Class D Subwoofer amplifiers, effectionately known as “Mono Block D’s” are very power efficient amplifiers.

Class D Amplifiers have an efficiency of around 70%, which means that a 2000 watt rated Class D will typically deliver 1400 watts to the subwoofer while wasting about 600 watts in heat. Still, this Class amplifier is the highest efficiency amplifier design available on the market.

The way these amplifiers achieve such a high efficiency is by first creating a high power full voltage square wave around the frequency of our input signal. A square wave is really easy to generate and control because it only requires a synchronous switching of the amplifier’s power transistors on and off to create that wave.

That high power square wave is then filtered by the low powered audio input signal (using low pass filtering, and feed back control loops) to shape the high power wave going to the subwoofers to match the input audio signal now both in its primary frequency (which was done when we generated the square wave), and in harmonics (which is taken care of by the filtering).

This is very similar to the performance of subtractive synthesizer that generates an square wave at your desired frequency , and then you subtract out the higher harmonics to create the sound shape that you want to play.

Which brings me to my main next point …

Amplifier stability :

Amplifier stability is the study of how hard you can push your amplifier and still have it work properly as a power amplifier (with high gain, low distortion, and a wide frequency range response).

In class D amplifiers, there are two things that affect amplifier stability:

1. Because the amplifier uses a feedback loop to filter the high power full voltage output signal, then there is always possibility for this feedback loop to get inverted and become a positive (rather than a negative) feedback loop.What this means is that rather than filtering higher harmonics, the amplifier may start to internally add its own noise to the system, and this will happen at some frequency that the amplifier internally resonates at.These types of phenomenon usually happen on power transitions, when the amplifier is switched on, when the power is interrupted momentarily or surged …etc As the amplifier charges its internal capacitors and electronics and approaches it’s stable steady operating state it goes through a transient period where it may become unstable.Of course internal compensation of these phenomenon using damping capacitors and control circuits is important, but sometimes external factors such as the load impedance of the subwoofer also comes into play into the modeling of the amplifier.
2. Thermal stability.As we said about 25 to 30% of the amp rated power is wasted in heat…. heat builds up inside the amplifier, the temperature of the internal components increases, the internal resistance of the amplifier blocks increases with temperature, the overall gain of the internal amplifier block drops as it’s resistances rise, which drives more heat, more resistance, less gain … until either the amplifier burns out, or the thermal overlaod protection circuits inside the power transistors and the amplifier power supply shut off and you are ‘embarrassed’ in front of your peers because your amp has just cut out.

Stability rating :

So what we typically see is an amplifier rating … “2 ohm stable” or “0.5 ohm stable”.

This means that the minimum subwoofer impedance connected to the amplifier should not be lower than 1 ohm (for example) for a 1 ohm stable amplifier.
This also the impedance that allows us to extract the most power out of our amplifier.

As I’ve explained before in our previous article on subwoofer capacitor ratings, the maximum RMS power you can extract from a certain amplifier will be as follows:

Power = V * V / R

Where V = the system’s rated voltage (typically 14 volts)
and R is the resistive impedance of the load or the subwoofer (typically 4 ohms for a single voice coil car subwoofer).

This makes the typical RMS power extracted from a single channel 14 volt amplifier into a 4 ohm load 49 Watts.

At the same time connecting another subwoofer in parallel with our original 4 ohm subwoofer will give us a total load resistance of 2 ohms.

With this new load value we can now deliver 98 Watts from the same 14 volt power source, so long as the amplifier is at least 2 ohms stable!

So how do we apply this knowledge practically ?

You can see from the example before that having an amplifier with the lowest possible stability rating gives you the ability and flexibility to add more subwoofers to your car’s audio system and extract more power from the same amplifier and the same power supply.

One great example of such an amplifier is the Kicker SX1250.1 which is a great and flexible amplifier. The Kicker SX1250.1 delivers up to 312 watts RMS to a 4 ohm load.
Working our same power equation backwards we find that it takes 35 volts to be able to deliver 312 watts RMS to a 4 ohm load. This tells us that our Kicker amplifier has a built in DC to DC step up voltage converter that will convert the incoming 14 volt power supply into a 35 volt internal power supply that the amplifier can then deliver to the subwoofers.

Furthermore. the Kicker is stable down to 1 ohm as follows:

So here’s the typical scenario in which such an amplifier showcases it’s flexibility.

