Andrew Roberts, a guitarist and a regular contributor to Pro Audio Review, is a sound reinforcement and recording engineer. He is the owner and chief engineer of a multisystem sound company and a project studio.
Contact: QSC Audio Products at 714-754-6175, www.qscaudio.com.
As most of us probably know, we audio professionals operate in a field where there is an accelerated evolution of the tools of our trade. Without some type of refinement or improvement, it is rare for any given product to have a life cycle beyond a few years. It has been nearly a decade since QSC introduced its PLX series of amplifiers. These less expensive brethren of the company's Powerlight series proved to be an instant success with musicians, institutions and even sound providers like myself. The source of their popularity was that they provided significant power and minimal weight at a very affordable price. My sound reinforcement business owns quite a few PLX amps and we have used them on hundreds of jobs in all manner of situations and operating conditions. I have seen them prevail through dust clouds, voltage drops and over-zealous band engineers. They have proven to be very competent and remarkably dependable. Therefore, it was with much anticipation that I received for review the PLX3602, a replacement for the venerable 3402, the former flagship of the PLX series.
Features
To those familiar with the original PLX line, this new breed will seem like familiar territory, but with more refinements than overhauls. One change is that there are now six models in the line, including two models specifically designed for 4 ohm and 8 ohm loads (PLX1104 and PLX1804).
Like its predecessor, the 3602 is two rack spaces tall. From there, the two amps have slight differences and, in some cases, more significant variations. For starters, the front panel looks remarkably different. QSC has eschewed the old gun metal gray for a new vibrant silver finish that looks like something from Mr. Scott's engine room on the Enterprise. Gone too are the old elephant ear style front panel handles. They have been replaced by discreet finger holds formed by flanges on the faceplate. As with the 3402, front panel controls are a power switch, volume attenuators and LED indicators (power, signal [-35 dB], -10 dB, clip, bridge mono, parallel).
The back panel is home to input connections (XLR and 1/4-inch balanced), speaker output connections (Speakon and binding post), and a host of configuration switches. Gone are the recessed DIP configuration switches from the first generation PLX units (which I spent many a moment trying to adjust - crouched over, Leatherman in hand, Maglite in mouth, trying to see the tiny toggles). The old DIP switches have been replaced by new switches that you can manipulate with your fingers. While I welcome this change, it could be argued that the amp is now less idiot-proof, making it easier to slip the amp into bridge mono mode or some other mode that may cause problems for inexperienced users. The new switches allow access to some previously seen functions (bridge/stereo/parallel, clip limiters, high-pass filters) and some new functions (low-pass filters).
As you may have gathered by the model number, the new amp is slightly more powerful than its predecessor. The 3602 is rated for 1,100 watts per channel at 4 ohms (20 Hz – 20 kHz, 0.05% THD) and 3600 watts in bridge mono (1kHz, 1.0% THD, 4 ohms). I would like to interject that, if the 3602 is as robust as its ancestor, it will be comfortable driving 2 ohm loads (It is rated at 1,800W for 2 ohms – Ed.). Continuing the PLX tradition, the 3602 is very thrifty too, consuming just 11.5 amps when powering a 4 ohm load at 1/8 power (the closest rating to actual use). It also weighs in at a remarkably light 21 pounds.
In Use
The most fitting assignment for an amp with this level of power output is sub-woofer duty and, being the conscientious reviewer, I felt compelled to oblige. I used the 3602 to power a pair of 4 ohm cabinets (each loaded with four 15-inch woofers) and a pair of single 18-inch 8 ohm subs. The first event was a concert featuring a local jazz group in a lively, confined acoustic environment. The 3602 performed wonderfully - dealing with kick transients and delivering robust lows from the 4 ohm subs. While I had a high-pass filter set at 25 Hz on my speaker processor's sub output, it was nice to have the onboard rolloff at 33 Hz, had I needed it. Later, in bridge-mono mode, the amp was brilliant in powering the 8 ohm cabinets, even for more aggressive musical styles. It displayed lots of headroom and delivered a deep wonderful bass sound.
At another event, where I was doing sound for a group of Nigerian musicians, we were presented with a drummer who needed some more thump in his monitors. Since we were short on processing for such a request, I pressed the 3602 into service. I put the amp into stereo mode and applied the 100 Hz internal high-pass filter to one channel (for mid/high) and a low pass on the other channel (for a sub). The drummer had a big grin on his face during the sound check and, as I listened over his shoulder, I too was impressed with the clean thump of the kick drum sound. Throughout the course of the band's boisterous performance, the 3602 delivered enough drum fill sound to dislodge tooth fillings (and contaminate the drum sound for FOH).
At another event, I used the 3602 to power a string of 8 ohm cabinets for public address. The event was at the house of former President Bill Clinton and his wife, Senator Hillary Clinton. With more than 150 people gathered in their back yard, we were required to provide discrete distributed sound for several speeches. This included powering three cabinets per amp channel. While this is not a particularly stressful chore for an amp, it was a test of the amp's flexibility. I put the amp in full range mode and applied the internal 33 Hz high-pass filter. There was clean intelligible speech heard throughout the listening area.
Summary
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With increased power, more features, an easier to use configuration section, and a lower price ($1,549) than its predecessor, the PLX3602 is sure to carry the PLX popularity torch a long way. This amp is a wonderful choice for institutional or worship use but, at only 21 pounds, it is a blessing for portable sound reinforcement. If its level of reliability is equal to that of its PLX ancestors, the PLX3602 should be a staple in sound systems worldwide. If you want lots of power, good sound, low weight, and proven reliability at an affordable price- this is your amp!
Review Setup
Midas Venice 160, 320 mixers; Audio-Technica, Shure, Audix and Sennheiser mics; Rane, TC, BSS, PreSonus processors; JBL SRX and Community loudspeakers.
Notes: Unless otherwise noted or implied, all measurements are made with 8-ohm loads from the balanced inputs, stereo mode with the volume set to maximum. Both channels driven for all distortion and power measurements. Measurements made on one channel are made on channel one.
-Bascom King
QSC PLX3602 Power Amplifier Bench Measurement
The PLX3602 is the currently most powerful amplifier in the PLX series. A switching power supply is utilized resulting in low weight relative to the amplifier's output power. The output stage uses a three-tier Class H topology resulting in higher efficiency over the
Frequency response as a function of open circuit, 8, and 4 ohm loading is plotted in Figure 1. As can be seen, the response is very independent of load suggestive of a high damping factor. Frequency response of the 33 Hz and 100 Hz low-cut and the 100 Hz high-cut filters are shown plotted in Figure 2.
THD+N for a 1 kHz test signal and for 8 and 2 ohm loading in the stereo mode are shown in Figure 3 and 4 respectively. Distortion is quite low in this design for this testing condition. THD+N as a function of frequency and power for 4 ohm loading is plotted in Figure 5. The frequently observed rise in high frequency distortion in many amplifier designs is in evidence here. A spectrum of the distortion and noise residue of a 1 kHz test signal at10W into a 4 ohm load is shown in Figure 6. Amazingly low is the amount of AC line harmonics. The signal frequency harmonics are dominantly even indicating good half-cycle symmetry at this power level.
Channel separation is plotted in Figure 7 for both testing directions. Unusual is the more or less flat nature of the separation vs. frequency. Much more usual is a characteristic that rises with frequency due to capacitive coupling in the signal circuitry.
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