JL Audio’s latest amp offers the ability to support three different loads with the same power rating.

Either JL Audio’s marketing department is incredibly good, or the company just knows how to produce products that raise a stink. Last issue it was the 15W3 subwoofer, now it’s an amplifier with a three-speed tranny. This amp has been praised, criticized, heralded, demonized...the subject of much debate to say the least. In case you missed it, this thing is supposed to make 75 watts x 4 into 4 ohms, 3 ohms, or 2 ohms.

Performance
The 300/4’s rather unusual power ratings are attributed to the fact that the amp has three internal DC rail voltage settings, which are approximately 57 volts, 50 volts, and 43 volts. The upper range is automatically selected when the amp is turned on, then, based on the amount of current drawn, one of the lower ranges may be engaged. Of course, the amount of current being drawn depends upon the load. So there you have it, three different loads with the same power rating - good old Ohm’s Law in action again. I have a feeling that most of the criticism surrounding this rail-switching feature stems from the fact that it switches down rather than up.

Car audio people don’t like less — they like more. The problem is that you’re getting more; you’re just looking at it from the wrong way around. Think of it as one of those “high current” amps that has enough sense to know when it’s not driving a low impedance load. The amp uses the higher rail voltages to increase current flow, and, therefore, power, into higher impedances.

So, how does it perform? Overall, very well, but we did run into a quirk or two during our 10.5VDC power response sweeps. When driving all channels into 4 ohms at this power supply voltage, the 300/4 goes into some type of mild oscillation just as the output signal begins to clip. While this characteristic doesn’t seem particularly ominous, it does prevent a solid measurement. With all channels driving 2 ohms and a 10.5-volt supply, the amp goes into a protection mode that reduces the signal just before it reaches clipping. This is why both of our “Power @ 1%” charts are short one trace. I should point out that despite the fact that we couldn’t get a clear cut measurement, the output power just before either of these conditions occurs is very close to what was measured with higher supply voltages — this amp is very tightly regulated.

As with any amp that acts a bit unconventional on the bench, we couldn’t wait to try the 300/4 with music and real speakers. We figure our readers really need to know how the amp is going to work with speakers and music as opposed to test tones on a lab bench. We cranked a wide variety of program material into every one of the 300/4’s rated loads while monitoring the amp’s rail voltage. As we might have guessed, it performed beautifully during this real-world test. Despite our best efforts to confuse it, we never got it to do anything that it wasn’t supposed to do.

Note one point to seriously consider: Take a close look at the reactive power vs. the resistive power. On a real speaker, as opposed to a bench resistor, this amp really performs, to the tune of almost a doubling of power. The adaptive power supply scores some extra points where it matters — driving speakers.

Beyond these points, I’m going to leave you with the spec sheet; it pretty much speaks for itself. While most of the numbers are very good, a few, like the turn-on/off specs of about 0.5dB SPL, make it all the way to superb.

Connections & Adjustments
Everyone already knows how this list goes, so I’m going to talk about what’s cool rather than what’s there. First, the input sensitivity is a (manually) switchable dual range set-up. Low sensitivity range is spec’ed at 800mV to 8 volts, and high range is speced at about 200Mv to 2 volts. This makes the control much less sensitive to small adjustments. Ever try to find the 6-volt sensitivity setting on a single adjustment that goes from 200mV to 8 volts? Second, the 300/4 has, by far, the coolest crossover in the automotive amplifier industry. It allows you to select either 12 dB- or 24 dB-per-octave filter slopes, it will operate in either high- or low-pass mode with a crossover point anywhere between about 55 Hz and 5 kHz, and there is a fully functional filter set on each pair of channels.

The best part, though, is the accuracy with which the crossover can be set. Most amps include some type of scale on all of their adjustments, which usually takes the form of a screen-printed legend on the amplifier chassis, or end plate. To say the least, this approach provides little more than a general idea as to the actual crossover point. JL went two steps further by implementing a detented adjustment and a 10x range switch. The 300/4’s manual lists the exact crossover frequency at each detent between 58 Hz and 542 Hz. Kicking in the 10 x range switch multiplies whatever setting you have by a factor of ten, again allowing a wide range of operation without an overly sensitive adjustment. While crossovers can be set by ear, and there are times when a mismatched setting might be beneficial, it’s hard to argue against knowing exactly where you are from the get-go.

Installation
But wait, there’s more. Ever notice that all of the RCA cables in this industry come with connectors that are way too tight and that all of the input connectors on amps are flimsy circuit board-mounted things? Guess JL did too, that’s probably why they chose to use real connectors on the 300/4. They are mounted on the circuit board, but they are also securely mounted to the amp’s chassis with a large nut around each connector rather than a tiny screw in the plastic between the connectors. Most amps require a gentle touch in this area, but the 300/4 would likely survive a hammer blow to the input RCA’s. The 300/4 is also thankfully devoid of chassis-mounted fuses. If you fuse the amp properly, at the battery, there’s really no reason for more fuses at the amp. Let’s see, it has a real heat sink, those neat looking captive-style power and speaker connections, and did I mention the crossover...

Manual
Straight up, I quit talking about owner’s manuals because most of them really, really suck. However, this is one that’s worth your time.