 1,100-watt X2 1100.1 and the monster X2 2500.1 2,500-watt unit. Both of those amps share the same basic design as the XDi family but add a few more features and updated circuitry.
If the power ratings on an amplifier you’re considering seem too good to be true, if the heatsink is tiny or the price seems like too much of a bargain, chances are you’re going to be missing out on some critical features or performance factors. The best amp isn’t always the one that makes the most power.