We’ve all seen shows where a magician makes a rabbit appear out of a hat or a dove from a handkerchief. We know these sleight-of-hand tricks that have been practiced for years. When it comes to car audio, we know that it’s impossible for an amplifier to make more power than it consumes, even if that power is averaged over a long period.
What if an amplification solution could make your music louder without requiring your installer to connect a power wire to the battery? The engineering team at Rockford Fosgate offers this solution with their PM100X1K Punch Series Element Ready monoblock amps. We’re sure you are as intrigued as we are, so let’s check them out!
Rockford Fosgate PM100X1K Features
Before we discuss how the PM100X1 amplifiers work, let’s examine their features and specifications. The amps, which are sold in pairs, are single-channel units designed to boost the power from a radio and drive a full-range speaker. Each amp (a PM100X1) is rated to produce 60 watts of power into a four-ohm load and 100 watts in a two-ohm load. That’s three to five times a typical radio’s power per channel.
Each PM100X1 amp is based on an extruded aluminum chassis that measures 1.3 by 3 inches. The end caps have mounting flanges, bringing the total length to 3.9 inches. There is a 7.5-inch pigtail on the left side of the amp with five wire connections. The connections include two signal input wires that would connect to your radio, two output wires that connect to the speaker and a ground wire that would go to the battery’s negative terminal.
The kit also includes an additional wire for the harness. Should the BTL voltage of the connected radio not be enough to activate the amplifier, this red wire can be connected to a 12-volt power source to turn it on.
Signal Processing Features
There are two switches on the right side of the amp. The first switch allows your installer to select a 60, 80 or 100 hertz high-pass crossover frequency. It should be noted that the crossover can’t be defeated. So, you won’t be using these to power a subwoofer.
The amp has no gain control as it’s easy to predict that most source units will provide 20 to 22 watts of power when supplied with 14.4 volts. We expect the amp to have just over six dB of gain. We’ll check this once we have one on the test bench.
Element Ready Design
These amplifiers are designed for marine applications, so they are Element Ready and carry an IPX6 water and dust intrusion rating. The right side of the amp has two switches, and Rockford Fosgate includes a rubber plug to seal things up after adjustment.
We did take the amp apart to examine its internal components. In addition to its impressively compact design, we noted a full rubber gasket on each end panel and O-rings around all the mounting hardware. This is serious attention to detail!
Rockford Fosgate’s Element Ready design goes beyond simply adding water-resistant features. They design and test products like these to withstand intense vibrations and impacts, like those you’d experience in a boat, personal watercraft or side-by-side. They also create the amp so that the materials and finishes resist degradation from prolonged UV exposure.
Rockford Fosgate FreeLoad
OK, let’s talk about how these amps work. If you’ve read any of our articles about Bridge-Tied Load (BTL) amplifiers, you might know that a typical car radio has a DC voltage on the speaker outputs. This voltage is typically half of the supply voltage. If the electrical system in your car or truck is at 12 volts, you can measure about six volts on each of the speaker wires. As there is no difference between the voltages on the wires, the speaker doesn’t move because there is no difference in potential.
In a conventional amplifier, a bank of MOSFETs chops up the battery power into pulses and then feeds them through a transformer to boost the amplitude. The transformer’s output is smoothed with capacitors to produce the rail voltages.
The FreeLoad design found in these amplifiers works in two ways. First, it captures energy from the DC offset voltage on the speaker wires. The ground wire on the amp is required to provide the internal energy storage circuit with a reference voltage level. The amp uses a boost converter to charge a pair of huge capacitors (roughly 15,000 microfarads at 15 volts) to store energy for the output circuit.
Additionally, the FreeLoad design stores additional power by presenting the source amp with a load and then converting that to stored energy. As the AC signal increases in amplitude, FreeLoad stores more energy. We imagine they use some form of rectifier circuit to perform this task. The rectification output can put additional energy in the storage capacitors mentioned above. If you want to know precisely how the amp works, look up Rockford Fosgate’s patent on the FreeLoad circuit. It’s not light reading, but it’s very interesting.
To summarize and put this into layman’s terms – the PM100X1 amps are designed to store the energy available from the radio amplifier into a pair of caps – similar in concept to the Boosted Rail design we saw in the PBR400X4D amp reviewed recently. That energy is then used to power the output circuitry of an amplifier to make your music louder.
Knowing that some factory-installed audio systems use speakers with lower impedance (two ohms or lower), the amp has a switch that optimizes the FreeLoad circuitry to draw additional current from this low-impedance design. With most car radios, your installer will leave this in the four-ohm setting. The two-ohm setting is available if the vehicle has a BTL amp designed for lower impedance.
