| Kicker SX400.2 - 12Volt Tuning
Over the last couple of years, more and more digitally controlled amplifiers have entered the market. This concept has been around since the late '80s and early '90s-PPI had the DCX controller and a series of digitally controlled amplifiers while Rockford Fosgate offered the Symmetry controller, among others. With its introduction at the 2004 Consumer Electronic show, Kicker is now offering their SX line of amps with a digital interface that controls all of the usual amplifier functions and then some.
For the most part, the SX400.2 is your standard class AB amp, but with a powerful DSP built in. Kicker packed a ton of features into their digital signal processing by implementing features like gain, equalizer, crossover, security lockout, diagnostics (SickBay(tm)), phase and compression. What's striking about this amplifier is there are no knobs or switches; everything is controlled through the digital interface (ISIS) on top of the unit. Power output is said to be 100 watts x 2 at 4 ohms, 200 watts x 2 into 2 ohms and 400 watts x 1 at 4 ohms bridged-more on that to come.
The main component of the SX400.2's chassis is an extruded, black anodized aluminum base. Built upside down from typical amplifier construction, the heat sink resides below the PCB and rises up to include horizontal heat-sink fins on the front and back sides. A silver sheet-metal shroud covers the top and incorporates a window for the DSP control module that contains a blue VFD display and push buttons. The endplates to the chassis containing the connections are constructed of sheet metal and have been powder-coated black. Blue, cast aluminum EndKaps(tm) that bolt to the top with gothic points stretching almost to the middle of the amplifier cover the connections. This is an incredibly cool-looking amp, especially with the blue letters scrolling across the display.
Power, ground and remote enter through large gold-plated brass terminals on the left side (power and grounds will accept 2awg cable). The speaker outputs also use good-size brass terminal blocks, and get this: all of the terminal blocks and the EndKap(tm) screws use the same 3mm allen wrench! (If you don't understand the significance of this, you haven't installed enough amplifiers that require a different tool for every screw.) The RCA inputs, speaker outputs and remote level control jacks are on the end opposite the power terminals.
Circuit Design
Inside there is a fairly systematic flow to the layout, with power supplies at one end and signal path and speaker outputs at the other. There is a daughter board that contains the RCA inputs and digital signal processing. No doubt Kicker uses this board for all 2-channel digitally controlled models, simply mating it to the larger or smaller amplifier main boards of the various size amplifiers.
The main amplifier PCB uses through-hole parts entirely. All resistors are high-tolerance parts, and film capacitors are used where it counts in order to keep the signal path clean. The input/DSP daughter board is entirely surface-mount except for the RCA input and remote level control connectors. The daughter board connects to the main board through a 26-pin header and is held in place by no less than five screws.
From the terminal block, the +12V goes through a pair of ATC 20A fuses and a heavy-gauge air-core inductor hash filter. There are actually two separate power supplies, both managed by separate TL494CN PWM controllers running at approximately 30kHz. One supplies the heavy lifting of the amplification itself while the other provides a stable power source for the digital stuff. The Litz-wound transformer for the "big" side is switched through two pairs of IRFZ44's capable of 150 watts each, for a total of 600-watt capability. There is a total of 4400F filtering on the primary side and 4000f on the secondary, in addition to a cap/inductor filter on each of the positive and negative rails. The outputs consist of two complementary pairs of TO-247 size devices for each channel at the business end of a Darlington-type circuit, each pair handling up to 160 watts at 25 degrees Celsius. That's 640 watts total, so the amp should easily be capable of delivering the promised 400 watts of dynamic power.
The two pairs of RCAs are paralleled together, in essence no more than a built-in "Y" connector. The outputs (PAST(tm)) are exactly the same as the inputs with no buffering or crossover filtering.
The brains of this amplifier are on the Input/DSP board. Even the input gain is adjusted through the digital interface. The input signal is buffered through low-noise op-amps and is then sucked into the digital domain by analog-to-digital converters (ADCs) where it exists as nothing more than a string of numbers. The primary component is a DSP chip that facilitates the parametric equalizer, crossovers, gain and compression by manipulation of the string of numbers. Think of the chip as a calculator performing various calculations on the "numbers" that make up the signal path. The modified digital number string is then converted back to analog by, you guessed it, a digital-to-analog converter, and is sent to the amplifier's output section. There is an 8-bit Micro controller with programmable flash memory that holds the instructions for the DSP chip, a vacuum fluorescent display and driver, and an array of push-button switches so you can tell it all what to do.
PerformanceFrequency response measured 20Hz to 23.5kHz, a little better than the 20Hz to 20kHz that Kicker states. There is a very pronounced roll-off at 23.5kHz (see frequency response graph) that is probably a result of the digital signal processing.
As an amplifier, our tests reveal that this is a good one. All measured parameters are as good or better than Kicker claims. It has good power, high damping factor and reasonable efficiency for a class AB amp. The fun part, though, was watching the crossover and EQ charts appear on the screen as though someone in the art department had drawn them for an ad. Welcome to the wonderful world of digital signal processing, folks!
Let's begin with the parametric equalizer. With a selectable Q range of 0.5 (really wide) to 10 (a sharp point), we chose the center frequency to be 2kHz. We first measured the curve at +18dB with the Q at 10. A spike appeared on our screen with the point exactly at 2kHz and up exactly 18dB. We added a -18dB curve, and it was a precise mirror image of the first. This is exactly what you would expect, but seldom see with analog circuitry. We changed the Q to 0.5 and ran the sweeps again, resulting in exactly symmetrical arcs that intersected the first two spikes at 2kHz, at both + and -18dB. Ever see a Spirograph? We ran a third pair of sweeps at + and - 9dB with a Q of 10, creating a smaller pair of spikes inside the first set, centered at exactly 2kHz and + and - 9dB. Most impressive!