Stage 1:

Let’s say you start with a basic install, coupling this amplifier with a 4 ohm single voice coil Mobile Authority MA120QS. The MA120QS is a 12″ subwoofer from Mobile Authority that can handle 300 watts RMS and 700 watts at peak and is a perfect match for our amplifier as a basic install.

This setup requires something like a 0.5 Farad capacitor, we draw a steady 21 amps of power (probably requiring a 35A fuse), and need 6 gauge wiring between the battery and the amplifier and 10 gauge wiring between the amp and the speakers.

The reason we need different gauge wiring here is that the amp is supplied with 300 watts @ 14 volts which is 21 amps. However, the amp delivers power at 35 volts to the speaker so the 300 watts get delivered using only 8.5 amps on the speaker side (due to the higher voltage) and so thinner wiring is allowable.

Stage 2:

After a while of bumping your 625 watt (peak) system around town and in competition, you may decide that you want to upgrade your system for more power.
A quick look around shows the Infinity 1262W car sub woofer. This subwoofer has two 4 ohm voice coils that can be wired in parallel for a total impedance of 2 ohms, at the same time the subwoofer has a power handling of 1500 watts peak (600 RMS) and is capable of taking everything that the amplifier can throw at it at a 2 ohm rating.

Typically to do this kind of upgrade (to double your power) you’d need a whole new system (subwoofers, amplifier, enclosure, wiring, capacitor …etc). However, since your amplifier is in fact 1 ohm stable (which means it is also stable at 2 ohms) you can retain the same amplifier, same enclosure and just upgrade your subwoofer, capacitor and wiring (if it isn’t already rated enough to deliver 1500watts).

Stage 3:

After a few months, you get more bass envy. You want more upgrades, you want a louder system and maybe want to go for an award at an SPL competition.

Your amplifier is 1 ohm stable and capable of delivering 1250 watts RMS or 2500 watts at peak. But you’re only using 1250 of those 2500 watts. A quick look around the shops comes up with the massive Pioneer TS W5102SPL car sub woofer.

This pioneer subwoofer is also a 12″ subwoofer, and if you had a deep enough enclosure to start with (or enough space on top of the enclosure to install it outside of the box playing inwards) would fit right in place of the MA and Infiniti subwoofers you had before.

What’s special about the pioneer is that it comes with two 2 ohm voice coils that when wired in parallel give a total impedance of 1 ohm and are capable of both extracting 2500 watts from our amplifier and withstanding the full force of that punch has the sub is rated for up to 6000 watts at peak!

Other considerations :

As I’ve explained, the amplifier will waste about 30% of it’s power on heating itself up. The kicker 1250.1 deals with this by having a fairly large amplifier area to help better diffuse the heat, as well as having a cooling fan on board, and a digital temperature display to monitor the stable operation of the amplifier.

Make sure you install the amplifier OUTSIDE of the subwoofer enclosure in an area where it sees plenty of fresh air and is able to evacuate its heat.

Make sure you have the right wiring / fusing / capacitor for your installation based on the current power level that you are running (rather than based on marketing figures, peak power numbers or out of the box ratings).

If you think that you’re possibly going to be upgrading your setup as discussed in this article, then it’s a good idea to err on the larger side for enclosure design, so that the enclosure can still be able to physically and audibly handle a larger subwoofer, requiring more space, and pushing more power.

Other cool features :

The kicker 1250.1 has other great features that make it a very flexible and professional amplifier that I want to touch on here….

Most amplifiers you’ll see come with a low pass filter to separate the bass signal from the rest of the music.
Some amplifiers may even have an adjustable low pass filter (crossover frequency) so that you can tune your subs to pick up where your midrange and midbass speakers drop off.
The kicker goes beyond that, it not only gives you an adjustable filter frequency, but it also gives you an adjustable crossover slope between a soft roll off of 6 dB/octave to a sharp wall drop of 48! db/octave)

As if that’s not enough, the Kicker also comes with an adjustable ‘rumble filter’. A rumble filter is a HIGH PASS filter that prevents imperfectly designed sub boxes from resonating and vibrating at subsonic frequencies. Sometimes (especially with a poorly designed vented enclosure) subsonic frequencies sound horrible from your bass port. A high pass filter can filter these frequencies out preventing your subwoofer from playing them and keeping your system clean.

The kicker 1250.1 has a variable frequency AND variable slope high pass rumble filter that allows you to get the most bass out of your system even in an imperfect enclosure.

