PE #300-804 200 Watt Sub Amp: A closer look by Darren Kuzma The Parts Express #300-804 is one of the more recent arrivals onto the subwoofer amplifier market. With a very modest external package, it is difficult to get a good idea of what the real-world performance of this amplifier really is. After receiving many inquiries into what makes this amplifier different from others on the market, I decided it was time to take a closer look at what it has to offer. General Description The amplifier is fairly compact and measures 9-13/16 square on the face, and 5 deep. This depth includes a plastic rear housing which surrounds all of the electronics to prevent air leaks into the cabinet and to protect the components. The cutout size needed to install this amp into a cabinet is 8-5/8 square. The amplifier weighs in at a healthy 12 lbs. Construction isn t exciting on the inside, but it is a very functional design. On the inside, the only thing that stands out is a pair of large 10,000 uf power supply caps. The 1-1/2 tall heatsink fins on the outside hint that it should be good at removing heat from the output transistors. Power Before we start throwing power numbers out there, we must first discuss one of the main features of this amp: the soft clip circuitry. This circuitry prevents the amplifier from clipping, eliminating the audible flops, farts, and wubba-wubba s that are normally a result. Amplifier clipping is probably the most noticeable form of distortion that can come from a subwoofer system, other than physical bottoming of the subwoofer driver. Not only are these ill-effects of clipping bad-sounding, they can also cause damage to subwoofer drivers or to amps themselves. The soft clip circuitry works by adjusting the entire output such that none of the sine wave peaks are chopped off. While it does take a certain amount of time for this circuitry to kick in, it is in the low millisecond range and for all practical purposes works extremely well. Some people may feel gypped because the circuit is essentially compressing their audio signal, but is a small price to pay for protecting your ears and your sub! Back to the power testing Since most amplifiers do not have this soft clip circuitry, as the output level is increased the signal begins to clip and massive distortion sets in. Hence, many amplifiers will often be rated max power at 10% distortion or some other very high percentage. But, because of the soft clip circuitry, this amplifier is always working in its low-distortion range of less than.2%, even at its maximum measured RMS output power. By using a classic power testing method with a scope and a true RMS voltmeter, I was able to measure the power output into various dummy loads. Into a 4 ohm load, the amplifier was able to maintain 31V RMS without clipping. This corresponds to a power output of 240 watts. Into 8 ohms it measured 34.5V RMS, or approx. 150 watts. At 16 ohms, voltage was the same at 34.5V RMS, or 75 watts.
Frequency Response: By running measurements of the amplifier into a dummy load, I was able to get a good picture of the overall frequency response of the amp. The stock amplifier has a response that is 3 db down at roughly 18 Hz on the low side and 168 Hz on the high side when the crossover is at its highest setting. A range of 18 Hz through 160 Hz is appropriate for just about any conventional subwoofer system. The low end: A built-in rumble filter attenuates the lowest audio frequencies to help conserve amplifier headroom and protect drivers. The rumble filter is actually a 12 db/octave high-pass filter that is centered at 20 Hz, the F3 is 18.2 Hz. Since most source material does not contain information below 20 Hz, usually we are not missing anything. But, in cases where information is available this low, we generally need to protect our drivers from this potentially damaging information. Since excursion demands increase with decreasing frequencies, even the beefiest sub can start to run into overexcursion problems. This is particularly a concern in vented subwoofer systems where overexcursion occurs rapidly below the tuning frequency. The high end: The upper end low-pass filter of the amplifier can be adjusted from a frequency of 40 Hz to 160 Hz. The settings on the knob are actually fairly accurate and correspond with the upper end Fc quite well. The low-pass filter features 24 db/octave attenuation, far superior to standard 12 db/octave filters. Side commentary: There are several problems associated with the common 12 db/octave filters found on other amplifiers. The shallow slope often does not allow good integration with mid-sized main speakers. It is difficult to achieve a good transition without getting overlap in the lower midbass region, which can cause a tubby or boxy sound. Also, many subwoofers have large response peaks in the 800 Hz- 2 khz region. If not attenuated enough, these peaks can bleed through into the output from the subwoofer, causing what could be described as a ringing sound. Not only is this annoying, but it can result in a fatiguing system and a subwoofer that does not disappear into the room. Connections The #300-804 has a full complement of the standard connections needed for most subwoofer applications. Power is supplied via a standard 6-foot power cord. A signal can be fed to the amplifier from either the high-level (speaker) input or a low-level (line) input. Output to the subwoofer is through a single pair of speaker wires, terminated in female disconnects. The disconnects can be cut off and the wires stripped for a direct connection to your driver. When the speaker level inputs are used, the speaker level output will transmit the signal to your speakers with the addition of a high-pass filter. The high-pass filter consists of a 220 uf electrolytic capacitor and corresponds to a crossover point of approximately 90 Hz @ 8 ohms and 180 Hz @ 4 ohms. The capacitor can easily be removed or the value
