JL Audio JD500/1

Product Overview:

Designed to produce a whopping 500 watts into 2 ohms (14.4V, 1% THD), the JD500/1 amplifier is a great solution for powering subwoofers in a cost-conscious system. Our proprietary NexD™ switching technology delivers large amounts of real-world power output with excellent efficiency and ultra-low distortion.

The JD500/1’s controls are located on the input panel. while the power and speaker connections can be found along the opposite end of its extruded aluminum chassis. Signal processing features include a 12 dB / octave low-pass filter, with continuously variable frequency selection from 50-500 Hz, and a variable bass boost EQ. Dual-range, differential-balanced inputs are also on-hand, capable of accepting line-level and speaker-level signals. Automatic turn-on capability (via signal sensing or DC-offset sensing) is standard, along with a set of preamp outputs so you can easily pass signals to another amplifier.

The JD500/1 is also equipped with an onboard LED-clipping indicator ring, surrounding the input sensitivity control. This useful feature lets you set your amplifier’s input sensitivity, quickly and accurately, without special equipment. With the addition of an RBC-1 remote level control (sold separately), you can control the subwoofer level from the driver’s seat.

NexD Switching Technology

Let's get a little nerdy... A traditional PWM switching amp design uses a fixed switching frequency and varies the pulse width based solely on the audio input signal to the pulse modulator. This basic approach assumes some ideal conditions, like a rock-steady power supply, that do not necessarily occur in a real, in-car installation. Power supply sag with signal causes distortion at all power levels with these designs, even well below clipping. The cure for this is feedback (a corrective signal sent back to the input side), but this becomes impractical as the audio frequency increases, making it a good solution for band-limited (subwoofer) amps, but not for full-range amplifiers.

Taking the above into account when designing the NexD™ subwoofer amplifiers, we have taken the simple fixed PWM approach and enhanced it with a very high switching frequency: 240 kHz, which is about 4x higher than most Class D subwoofer amplifiers. This pushes the amplifier's bandwidth to at least 500 Hz with minimal distortion and improves efficiency, while keeping typically bulky circuity more compact. Your subwoofers will reward you with tight, rock-solid bass performance.

For the full-range NexD™ amplifiers and channels we applied a variable, ultra-high speed version of the NexD™ technology (switching at well over 400 kHz). In these designs, a self-oscillating modulator with "feed-forward" sends information about the instantaneous power supply voltage to the modulator, combining this with the input signal to cancel out any distortion due to supply voltage fluctuation. This reduces distortion prior to applying feedback, simplifying the overall feedback loop design.

Differential-Balanced Inputs

Today’s vehicles are equipped with sophisticated electronics capable of generating noise and interference that can intrude into the audio path, ruining the listening experience. Most audio equipment uses single-ended, ground-referenced RCA jacks to receive audio signals. This means that the RCA shield is connected directly to the signal ground. This is a simple, cost-effective input design that works well for short runs in noise-free environments. In noisier environments, this method cannot reject the common mode noise that could be on both RCA conductors (shield and center conductor).

By having more than one chassis-grounded RCA shield on a device, single-ended input designs are also more susceptible to ground loops. If noise is present on both conductors while the shield is connected to ground, the input amplifier will amplify the noise relative to the grounded RCA shield. When there are multiple RCA connections or any other conductors subjected to a magnetic field, this will cause a difference in potential across the RCA loop to be amplified at the grounded RCA as noise (a classic ground loop).

JL Audio’s Differential-Balanced Input circuits measure the center pin of the RCA connection relative to the RCA shield, even when the shield is not connected to ground, and even when the voltages at both conductors are moving relative to ground. Because both the center conductor and the shield see high impedance to ground at the input, a ground loop cannot be formed, as there is no current flow to induce a voltage as noise. This works equally well with differential-balanced, or unbalanced signal sources, making this input architecture compatible with all standard car and marine audio equipment, using standard, unshielded, twisted-pair RCA cables. No special cables are required, unlike the fully balanced connections used in professional audio.

Another benefit of the Differential-Balanced Input design is that it easily accepts low-level and high-level signals from a variety of factory head unit outputs and amplifiers, including single-ended “ground referenced” sources, as well as Balanced or Unbalanced “Bridged or BTL” sources. This eliminates the need for Line Output Converters (LOCs) in most installation scenarios. The obvious question is: “Why don’t all car audio amplifiers use Differential-Balanced Inputs?” The answer is simple: it costs more. Executing a proper differential-balanced input section requires more components and better quality components than a basic single-ended design, so you generally won’t find them on less expensive amplifiers.