Preamp
The crossover for the JPX720 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 JPX720 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.