Author: Wayne Harris
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The Symmetry EPX (code name STEALTH) is the culmination of many months of painstaking Research and Development activity. Technology that was originally developed for the Symmetry CSV, has been successfully migrated downwards to create a new and exciting product that is rich in features, yet affordable in cost.
The EPX has many of the same features as it's big brother, including a Preamplifier, a 14 band Equalizer, and a 3 way fadable Crossover. Unlike the CSV, however, the EPX is a "closed" system, in that it cannot be expanded beyond the capabilities it already possesses.
To fully appreciate the benefits offered by the EPX, it is necessary to examine each of it's key components in detail.
The power supply section provides the fundamental voltages required for circuit operation. In simplest terms, a power supply converts the standard automotive 12 volts DC into the bi-polar voltages required for audio applications. Unfortunately, this is the ONLY design criteria used by many manufacturers when designing power supplies for their equipment.
The supply in the EPX is different. Sure, it converts voltages just like the others. But that's only the beginning. The EPX power supply features a Self-Oscillating High Voltage Switching topology. This type of design provides a very important characteristic that is sorely lacking in most other power supply designs. COMPLETE "Power Ground / Signal Ground" isolation. I'm not talking about the measly 100 to 200 Ohm isolation found in most other products; what we're talking about here is a full 1,000,000 Ohms of ground isolation. "Big Deal!" You say. When you're an installer, it IS a big deal. Input ground isolation is the single most effective way to prevent alternator whine in an auto sound installation.
Reduced RF radiation is another benefit provided by Self-Oscillating power supplies. "What does RF stand for anyway ... Rockford Fosgate?" You might ask. Actually, RF stands for Radio Frequency as in Radio Frequency Interference, or RFI for short. RFI "desensitizes" the tuner section of your radio. This results in poor radio reception. Self-Oscillating power supplies typically generate significantly less RFI than other switching power supply designs.
In addition to performance, reliability was another objective during the design process. ALL of the components used in the EPX power supply operate WITHIN the manufacturer's specified maximum operational limits! "Is that all?" You ask. Well, I could tell you about the Electro-Static Discharge or ESD protection built into the unit, but you might be "shocked." "How about color-blind installers?" No problem! The EPX power supply incorporates Reverse-Polarity protection "just in case."
The preamplifier section is the heart of all EPX signal processing activity. It provides source selection from one of two separate input sources, volume control over an 80 dB range, independent bass and treble controls with +/- 12 dB of boost or cut, and balance and fade capabilities. Features that, all in all, are not uncommon in the marketplace. "So what makes the EPX preamp special?" You ask. I was hoping you would ask that question.
First, the EPX preamp has special input circuitry. This circuitry, called "differential inputs," helps prevent noise problems from creeping into the audio system. "How?" You ask. Well, simply put, a circuit that incorporates differential inputs will only amplify the DIFFERENCE between two conductors or wires. In car audio, most components are connected together with RCA patch cables. Each cable consists of a center conductor (the hot lead) and a shielded outer conductor (usually audio ground.) Under ideal circumstances, the difference between the hot lead and the outer shield conductor is the musical signal. Cars are anything but ideal, however. Electrical noise is generated by the alternator, fan motors, windshield wipers, brake lights, and who knows what else. These noises can be inductively coupled into the RCA patch cables. When this occurs, the noise spikes appear on BOTH the center conductor and it's outer shield. Since differential input circuitry amplifies only the difference between the two conductors, the noise spikes are NOT amplified, and they are consequently eliminated from the system.
To further reduce the possibility of noise, each set of RCA inputs has it's OWN independent audio ground. This is critically important when dealing with systems that contain multiple source unit's. "Why?" You ask. Because in every system there should be one, and only one, point where audio ground and power ground are common. And this point should be established by the source unit. When multiple source units are used, each will establish it's own common ground point, with the result being the dreaded "ground loop." (Ground loops are the primary cause of engine noise in a system.) By utilizing independent input grounds, the EPX preamp prevents ground loops and their associated headaches.
To further enhance the performance of the preamp, individual input sensitivity controls have been incorporated into the input circuitry. This, in conjunction with our unique ON-LINE level setting procedure, allows the installer to perfectly integrate the EPX with the rest of the sound system. Speaking of ON-LINE level setting, this is an industry first. The EPX system actually steps the installer through the level setting procedure. This process is more than just a series of instructions, however. The EPX actually adjusts itself during this maneuver to insure the best possible signal to noise ratio, while still maintaining adequate signal headroom. Finally, the EPX is capable of producing an incredible 10 Volts RMS of output signal. The result; reduced system noise (hiss) and maximum dynamic range
The "E" in EPX stands for the unit's integrated 14 band equalizer. "What's an equalizer?" You ask. Equalizers, or EQ's for short, are signal processors that are used to correct or compensate for the acoustical deficiencies inherent in most auto sound installations. EQ's are also commonly used to "tailor" the sound of a system to the listener's own personal listening preference. The EQ in the EPX is one of the finest auto sound equalizers available.
