Cosmetics
Jensen packaged the JXP-720 amplifier in a relatively small chassis, considering its power output. The amp measures a mere 10" x 11" x 2.2". Exterior cosmetics are definitely unique, a love it or hate it design, if you will. The main portion of the amplifier is made up of an extruded aluminum heatsink anodized in a brilliant blue finish. Looking down upon the JXP-720, large fins extend outward to the top and bottom. These fins are responsible for dissipating the majority of heat generated during operation. Atop the amp is a chrome wire mesh that flows over the majority of the real estate, and held in place with six, chrome-plated pan head machine screws. In the center of the amplifier, encompassed by plastic cladding, is the Jensen logo; this logo glows in a consistent cosmic blue when the amplifier is powered up. Just north of the logo are the power/protection L.E.D., and the level, crossover, and bass boost controls. Access to these controls requires the removal of a translucent blue acrylic plate mounted using a single, hand-tightened setscrew. To the sides, the charcoal gray injection molded plastic end plates are superbly finished and subtly incorporated into the JXP-720's visual character.
Layout
Looking inside reveals a PCB layout that uses through-hole components and a single sided circuit board. Because of the board design, there are almost 50 printed circuit board jumpers. While this type of construction saves the consumer money, it can reduce reliability. Single sided boards lack plated-through holes, allowing components to sometimes vibrate loose.
The internal construction of the JXP-720 is otherwise clean; the power and speaker connections are on one end, with audio on the opposite end. The gain and crossover controls are on the top of the unit, underneath the blue translucent cover, as mentioned. These controls are mounted to a small PCB, which is connected to the main board via a 10-pin jumper cable.
On the left side of the PCB you will find the power and speaker connections protruding through the chassis. These connections are standard gold-plated lug terminals enclosed by a plastic housing. The power terminals will accommodate #10 spade or bare stranded wire, sufficient for this power category. The manual specifies #8 gauge wire, but you may have a difficult time with this size wire. The speaker accepts a slightly smaller spade#12 wire.
For the audio connections, Jensen opted to use two sets of standard gold-plated style RCA receptacles; one set for input and another for output. These are located on the opposite side of the heatsink from the power terminals. High level inputs are also available as an input source when no low level (RCA) connections are available.
Power Supply
Jensen uses an unregulated power supply in the JXP-720. This power supply employs six IRFZ46, 46 ampere MOSFETs. These are sufficient to supply the needed power. An input RF filter is included for the 12-volt power.
We give Jensen big thumbs up for the use of 105* C capacitors on the three 2,200uF filters used in the power supply section. As we have stressed many times before, capacitors can degrade over time in areas of extreme heat, so it is important to have high temperature capacitors in areas such as the power supply where temperatures may rise excessively.
Power Amp
The output of the amplifier uses four 2,200uF capacitors. Unfortunately, Jensen chose to use a lower grade 85* C caps in this section.
Powering the output are four 125 watt output devices per channel.
Preamp
The crossover for the JXP-720 can be configured high pass, low pass or full range. Slopes are fixed at 12 dB/Octave, and the crossover frequency is variable over the range of 40-400 Hz. Oddly, there are two potentiometers to adjust the frequencies, one for left and one for right. It is obvious that Jensen made this arrangement because of the depth required by a dual pot. However, using the individual controls can be a bit frustrating while trying to match the two.
The preamp section also includes a variable bass boost control said to be centered at 45Hz. This control, as can be seen in the graph, behaves in an odd way: the center frequency shifts from about 30Hz to 45Hz as the bass boost is increased. Also, the Q shifts dramatically with bass boost. Most users that need bass boost will no doubt use its full gain of 12 dB. In this full boost position, the frequency and Q would be about right for most systems.
For added adjustment when using the amplifier in lowpass mode, Jensen includes a remote level control along with a 25 ft. phone cord and mounting hardware. This can be easily mounted up front in the vehicle and allows the user to attenuate the amplifiers output from 0-40dB from a remote location.
Protection
On the front panel next to the power connections are two 30-ampere fuses. These fuses may be a bit excessive, as we only encountered maximum current draws in the range of 50 amperes. And though the amplifier is protected thermally, we never got it hot enough during our tests to shut down despite its smallish size. Also notable, its thermal shutdown is set at a relatively cool 70* C.
Like many amplifiers we have tested recently, there is no over-voltage protection. This is not a large concern, but better safe than not.
The Jensen is rated to play safely into 2 ohms, which it did without a hitch. Impedance loads below 1 ohm sent the amp safely into short protection.
Manual
The manual is well written, but sparse. It covers three models and three languages. Given the simple features of this amp, it provided all the information needed for the average installation.
