Precision Power Inc., or "PPI," has been a well-respected amplifier maker since the early 1980s. PPI has presented plenty of innovations over the years, like the digital control pieces in the late '80s, the flat piston subwoofer in the mid '90s, and the $5,000 2500 F1 chrome-and-glass amplifier in the late '90s. Even with all of the cool stuff, they always seemed to understand that the mainstay of their business was high-quality class A/B amplifiers--both in sound quality and reliability. Let's see what they're up to in 2005 by taking a closer look at the DCX300.2
The PPI DCX300.2 is an unregulated class AB amplifier rated at 2 x 75 watts into 4 ohms, 2 x 150 watts into 2 ohms and 300 watts bridged into 4 ohms. The manufacturer states that all power specs are at 12.5 volts, meaning that you can expect more than its rated power because your alternator will regulate its output somewhere between 13.8 and 14.4 volts while your car is running. The amp is equipped with highpass/lowpass/full-range crossovers for the speakers, as well as RCA outputs with a second highpass/lowpass/full-range crossover completely independent of the first. This is a great tuning feature if you're trying to clean up that subwoofer-to-midrange muddiness--set your mids on highpass at roughly 110Hz and your subs with a lowpass setting between 85-90Hz. Coupled with PPI's "QBASS Plus," which allows up to +18dB of boost at one of four different frequencies, you may have a hard time deciding whether to use this amp for your subs or your full-range speakers.
Cosmetics
It's an attractive amplifier (especially if you are into sci-fi), with highly styled shrouds making it look something like the starship Enterprise. The shrouds are a fairly elaborate affair, comprised of both molded plastic and cast aluminum parts. These pieces bolt to an acrylic top cover and aluminum heatsink. Speaking of heatsink, this amp is built upside down; it uses a typical long U-shaped extrusion, but with the open side up. The main circuit board is suspended slightly above the inside of the heatsink, which has a series of small fins on the inside and vents in the endplates to release hot air. The translucent, dark red acrylic top cover has a silk-screened PPI logo and model number in red and white atop it. To dress it up and hide your wires neatly, two center strips and the end shrouds are bolted to this piece. There are four indicator lights visible through the acrylic top: Power on, Clip out (which matched the clipping signal of my 'scope at all impedances), Short and Thermal protect.
After you manage to remove the several screws holding the shrouds, for which there are no instructions, you can access the endplates to wire up and adjust your amp. On one endplate you'll find the RCA inputs and outputs; -12dB attenuation switch; two crossovers with frequency range knobs and Full/LP/HP switches; the speaker and remote connection plug; and finally the QBASS Plus switches. On the other end reside the power/ground connector plug, the QBASS boost knob and the QBASS remote jack. Of special interest are the crossover frequency adjustment knobs: they have 40 little "clicks" that give you 41 precise settings that correspond with the frequency chart in the owner's manual (page 18) so you don't have to try to guess the frequency by the numbers written around the adjustment hole. It seems odd to have the QBASS frequency switches on one end and the boost control and plug-in for the remote at the opposite end.
Circuit Design
Removing the four remaining screws in the acrylic top cover, I was inside this amp in no time. My initial impression was, "Wow, it's roomy in here!" Perhaps it's because the heatsink where the hang-off devices sit is flat instead of vertical, but it looks downright spacious. Don't get me wrong--there are lots of parts in here, but you're unlikely to encounter any thermal issues with this one. My second thought was, "Where's the fuse?" Precision Power doesn't put any fuses in their amps! They must be crazy! Everybody knows you have to have fuses in an amplifier, don't they? Well, in a word, no. And it may actually be a good thing. See the mini sidebar below.
Q: What are fuses for?
A: To prevent too much current flowing through a given component (e.g., wire, light bulb, amplifier) so that it doesn't get really hot and set something on fire. That's it.
Q: How does a fuse work?
A: The fuse element presents a resistive component in the current flow. Resistance causes heat, determined by how much current is flowing through the element. The element will melt at a specific temperature caused by a specific amount of current flow, thereby disconnecting the current source (your battery) from the current drain (e.g., amplifier, light bulb, shorted-out power cable). Certified installers, IASCA and the PPI owner's manual all tell you to install a fuse within 18" of the positive battery terminal. Always do this! This will keep you out of trouble if anything overloads the power cable, whether it is the system or a stray mounting screw. The fuse will blow, disconnecting the line. OK, here's the good part. A fuse gets hot because it has resistance. We use big power cables in our systems because they have much lower resistance than small power cables. Less resistance equals better performance from your amplifier. So why, if you put the fuse up by the battery, do you need a second current-restricting fuse at the amplifier? Hmm...
Back to the circuitry. Over 12 volts enter the amplifier through the large brass connector pins, go by 6,000uF of primary filter caps and directly to the transformer. The Litz-wound transformer looks a little small, but again, it just seems small due to the big heatsink. The SG3525A pwm chip drives the six 50NO6 switchers (120 watts dissipation each = 720 watts capability) at 40.25kHz and also determines behavior when the protection circuits are engaged. Two thousand microfarad capacitance, each on the +40-volt and -40-volt rails, stabilize the voltage available to the output devices. Speaking of outputs, there is a single pair of TO-218-size devices for each channel, each pair theoretically capable of 250 watts, more than enough for rated power.
