The JL Audio M series amplifiers use a Class D circuit design to power the speakers. Class D amplifiers are popular for marine applications because they are efficient and they do not strain your boat or vehicle's charging system like a Class AB amp might. Lower current draw means less heat. For most marine applications, you are using your boat when the weather is nice and it is usually during the summer months. These also happen to be the hottest months of the year and you want your amplifier to stay as cool as possible so that you can play your music as loud as you like, for as long as you want, without having to worry about it shutting down and going into protect mode. The M series amps use JL's NexD switching technology and offer unbelievable thermal management. In addition to the thermal management NexD switching is constantly analyzing the power supply voltage. It uses the power supply voltge and the input signal to cancel out distortion due to voltage fluctuations. This in turn reduces distortion for better, cleaner sound. The M series amps are relatively compact and they deliver nice, clean power to your speakers.

THe JL Audio MHD amplifiers also have Class D power circuitry, but they manage and eliminate distortion in a different manner than the M series. Instead of using NexD switching technology, the MHD amplifiers use JL's Single Cycle Control switching technology. They both essentially do the same thing, they eliminate distortion caused by changes in teh power supply voltage, they just do it in different ways. Single Cycle Control was first, NexD is the newer technology.

The JL Audio RIPS power supply ensures constant power output across all impedances. This means that you have the same output at 4 ohms, 2 ohms, 1 ohm, 8 ohms, or even 16 ohms. As the speaker cone moves, the impedance load on the amplifier is contantly changing. That also means that the power output is constantly changing for all amplifiers that don't have the RIPS, constant power circuitry. Now, if you are wiring 4 speakers in parallel off of two channels to get to a 2 ohm load, it would not be advtantageous with the MHD amplifiers. But, also think of it this way. The M400/4 does not put out 100 watts per channel at 4 ohms. It puts out 100 watts per channel at 2 ohms. With the MHD amplifiers, you are getting the full rated power of the amplifier at 4 ohms and at 8 ohms. Instead of thinking of the MHD600/4 as a 600 watt amplifier, think of it as a 1200 watt amplifier because that is what it would be if it was in the M series. The RIPS power supply gives you a difference in sound quality and clarity that you can hear. Think of the MHD900/5 as an 1800 watt 5 channel amplifier, not a 900 watt 5 channel amplifier. When you shift your mindset, you realize why these amplifiers are so expensive.

The crossover slope on the amplifier will tell you how sharp or how steep the fall off is at the frequency you decide. When an amplifier has a 12db/octave slope on the crossover it is not as steep as a 24db/octave. This means that if you set your high pass crossover point at 80 Hz, the db output at 70 hz will be higheer with a 12db/octave amp vs. a 24 db/octave. We are setting that crossover point at 80 because we don't want our speakers to play below 80. In general, the steeper you can make that crossover slope, the better. The M series amps have 12dB/octave crossovers while the MHD has a selectable 12 or 24dB/octave crossover. This means that you can be much more precise with the crossover on the MHD than you can on the M.

The signal to noise ratio tells you how much of what you hear is the actual music vs. background noise. There is electric/electronic noise everywhere in the world. It is comeing from your head unit, the engine, anything that uses electricity in same way has noise associated with it. Most of it we can't hear. But when we do hear it, we notice it and it is annoying. Think about the humming of a ceiling fan. Have you ever heard a light make a buzzing sound when it is turned on. That is all noise.

With an amplifier, when you are boosting the audio signal, you are also boosting the sound of the noise and some of that noise is coming from the amplifier itself. The Signal to noise ratio or S/N ratio tells you how much of what you hear is the actual audio signal and how much of what you hear is noise. The higher the S/N number, the better it will sound.

Virtually all amplifiers accept low level inputs, usually up to 4 volts. This is the signal that leaves your head unit or controller via RCA cables and goes into your amplifier. The other type of input is a high level, or speaker level input. This is the signal that is boosted by the internal amplifier of your radio or head unit. Instead of using the RCA pre-outs on the head unit, you would use the harness for the speaker wires. The high level inputs are usually at voltages above 4V and can be as high as 10. Generally speaking the higher the input voltage, the better the music will sound. If you can use high level inputs, 8 volts from a high level input will sound better than 4 volts from a low level inputs.

The M series has a much larger product offering than the MHD series. If you are installing a large system, the larger product line allows for more flexibility with your amplifier setup. The M series has everything from a monoblock to an 8 channel, with the exception of a 7 channel amp because those don't really exist in the mobile audio world. The MHD line has only three options, a 4 channel, a 5 channel, and a monoblock.

When it comes to speakers, if you are running all of your speakers at 4 ohms, the MHD amplifiers are night and day better than the M-series. The constant power circuit that delivers the full rated power across all impedances will sound significantly better than the M series. The signal processing is much better, the S/N ratio is much better, the MHD is just a better amplifier. However, they are also significantly more expensive. The key is to not look at the amplifer as a 600 watt, 4 channel amplifier, but instead as a 1200 watt amplifier. The same is true with with the 900 watt 5 channel. If you think of it as a 1300 watt, 5-channel amplifier, it doesn't seem as expensive or overpriced. If your speakers can handle the power of MHD and you have the budget for the MHD amplifiers, the MHD amps are probably the best sounding marine amplifiers in the world, if you are powering your speakers at 4 ohms.

If you are powering marine subwoofers, I like the M series better, unless you are powering a single M7 or a different brand's, high powered marine subwoofer. If you have a single 4 ohms subwoofer that can handle 300 watts, buying a 750 watt amplifier doesn't make sense. If you are powering a pair of 300 watt marine subwoofers, you can wire them in parallel to a 2 ohm load, and the M600/1v2 is going to be a perfect match and cost significantly less.