As for the crossovers, this selection took a long time to test, mostly due to the fact that you can choose from six crossover slopes: 6dB, 12dB, 18dB, 24dB, 30dB and 36dB. The crossover points measured exactly where we set them and correspond with the range Kicker gives in the specs. We graphed the crossover curves at 100Hz and 2.5kHz in order to display them without any interaction with the roll-off curves at the extremes of the frequency response. As you can see, the crossovers are very clean and symmetrical. We found a discrepancy here-the actual measured slopes are a little shy of the specs, actually by about 1dB/octave. For example, the 12dB/octave slope measured closer to 11dB/ octave. Interestingly enough, this is not a cumulative error, but seems to be a constant 1dB difference throughout the range of slopes. For the record, we wouldn't lose any sleep over it. Even with 1dB error, this is still the most precise signal processing we've ever seen in a mobile amplifier.
The remote level control (BLAST(tm)) is an attenuator with 22dB of range, controlling the overall level of the amplifier. There was no measurable turn-on or turn-off noise.
ManualThe owner's manual resembles the instructions for a head unit with all of the digital menus and readouts. It does a pretty good job of telling you how to install the amplifier, with wire gauge charts and system diagrams and good end panel pictures. Everything is pretty clear, but you really have to get your head into the digital interface instructions. Not that it's presented badly, because it isn't. It's just that there's a lot of information. We found it easier to read a couple of pages and then fire the amp up and explore the menus, occasionally looking back at the manual.
We found the manual pretty complete as far as how to adjust the various features, but it assumes a fairly high level of knowledge on the part of the installer. For instance, it explains how to set the gain (which buttons to push) but not how to determine the correct level. Same thing with the crossovers and parametric equalizer, the book expects you to know what you're after and explains how to make the setting. On the other hand, there are three settings for the Kompressor(tm) (besides OFF) with suggestions as to which setting you might want to use, but it would be nice to see some specs (i.e., threshold, compression ratio, etc.)
The manual also contains information regarding the warranty, which covers materials and workmanship for 90 days or two years when purchased from an authorized Kicker dealer.
Listening TestAfter installing the SX400.2 in the Buick, we sat in the driveway with the A/C and baseball game on for few minutes to cool down. It took a few extra minutes to learn the gain menus. We leaned into the trunk in 98-degree heat long enough to get that trunk lip mark across our foreheads again. The amp is connected to the 6.5-inch separates in the front doors at 4 ohms, so it should be delivering about 88-watts per channel.
We started with The Waybacks' Way Live album. The Waybacks are an energetic bluegrass band with a great drummer (think speed-grass). The CD is recorded with various solo instruments panned pretty hard left or right, so you can really hear the guitars, mandolin and fiddle quite clearly. All the instruments sounded true, right down to the picking and bowing sounds. It was just as if you were listening to them being played right in front of you. The male vocals were also very convincing.
Next up was Donald Fagen's Nightfly. On the intro to "Snowbound," we could really hear the individual low notes from the Fender Rhodes, but the cymbal work starting off "Tomorrow's Girls" was really just OK.
We put in Mary Chapin Carpenter's Come on, Come on to see how the Kicker would handle female vocals. Her voice was well centered even through some of the over-processed tracks. There is a song called "Only a Dream" that features a well-recorded piano. The low-end and mid-range sounded solid, almost too solid. The highs seemed a little indistinct on this track as well. Not like it's been equalized, but just not quite there.
During this phase of the listening we noticed a clicking sound when adjusting the volume at low levels. Imagine a slow "zipper" sound in which you can hear the individual teeth of the zipper, which stops when you stop turning the volume knob-strange, but low enough to not be terribly annoying.
We played around with the Kompressor(tm) a little, but it seems to be geared more to the subwoofer side of life. No problem, we quickly rewired the Kicker on a 12-inch dual voice coil in a sealed box, reconnected the separates and grabbed an old CD sampler from some car audio show. (We honestly don't remember what it is, but we think we picked it up at an event in Idaho years ago. There's a kid named Billy and his deaf Grandma yakking between tracks of sine bombs, sweeps and various bump tracks.) The Kicker and the 12-inch made for a fun combination with no lack of bottom end. The amp was able to reproduce the lowest of the sine sweep frequencies easily, and the Kompressor(tm) worked wonders. In a nutshell, a compressor decreases (compresses) the dynamic range. It makes the difference between the softest and loudest signals smaller. This means you can hear more of the softer stuff in between the loud stuff. Trust me, it's a good thing.
ConclusionOverall we thought the SX400.2 performed well. It has a lot of digital stuff and operates glitch-free. The DSP functions are accurate and clean, and the Kicker engineers have solved a lot of I/O problems that other companies are still wrestling with. The interface and menus are well thought out and easy to navigate. The amp lacks sizzle in the highs, but low-end and mid-range are as solid as they come. The retail price on the SX400.2 is $599.95, and at a maximum power of 425 watts, you're paying $1.41 per watt-definitely on the high side for this power range. On the other hand, you're getting the most-impressive crossover available in car audio, a single band parametric equalizer and selectable compression, plus a few goodies that are just fun. We're not sure what that's worth in car audio (check out some pro gear catalogs), but it seems like a good deal to us.