FreeLoad and The High-Pass Filter
You might be wondering why the amp has a non-defeatable high-pass filter. Given how the FreeLoad circuit works, it needs to store energy from the limited supply available from the factory-installed radio for use later during louder musical passages. Low-frequency audio information requires much more energy to reproduce than midrange and high-frequency signals. Also, low-frequency information is often sustained, like a pipe organ pedal or key on a synthesizer. These long, drawn-out notes would deplete the reserve energy in the aforementioned storage capacitors very quickly. So, low-frequency information is filtered to maintain a high average power level.
Given the typical applications for these amplifiers, like in a boat or side-by-side, you likely aren’t in an environment where pushing a 6.5-inch speaker to produce low-frequency information is a wise choice. You won’t hear the bass over the engine, and the speaker will sound stressed while trying to reproduce that music. All in all, it’s a good feature that benefits the net listening experience.
Bench Testing the PM100X1
We connected one of the PM100X1 amps to the output of a high-end multimedia receiver. When provided with 14.4 volts, this radio provides about 22 watts of power into a four-ohm load. We connected the output of the PM100X1 to our bank of four-ohm ceramic load resistors.
Measuring power proved challenging. However, we did confirm that the amp can deliver at least 61.4 watts into a four-ohm load. This required making very short burst measurements. Nevertheless, the number is quite accurate.
We also confirmed that the amp provides 6.52 dB of boost to an audio signal. This matches our prediction on taking roughly 20 watts of power and making it 60, into the same load impedance.
While making distortion measurements, we noted that the amp presents a challenge to the radio output. We’ve measured distortion and frequency response characteristics on this radio several times. While driving the PM100X1, though, it showed moderately poor performance, with a broad peak around 2kHz.
Given this as the source, we expect the amp to seem bright during our listening session. If your radio has an EQ, pulling out a dB of gain at 2 kHz and above will flatten the response. The graph below shows the net frequency response measurements into different loads when the radio response is subtracted.
It’s interesting and commendable that the amp is optimized for driving real-world loads, as the black trace above shows. In this instance, with the crossover set to 60 Hz, we measured the -3dB frequency response to be flat from about 75Hz to just a hair over 20 kHz. The -6dB point of 60 Hz on the bottom end indicates that the high-pass filter is a -24dB/octave design.
Auditioning the Rockford Fosgate PM100X1K
After several hours in the lab gathering pertinent data, we took the configuration to the listening room to hear how the PM100X1K sounded. The amps drove a set of 2.5-way home audio bookshelf speakers with a nominal impedance of four ohms.
First was Boy in the Bubble by Paul Simon from his album Graceland. This is an impressively dynamic and very well-recorded track. The second the music started, we turned up the volume well past what would be considered a comfortable listening level. The neighbors protested almost instantly. Nevertheless, the pair of PM100X1 amps did their thing and enhanced the listening level dramatically.
As predicted, the upper midrange was emphasized, making the T and S sounds stand out. The midbass was dynamic and impactful, making the percussion fun.
Next, we played New Order’s Blue Monday from their The Best of New Order album. This particular track has an incredibly dynamic bass line. The little marine amps did it justice at impressive listening levels. It’s hard to believe this much energy comes from a radio and two amplifiers fed by 16 AWG power wires. It sounds dramatically louder.
Next was Bulletproof by La Roux from their self-titled album. This track has significantly more low-frequency energy than Blue Monday. So, even with the high-pass filter set to 60 Hz, we needed to drop the volume by two notches. The system responded with much cleaner and clearer vocals.
Conclusion on the Rockford Fosgate PM100X1K Amplifier Kit
Overall, the pocket-sized Rockford Fosgate PM100X1K amplifier kit does precisely what it claims. It will make your music audibly louder with minimal installation effort. Do they have the clarity, impact and composure of a full-size Power series amp? No. However, not many amps on the market do. Are they hands-down the easiest way to make a 12-volt audio system without an amplifier play louder? You bet they are!
If you want to upgrade the audio system in your boat, side-by-side or even your car and don’t want to spend money on purchasing and installing large power wiring, drop by a local authorized Rockford Fosgate retailer and ask about the PM100X1K kit. This pair of FreeLoad amplifiers will boost the playback level and bring your system to life!
You can find a retailer using the locator tool on the Rockford Fosgate website. Be sure to follow the mobile audio fanatics from Arizona on Facebook, Instagram and YouTube to learn about the products they offer and the incredible events they attend.