changed if desired. The speaker level input posts will accept bare wire, or by removing the small plugs at the end of the posts, banana plugs are accepted. The low-level RCA input features stereo connections, allowing both left and right channels to be connected to the amplifier. The left and right channels are summed internally and both are fed to any single coil subwoofer. Either channel can be used for connection to a mono subwoofer signal. The low-level line out transmits the complete audio signal that is entering the amplifier. There is no high-pass or low-pass circuitry implemented in this feed, therefore the full range of original signal will pass through. This feature can be used when daisy-chaining multiple subwoofer amplifiers. Auto-on circuit The auto-on feature of this amplifier allows it to automatically turn itself on and off as needed. The auto-on circuit is activated via the input from the speaker or line level inputs. The turn-on sensitivity through the line-level inputs is approximately 5 mv, which is very low and should trigger with even the faintest signals. Once a signal ceases to be sent to the amplifier, it will take roughly 30 minutes for it to turn itself back into off status. This extended shut-off time is designed to prevent the amplifier from shutting off during periods of inactivity in movie soundtracks. When the amplifier is in standby mode, the power indicator LED will be completely off. Protection circuitry The #300-804 amplifier features a triad of protection circuits that make it very durable. As mentioned previously, soft clip circuitry is implemented to protect the driver and amplifier from rapidly changing dynamics. Not only does this make the amplifier sound better, it also makes it operate in a much more stable manner, without putting extra strain on the power supply. A very effective thermal protection circuit automatically cuts off the output of the amplifier should the heatsink become too hot. Because high temperatures are what destroy amplifier components, this feature ensures that unsafe conditions are never reached. After the thermal protection has been engaged, the amp will automatically reset itself once it reaches a cooler and safer temperature. There is no need to manually reset anything on the amplifier-- it will turn on back to its original level once it is cool. However, should the thermal protection circuit repeatedly trip, it is usually indicative of other problems such as low driver impedance or poor circulation. Lastly, the amplifier is fully overload and short-circuit protected. Should the output wires of the amp ever touch each other or a driver melts-down, the amplifier will go into a protection mode, indicated by a red power LED. This does not indicate damage, but merely that the amp has entered a state of self-protection. Should the amplifier enter this mode, it will need to be unplugged for 30 seconds or so to allow the circuit to reset. I decided that to do a full evaluation of this unit, I would have to test this short-circuit protection. While running a sine wave at full power of 240 watts into 4 ohms, I directly shorted the output wires of the amplifier. Besides practically welding the wires together, the amplifier immediately went into its protection mode like it should. Sure enough, after unplugging and plugging the amp back in, it went back into normal operating mode and was working just fine.
Let me tell you, for all the testing I put this poor amplifier through, it kept going without any problems. I never even blew the fuse-- truly a testament to its well-designed protection schemes. Load Stability One of the problems often encountered with other subwoofer amplifiers is their lack of stability into difficult 4 ohm loads. To get an idea of the stability of this amplifier, I decided that the best test would be to run it into a 2.66 ohm resistive load. This represents a more difficult load than anything that will be encountered with a normal 4 ohm nominal driver. To make the test as grueling as possible, a 100 Hz pure sine wave was used as the input signal. Running pure sine waves is about the most destructive thing that you can do to any amplifier. Since most subwoofer systems see only occasional output at full power, this is much more rigorous situation than will be found in normal use. So, running at full power of 294 watts into 2.66 ohms, I wanted to see how long the amplifier would last. Starting from room temperature, it ran for approximately 6-1/2 minutes before shutting down. The thermal protection kicked in at this point and put the amplifier back into standby mode. Once it cooled down, the amplifier picked right up where it left off and continued playing. Obviously since the amplifier was already hot, it didn t take too long to go into thermal overload again. However, it is good to see that it can last even this long, and that the protection circuits are working very well. 6-1/2 minutes of straight runtime at full power into 2.66 ohms is really quite impressive. A standard 4 ohm subwoofer should be any easy task for this unit. As a contrast, when the #300-794 amplifier was run through the same test at the same 294 watts of power it only lasted 4 minutes. On top of that, I found that a blown fuse was actually responsible for shutting the amp down, and not any of its protection circuitry. Hence, if this occurred in a real listening situation, you would have to stop the movie, pull the sub out and meticulously replace the fuse-- obviously not an ideal situation. Power consumption In the interest of learning as much as possible about the amp, I decided to do some quick tests of the power consumption. At full power, it uses about 540 watts, which is a current draw of about 4.5 amps. When turned on, but no signal is present, it uses 17 watts of power. When in standby or sleep mode, 15 watts of power are dissipated. Conclusions While this amplifier may not have quite the raw power of some other sub amps on the market, it more than makes up for this with consistency, stability, and great sound quality. Compared to the 300-794 and similar amplifiers, the maximum SPL the 300-804 can achieve will be within 1dB, which is more or less an unnoticeable difference. The soft clip circuitry is an extremely valuable advantage of the #300-804, and will surely result in a cleaner sounding sub. To me, the peace of mind of having a nearly indestructible amplifier alone gives this item great value. Add to that the great low impedance stability, small size, and great price, and this amplifier is a winner!
Specs: Measured Power: <0.2% distortion 240 watts RMS @ 4 ohms 150 watts RMS @ 8 ohms 75 watts RMS @ 16 ohms Amplifier Frequency Response 3dB down points (highest XO setting): 18 Hz and 168 Hz Rumble filter response: F3 = 18.2 Hz, F6 = 13.7 Hz, F10 = 10.7 Hz Low-pass frequency: Continuously variable from 40 to 160 Hz Output Frequency Response Speaker level: 6 db/octave high-pass, 90 Hz @ 4 ohms, 180 Hz @ 8 ohms Line level: 10 Hz- 20 khz, full bandwidth Auto-On Sensitivity ~ 5 mv, 30 minute shut-off time Max Power Consumption 540 watts, 4.5 amps