That’s not all. If you’ve ready my article on DIY subwoofer equalization, then you know how important it is to EQ your bass frequencies to get linear performance out of your subwoofer system both for best sound pressure as well as for perfect sound quality. The Kicker 1250.1 comes with an adjustable bass cut/boost of -/+18 dB at a user selectable frequency. Want more punch at 40hz, then set up your frequency at 40hz and boost it up. Want more kick from the system ? move the frequency to 200 hz and boost it up there.

This amplifier is rediculous.

I’m not done yet, there’s one more ridiculous feature on here….

Do you like to listen to live recorded music ? Like bands playing live and special edition CDs and Live DJ mixes ?
Does it bug you that sometimes during live recording that the leveling for the show isn’t exactly perfect ? Sometimes the bass varies in loudness from track to track ? Sometimes the EQ of the live concert recording isn’t perfect ?

The Kicker 1250.1 comes with a built in signal process comprised of a comrpressor/expander network (also known as a compander). What this circuitry does is that it pre-processes the bass signal before amplifying it so that it is neither too loud and clipping, nor too quiet and weak. This compresses the signal when it’s too powerful, and expands it when it’s too soft giving you an always perfect and always full and rich bass sound !!!!!!!!

Note: Click here to get the feature packed bass master Kicker 1250.1, improve your sound performance, tune your system and save $600 off of MSRP!

Technorati Tags: 1 ohm, 2 ohm, 4 ohm, Car Subs, Crossover, dB, DVC, SVC, Vented

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1. Subwoofer Tube Enclosures – Explained

The objective:

One big downside to traditional PA speaker systems is that they relied on two drivers: A subwoofer, and a horn driver to cover the entire aural spectrum. The problem with this setup is its loss of fidelity for midrange frequencies which are very essencial to genres such as psychadelic trance, vocal trance and jazzy house. So the plan was to build a capable cabinet system that combined enough thump to fill up a house party littered with DJ’s and local talent, all the while being able to give a more rich audible experience than the typical 2 driver PA speaker (which is obviously designed more for filling a bigger room than filling it beautifully with perfect music).

The speaker design was simple and effective based on cheap (I was a college student at the time) but effective components:

Components:

A pair of crunch audio 15 inch subwoofers (one per cabinet), rated at 300Watts.
Four Midrange 5.25 inch speakers. Each of these would be able to handle 150Watts and so 2 were wired up per cabinet matching the 300Watt sub with a 300 Watts of midrange.
Two 3″ titanium bullet tweeter horn drivers, capable of an insane 300 watts each with a frequency response down to 3000 hz. Again one per cabinet to match the 300Watt mids and Lows.

At the time I couldn’t find a single 300 Watt midrange speaker so I used two 150 watt speakers instead of a single 300watt midrange driver.

The crossover:

Clarion CD360 pre-amp crossover

Using my knowledge from electrical engineering classes I was taking at the time, I designed a simple 3 way crossover with a slope of 12db/Octave based on the impedances of the components that I had already acquired. I threw up the design into PSpice and simulated the crossover’s performance and optimized it, when that was done, I went shopping and picked up the closest inductors and capacitors I could find to the design values to build my Xover. Nothing in the real world is as perfect as it is when it’s designed on the computer. But the result was close enough to be awesome.

One of the other constraints of building this crossover is the fact that we’re crossing over the amplified signal. This means that all the components in our crossover need to have the current handling to be able to withstand 900 Watts of power without overheating and melting. As a solution to this design requirement, you’ll find that some home power audio manufacturers try to cross over the un-amplified audio signal between the signal source and the amplifier rack using a cheaper crossover network. This approach also opens up the opportunity to using different amplifiers for different goals within the system, such as using a power efficient class D MOSFET amplifier for the bass, while using a minimal distortion Class A or Class AB JFET amplifier for the mids and the highs.

Since my system was designed to be a versatile system to be used at home and on the road for events and parties, I chose to use an amplified crossover inside the cabinet housing to simplify using the speakers with any amplifier setup.

The cabinets:

Ported MDF woofer cabinet.

The whole concept of this cabinet was the combination of high SPL with High Quality audio. With a typical 15″ subwoofer you find the setup always lacking some response, with a bigger bass sound but less definition on the faster bass hits. To avoid this stereotypical view of 15″ subwoofers I decided to make a fortified sealed box enclosure to place the subwoofers in rather than a louder vented enclosure. The box was built of 1/4″ MDF (medium density fiberboard), lined with fiberglass sound deadening to reduce enclosure noise, and sealed entirely with silicone caulking to prevent air leakage. If I recall correctly, the design parameter for minimum enclosure space was around 2.5 cuft, so I went with a larger ~3 cuft enclosure.

