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

Hand Made Skin Drum

Tube enclosures are very efficient enclosures for bass creation and subwoofer applications. If you think about it, tribes used to use hollow logs and tube shapes to create their ancient drums. To this day, with advanced technology, acoustic modeling, and ‘if you can dream it , we can build it’ machining, we still have circular drums. Think about it, have you ever seen a square bass drum before ?

Now there are two reasons that a tube is very efficient at creating and amplifying bass frequencies:

1. A tube matched to the size of the drum surface (the drum skin) or in our case to the size of the subwoofer has it’s entire surface area in contact with the subwoofer surface.

So, any motion in the subwoofer, moves the entire air column in the tube, this is unlike a square shaped enclosure that allows for stagnant air to be trapped or turbulent in the corners of the subwoofer enclosure, a phenomenon that can create enclosure noise and bass distortion.

2. In a tube shaped enclosure, no two surfaces are parallel to each other except for the tube end plates (one of which has the subwoofer mounted to it). This prevents any standing waves of any frequency to exist inside the enclosure and is the primary reason a tube system requires less power (is more efficient) to create more base.If the enclosure were to resonate internally due to the parallel faces creating standing waves, then the subwoofer would have to overcome the pressure of these standing waves to create it’s own pressure waves in accordance with the music. This energy is wasted by the subwoofer and requires a higher power amplifier to create the same level of loudness.

If you’ve seen certain subwoofer boxes that are fairly deep (deep enough to create audible internal standing waves) you’ll find the front panel of the subwoofer box is mounted at an angle which prevents the same internal resonance phenomenon from occurring in a typical box enclosure.

Now here’s a product that takes advantage of this design:

Bazooka BTA 8100

The bazooka BTA8100 stands for Bass Tube with Amplifier 100watts. The BTA8100 uses a dual voice coil 8″ subwoofer powered by a  ’small’ two channel 100 watt RMS amplifier (also verified by the 7 amp fuse on a 14 volt system), that can peak at 200 watts momentarily to create a significant amount of bass from a very small enclosure.

The way the Bazooka bass tube creates such a big boom from a small enclosure is that it:

1. Uses a bass tube which is very power efficient
2. Focuses all of the energy on a very narrow frequency range of 35 to 85hz leaving out midbass and subsonic frequencies and focusing all the power on where most music ‘booms’ which is a pretty efficient way to add bass that most people will appreciate.
3. Uses a ported enclosure to further amplify the system response around the bass port tuned resonant frequency by 3+ db as per a typical ported system.
4. Brings the bass port all the way to the front to exit in the same plane as the subwoofer, usually a bassport tuned for a 1/4 wavelength and exiting in the same plane as the original speaker driver helps improve the subwoofer’s response in a vented or ported system to bring it closer to the response of a sealed system, while having the 3db gain advantage of the ported design.

Now we know the bass tube is 18.5″ long and so the length of the enclosure is double that (as the wave travels from the subwoofer surface, to the rear faceplate and backfowards to the port exit) of 37″. In free air, with no internal padding, at 30*C this 37″ corresponds to 1/4 of a 148″ wavelength which corresponds to a tuned frequency of 93hz which is very close to the highest cross over frequency of the 85hz low pass filter.

So the result of all of this efficiency, design and bass reflex tube tuning is an impressive 102dB sensitivity at 1W @ 1 meter.

Being a 100W RMS system, this system is best used in an open cabin such as in a hatchback or behind the rear seats of a Jeep. This allows the bass to travel easier into the compartment than it would if the system was installed in a sealed trunk in a sedan.

One thing Bazooka recommends to improve further improve how hard the system hits is to ‘corner load’ the subwoofer by installing the subwoofer (and the bass port) in a way where it is playing into the corner of the rear hatch or trunk area about 3″ to 5″ from the walls. Typically a subwoofer is rated (to 102dB in our example) by placing it in an open space and measuring it’s sound pressure, using 1 watt of power, with the microphone or pressure meter 1 meter away. This arrangement allows the subwoofer to play bass in all directions in 360 degrees of space.

Corner loading reflects half to 3 quarters of the sound wave to play in the opposite direction giving a bass boost.

By corner loading a subwoofer (and placing it in the corner), half of the audio is played out into space while the other half is reflected off of the wall (the corner) and played into the SAME space. What this does is that it allows both halves of the sound (the primary wave and the reflected wave) to stack up on top of each other giving between 6 and 12dB of additional bass gain using corner loading and room gain.

So what is the best application for this bass tube?

Because of how well the Bazooka tube is designed you will typically find TWO types of reviews on the internet for it:

1. WE HATE IT!

   The reason some people hate the bazooka tube is that it does not perform like a larger 15″ subwoofer in a 3 cubic foot enclosure. The bazooka tube does not play subsonic bass (typical of some genres of music) nor does it play good midbass in the 125 to 500hz region. So people that buy the Bazooka tube with the expectation that it is the same as having a full custom subwoofer box in their trunk, that they will be able to rattle the doors off of their car, or that they will be able to rumble the guy next to them at the stop light with 16 hz subsonic frequencies will be dissapointed.