At the other end, the music signal comes in via gold-plated RCAs and goes through an isolated and buffered input and on to the crossover and QBASS sections. There are lots of film capacitors and 2 percent resistors in here, very nice. The low-level signal is more than adequately far away from the power supply components, and even the speaker output traces are far away enough to prevent any noise back into the signal path. The speaker outputs consist of large brass pins that mate with the molded connector block.
Speaking of noise issues, I was concerned earlier to see the QBASS boost knob and remote jack right next to the power supply, but now I see how they did it. The level controls consist of a 16-step rotary switch (encoder) rather than the usual gain potentiometer. This encoder is connected to a series of diodes, which in turn "switch" voltage back through a grid of resistors and on to a voltage-controlled gain circuit. Instead of continuous boost from nothing to 18dB, you actually get 16 steps. Since there is no signal on the control lines, there is no noise picked up.
Performance
Frequency response is very conservatively rated at 20Hz to 20kHz + 0.25dB. When you're talking about crossover points you're looking at the -3dB point, and at CA&E we measure overall response at -1dB. My tests show a much better range of 7Hz on the bottom end to 44kHz on the top. I did notice some slight ripples (less than 0.2dB) in the 30Hz to 60Hz range. It turns out that the ripples moved when I switched the QBASS frequency. It seems that the circuit doesn't turn off completely--but I was looking very closely.
The crossover curves were pretty standard 12dB/octave with good shapes, but when the crossovers were turned on, the signal was about 2dB higher than when they were off. (See Frequency Response graph.) The crossover frequency range (the click points) was off a little. The chart in the owner's manual shows the lowpass starting at 28Hz and going to 2,980Hz; my tests revealed 35Hz to 3,500Hz. The highpass in the manual is stated 52Hz to 4728Hz; my test showed these to be 21Hz to 3,200Hz. QBASS tested much better, dead-on in fact. The frequency centers were where they are supposed to be, and full boost is exactly 18dB. (I had to check it again; I never have seen bass boost perform exactly as advertised!)
Protection circuits worked pretty much as the manual explained them. When the speakers were shorted out, sound came in blips as the power supply would re-start and again turn off until the short was corrected. The low-voltage circuit turned off the amp when the battery voltage dropped a little below 9 volts. In spite of the warnings about airflow through the vents in the endplates, I was not able to shut this amp off thermally; it simply has too much heatsink. The DCX300.2 made no turn-on or turn-off noise at all, except when shorted out, and that is acceptable.
Manual
This owner's manual has a lot of information in it, though it is certainly not a step-by-step installation tutorial. It has diagrams for the endplates, fuse charts, illustrations for wiring and reasonably clear instructions on setting the gain. However, nowhere does it tell you how to get the shrouds off the amp. (They did put in an Allen wrench that only fits the bolts in the shroud, so I figured it out myself.) Enough whining--other than a few strange or incomplete sentences, you will find that the manual is very helpful.
Precision Power gives you a 1-year warranty for over-the-counter units and 2 years if installed by an authorized dealer.
Listening test
With the amplifier in the trunk, CDs in hand, I climbed into the Buick on a perfect Arizona winter day--dead calm and 73 degrees. I started with an old favorite, recently found again: DADA's Puzzle from the early '90s. The best cut has to be "Dorina," and after browsing for a while I eventually came back to it. The song starts with a solid bass hook and a Fender Strat lead, joined by the drums as the tune picks up steam. I was impressed by how distinct every instrument sounded; the drums were strong but tight, and definitely under control. It seemed like I got a lot more low-end out of my separates than usual before they started popping. (The DCX300.2 was running full range to the front doors.) Next up was Dee Dee Bridgewater's rendition of "Killing me Softly." Bridgewater's voice was as sweet and airy as ever, but I particularly noticed the vintage Rhodes sound in the first verse--very rich and warm. Maybe a little too warm? I checked to make sure that the QBASS was turned all the way down and went back in for some Donald Fagen. "Snowbound" from the Kamakiriad CD has a great Fender Rhodes keyboard and you can hear it cracking on the lowest bass notes just like the real thing. Guitars all sounded good, and the cymbals were excellent in "Tomorrow's Girls." How about some female vocals? Mary Chapin Carpenter sounded a little thick on "Only a Dream" and the title track, "Come on, Come on."
Time came to switch it up, so I threw the PPI on a 12" subwoofer in a sealed box. I ran back through the previous tracks along with Yello's "Resistor" and Max Killa Hertz "Sine Bombs." The amp played low and tight, reproducing everything in my arsenal. All in all, it's a very well-rounded amplifier, well suited for any frequency range. The only noticeable mention was that it seemed a little warm in the low-mid range, but overall it's very nice to listen to. There was no hissing or turn-on noise at all, and with the volume at zero I couldn't tell if the amp was on or off by listening.
Conclusion
The DCX300.2 performed well on the bench and in the car; it is definitely underrated by PPI with both power and frequency response well exceeding their claims. The crossovers adding gain and the residual QBASS ripples may have you question why they didn't fix those issues, but they do not represent real problems. I'm sure changing a resistor value or two would solve both issues. The extended frequency response, excellent damping factor and high S/N ratio overshadow the minor infractions. The amplifier sounded great in the car and its RCA output crossover makes it a very flexible building block. It drives subwoofers as easily as full-range speakers, so you could use two or three of these for a great system. At 1 percent distortion I was able to squeeze in excess of 400 watts out of the DCX300.2. According to the buyer's guide, suggested retail is $499, so at retail you're paying $1.25 per watt. It's on the high side, but I think the quality and sound of this amp are well above average. At least with this amp, you get what you pay for.