In order to prevent interference and distortion between the drivers for the bass, midrange and highs, the internals of the enclosure were baffled to section off each portion of the cabinet with 1/4″ MDF and also sealed with silicon caulking after running the wiring between the different internal chambers.

Testing:

Once the cabinets were finally completed, there was a nice amount of ‘theoretical’ debate between myself, my electrical engineering colleagues, and my DJ friends as to weather the final result was in fact a 300 Watt or a 900 Watt system.

After a lot of debate and back and forth conversation I reluctantly agreed to put my product to the test … If you’re familiar with ‘type testing’ procedures, all regulated industries usually ‘type test’ their products to determine maximum tolerances, maximum power handling, limits of destruction and tolerance…etc. I feared that this outing to my friend’s house would end up as a ‘type test’ for my cabinet and that my two weeks of hard work cutting / drilling and assembling MDF would end up with a blown cabinet just to prove the theoretical limits of this cabinet.

At my friend Joe B’s house we hooked up one of my ‘300′ watt speakers to his 1000Watt per channel Mackie amplifier. This amplifier was the corner stone of his own home style DJing and practice station, one that he also took to smaller style events to drive the PA system. We hooked up my speaker to the Mackie, and JoeB put one of his favorite records records on the Tech-12s and started to play… slowly and hesitantly JoeB turned up the gain on his amplifier looking for where and when the speaker would start to complain or distort…

As the knob turned farther to the right, I became more and more anxious , but the midrange and titanium tweeter drives came more and more to life and the system was just insane. The goal of having a high power and clean sounding system with liquid midrange , hard hitting bass, and crisp highs was obviously over acheived and I had apparently over engineered my subwoofer for the target. The knob kept turning right and my fears of blowing my speakers turned to elation and euphoria … Eventually Joe accelerated the rate at which he turned up the volume and cranked the system to MAX gain !!!!! 1000 Watts of Mackie power were now pumping through my ‘home made’ and ‘first ever’ made cabinet and the EQ had to be adjusted with negative corrections past 2000hz to make up for how vivid the mids and highs were… and all in all, the speakers did not complain, distort, or skip a beat.

Despite being a heavy MDF cabinet, on a hard wood floor, the sheer amount of power driving through the speakers made it start to walk across the room floor .

I later took my speaker home and realized that our little test on JoeB’s amp had finally broken in this cabinet and the system had never sounded so good.

Takeaways:

Simulation of a 3rd order crossover network

The only thing I didn’t know at the time about speakers and subwoofers was the true meaning of sensitivity in db. In order to achieve my goal of matching my subs to my mids to my highs in order to build a cabinet that could in all of its components take the abuse of 900 watts, I ended up buying the highest powered midrange speakers I could find at the time (10 years ago) with a rating of 150Watts each. These speakers although had good power rating, had significantly lower sensitivity than my subwoofer and my tweeters.

The result of this mismatch is that when using the cabinet at a lower power level (such as when we had friends over for a DVD night) that the midrange on the speakers – which is where the dialog is in the music spectrum – had a little less gain than the rest of the system. The bass, effects and rumble of the subwoofers were great, the highs were clear and crisp, but unless you were driving the system hard at a house party, at lower power levels the mids were a bit flat.

If you are working with a low power source such as a stock head unit or are going to be using your system at lower power levels often, try to choose speakers and subwoofers with a higher senstivity rating. Speakers with a higher sensitivty rating are able to produce higher sound pressure levels (measured in dB at a standardized distance of 1 meter away from a speaker driven by 1 watt of power) from the same amount of power. That is a speaker with 92db senstivity is about 11 times louder than an 88db speaker running at the same power level.

So if you’re going to power your speakers from a low power source, use higher sensitivity speakers. And if you’re building a balanced cabinet or a balanced system, choose speakers with close or the same sensitivity so that you don’t have to re-equalize your system every time you crank the volume knob.

Note: Interested in a pre-amp crossover for your system, check out this clarion MCD-360 two way / three way crossover, save 127 dollars off of retail and build a subwoofer system to enjoy clean undistorted bass.

Technorati Tags: 4ohm, Crossover, dB, SVC

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