Performance
The Jensen JXP-720 came very close to the factory's distortion and power spec: distortion was 0.85% at 180 watts per channel into two ohms. While 0.85% is high, reducing the power to just 150 watts resulted in distortion numbers below 0.1% from 20Hz to 20kHz into two ohms.
The slew rate of 7 V/uS is quite slow and could affect the high frequency performance of the amplifier. As expected with slow slewing amplifiers, we measured a relatively high inter-modulation distortion of 0.2%. However, IMD does not affect the sound quality for low frequency use.
The damping factor was lower than we like to see. It typically measured less than 75 into a 4 ohm load. This could effect the accuracy of the low frequency response.
A-weighted Signal to Noise Ratio (SNR) was very good at 105 dB, below clipping. We could not detect any audible noise with our most efficient test speakers. However, channel separation did not fair so well, and at 1kHz measured a poor 59dB.
Efficiency at full power into 2 ohms was 52%, a tad low for a class AB amp. At 1/3 power the efficiency fell to 30%, 2 ohms.
Idle current measured a typical 0.9 amps.
This amplifier has both high and low level inputs. The low level input sensitivity was 240mV to 6.6 V to drive the amp into clipping at 4 ohms. The high level input had an odd range: 2 volts to 50 volts! Obviously you won't find a source that delivers more than about 12 volts RMS, so the gain control can never be set near the 50 volt position.
Turn on time was 1.8 seconds, just a bit shorter than we like to see. Turn off time was 2.5 seconds. This will be a problem for some installations as many head units and signal processors will produce a loud thump when turned off. The power amplifier typically needs to mute in less than 10 milliseconds.
As can be seen by the graphs, this amplifier did not pass our reactive load tests. We saw a good deal of oscillation when paralleling the speaker outputs with a 0.1uF capacitor. We tried this test again with just 1 foot of #12 speaker wire in series with the caps. In this case the output was clean, so we don't expect these oscillations to show up in any installation.
Listening Test
We heard a slight turn-on "pop", and a somewhat loud turn-off pop. The sound was due to the source deck's turn-off noise not being muted by this amps 2.5 second turn-off mute time.
The amp sounded clean and tight over the full frequency spectrum, despite its poor slew rate. During our blind A/B tests, we believe we heard high frequency harshness, but it was too subtle to be certain.
Conclusion
The Jensen JXP-720 is a good value at $199.99. It makes a substantial 372 watts into a 4-ohm mono load; that equates to a very reasonable 54 cents per watt. While this Jensen amplifier may not contain all of quality components found in some of the more expensive amps we have tested, it should be powerful and clean enough for most systems. And, given its built-in crossover and bass boost, we expect this amp to satisfy any budget-minded audio enthusiast's needs.
FEATURES
Full range stereo.
Low pass or high pass internal crossover
Variable Bass Boost control
PROS
High level inputs
Input RF inductor
Variable Bass Boost
Remote level control
Good S/N ratio
Low distortion
CONS
85* C. secondary capacitors
Single sided PCB
Bass boost center frequency moves from 30-45Hz depending on gain setting.
Long turn off delay
No over voltage protection
Low damping factor
High inter-modulation distortion
A Word About Capacitors
In the automotive environment where internal operating temperatures easily exceed 100 *C, high quality 105*C capacitors are a big plus to ensure the longevity of the amplifier. In high current areas, such as 12 volt filtering and the power supply output stages, low ESR (Equivalent Series Resistance) capacitors should be used as well. Low ESR capacitors not only do a better job of filtering, but run cooler. Capacitors have an electrolyte, and can easily be destroyed by high temperatures.
Damping
Damping is simply the amplifier output resistance specified as a ratio of this resistance divided into the load resistance. For example, an amplifier that has an output resistance of .01 ohms (perfect would be zero ohms), and rated into 4 ohms, the damping would be 4/.01 = 400. The ability of an amplifier to actually dampen cone motion is degraded by the speaker's own internal DC resistance. A typical 4 ohm speaker may have 3.2 ohms of resistance. So, in this example the damping could not exceed 4/3.2 = 1.25 regardless of the amplifier's damping.
Slew Rate
Slew rate is stated in volts per micro-second and measures the high frequency transient response of an amplifier. In this drawing, the amplifier is tested with a very fast rise-time square wave and the output is measured on an oscilloscope. In this example, the amplifier rises 10 volts in one microsecond. Good clean design dictates that an amplifier should "slew" ten times faster that the fastest audio signal presented to its inputs. If an amplifier is going to be reproducing high frequencies, its slew rate should be in the 15 - 50 range, depending on its power rating. Because amplifiers employ negative feedback, poor slew rates usually increase TIM (Transient Intermodulation Distortion). Slew rate is only an important parameter in high frequency applications.
www.Jensen.com
Brand: Jensen
Model: JXP-720
MSRP: $199.99