Here's why ...
Equalizer operation is exceptionally user-friendly. All adjustments may be made from the comfort of the listening area. Climbing into the trunk of the vehicle, tweaker in hand, is no longer required when performing EQ adjustments. This means that the user can actually hear changes as they are being made. What a Concept! Also, all equalizer settings are displayed both graphically and numerically on the system's large, easy to read display. This allows the user to instantly SEE exactly how much boost or cut is being applied to any particular band.
No more potentiometers! In the past, pots have been used as the control elements for equalizers. Being the electro-mechanical devices they are, it was only a matter of time before they became dirty or just plain wore out. The EPX's equalizer utilizes state-of-the-art solid state devices as the control elements. They don't wear out, they don't get dirty, and they'll be just as accurate tomorrow as they are today.
Fourteen bands of adjustment to satisfy even the most discriminating of listeners. Half-Octave centers in the difficult-to-adjust region below 1 KHz. Octave centers at 1 KHz and above. Each band may be adjusted over a +/- 12 dB range in exact 1 dB increments. And when that perfect sound is finally achieved, the user may then elect to save the current settings to one of the EPX's four equalizer memory presets.
The EPX's active electronic crossover is, arguably, the unit's most outstanding feature. It's primary purpose is to do to sound what a prism does to light. That is, it splits the composite musical signal into discrete frequency "bands". "Why would you want to do that?" You might ask. The main reason is performance. Most loudspeakers are not capable of reproducing the entire musical spectrum efficiently. By splitting the music into discrete bands, individual components of the musical signal can be routed to loudspeakers that are specifically designed to reproduce that part of the musical spectrum. For example, if a system were to divide the composite musical signal into 2 parts, it would be called a 2 way system. In this scenario, frequencies below the crossover point (the frequency where the splitting process occurs) would go to low-frequency speakers while those above the crossover point would be routed to speakers optimized for high-frequency reproduction. The EPX features a FADABLE, 3 way active crossover network. 10 discrete outputs are provided.
They are ...
The crossover points for each of these five groups may be adjusted independently of the others. Best of all, crossover point adjustments may be made in real time! This is an EXTRODINARY development! With the EPX, installers can actually adjust the system's crossover points while listening. In addition, the user is not limited to the handful of crossover modules offered by most crossover manufacturers. In fact, each filter in the EPX can be set to 1 of 256 possible frequencies, for a mind-blowing total of 72,057,594,000,000,000 (72 quadrillion) unique crossover combinations.
All crossover points are represented both graphically and numerically on the LCD display. Highpass, Bandpass, and Lowpass transfer functions are realistically simulated on a calibrated logarithmic frequency scale. The exact crossover frequency is displayed numerically.
Notice that the EPX features a FADABLE crossover with constant bass lowpass outputs. "What does that mean?" You ask. The answer is simple. EPX has outputs for both the Front and Rear of the system. Front and Rear crossover settings for these outputs can be adjusted independently. A fadeable crossover allows the user to fade the system from front to back for just the right amount of front stage / rear fill. The constant bass lowpass outputs provide lowpass information regardless of the position of the fader. Most crossovers do NOT have this capability.
The RDAT is what makes the EPX such a unique and powerful signal processor. Don't be fooled by it's small size. The RDAT is in reality a very powerful computer. It coordinates the operation of the power supply, preamp, equalizer, and crossover sections of the EPX, so that they all work together in perfect harmony. The RDAT also provides the all-important link with the user. To insure ease of operation, a menu driven software architecture was selected for the user interface. In this type of system, options are presented to the user on the RDAT's large 40 character by 2 line display. To select a particular option, the user need only to press the function key that corresponds to the desired menu selection. Nothing could be easier.
Did I mention powerful? The RDAT contains a 16 bit microcontroller that runs at almost 20 Mhz. It has a program capacity of 32 K bytes and an 8 K byte bank of non-volatile storage. Since all software is written in assembly language, program code is tight and performance is optimal.
Although less glamorous than some of the preceding circuitry, the EPX communications interface is just as crucial for proper system operation. It's sole purpose is to facilitate error-free communications with the RDAT described above. It accomplishes this task by using RS-232 serial interface circuitry on all lines carrying data. Furthermore, to maintain ground isolation, all data lines are optically isolated from the EPX's audio circuitry.
The Power Supply in the EPX is GREAT because ...
The Preamp in the EPX is GREAT because ...
The Equalizer in the EPX is GREAT because ...
The Crossover in the EPX is GREAT because ...
The Symmetry EPX is a GREAT signal processor because ...