2. WE LOVE IT!

   The people that love the Bazooka tube love that it is portable and easy to install and remove. Think of a Jeep Wrangler owner that loves to take his car apart, take the top off, remove the doors, take it in the mud, have a bass tube in the back that you can install and uninstall in no time. Have a plastic bass tube that isn’t carpeted that you can get wet or muddied (Bazooka even sells marine bass tubes that you really don’t have to worry about with water , rain and the elements).

   Or maybe you have a sound system that is completely flat on the low end and has no bass what so ever ? You want a quick , cheap, and very effective solution at adding some boom to your ride. Enough boom to be noticeable, to dramatically increase your enjoyment and your listening experience and something that your passangers will defiantely notice. At the same time you’re not looking to shatter glass, break any SPL competition records, or have your bass heard anywhere OUTSIDE of your cabin.If you’re this kind of person, you will love the bass tube, as it compliments your factory speaker arrangement (which typically does well for midbass, midrange and treble if you already have high quality speakers in there), and it will fill out the missing POWER in your music.

Here’s a great video of a larger application of a tube subwoofer

It’s really important to understand how a product works, and what it’s designed for, to get the most out of it and to know what to expect with it …

Note: Click here to get your Bazooka Subwoofer Tube and get $70.00 off!

Technorati Tags: 4ohm, Car Subs, dB, DVC, Vented

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The thing is, a lot of first time diy’ers build their first DIY Subwoofer box using a single undivided enclosure with dual subwoofers and a 2 channel amplifier. Compared to having no bass at all, then a single enclosure with dual subs will sound like the greatest addition to the car’s sound system since sliced bread. I know I for sure loved mine, just as much as my friend Julian loved his dual 10s in his CRX hatch, and as much as my friend Dan love his dual 12s in the trunk of his Oldsmobile.

If you think about a typical single sealed box subwoofer install, then the amount of air trapped inside the enclosed and sealed box is fixed. The volume of the box is not fixed though because one of the box walls has a variable excursion part to it which we call a subwoofer. As the subwoofer moves in and out with reference to the box wall when the music moves the speaker cone, the volume of the box increases (when the sub moves out) and decreases (when the sub moves in).

As we know from physics density = mass / volume. So a fixed mass of air is manipulated inside a variable volume box and what this does is create a variable density area behind the speaker cone inside the box. As the sub moves outwards, the desnity and pressure of air inside the box drops. This low pressure behind the speaker (with regular pressure outside of the box) creates a suction on the speaker which tries to dampen it and return it back to its resting position. The opposite is true when the speaker dips into the boxes volume, as the pressure increases inside the box trying to push it back out.

This ‘overdamped’ nature of sealed boxes is what helps them create TIGHT bass with minimal distortion even when the speaker is slightly overpowered as the physics of the box dampen the possible distortions of the speaker motion and accelerate its return to its resting position to prepare it for the next bass hit, even when faced with a series of fast paced rolling bass lines.

Now although sealed boxes are great for clean tight bass, they don’t always hit as hard or as low as we’d like them to, especially when compared to a tuned vented box or a band pass system. One solution to helping a sealed box reach max excursion and create more bass is to neutralize the high and low pressure waves existing inside the sealed box. This gets even worse on a typical ‘newbie’ DIY box with TWO subwoofers in a single enclosure. As both speakers move inwards and outwards in phase, the pressure waves inside the box are exaggerated and neither speaker is allowed to go to maximum excursion.

Isobaric literally means of equal (iso) pressure (baric). And the way we achieve this ‘isobaric’ region is to operate two identically matched subwoofers within a tight enclosure. The two cones of the speakers are arranged so that they can move OPPOSITE each other in a way that the total volume of the box is fixed. Since the amount of air trapped in the box is fixed, and the total volume of the box stays fixed, then the pressure inside the box stays fixed (iso-baric) and the pressure inside the box is always equal to the pressure outside of the box. This way both subwoofers are surrounded on both faces by equal pressure and are no longer over damped by the box physics but rather damped by their own physical damping (electrically by the voice coil and mechanically by the design of the basket and surround material).

There are many different subwoofer arrangements that will allow you to reach an isobaric condition in your sub enclosures, but the simplest are by altering:

1- The direction of subwoofer mounting (flush into the box, or protruding with the basket ouside of the box)
2- The polarity of the wiring of one of the two subwoofers so that it moves in opposite direction to the other subwoofer

So what is the advantage of using an isobaric (or push pull as some may call it) subwoofer network ?

Well one advantage is that as stated before the subwoofer is no longer overly damped, which means more excursion and louder bass.
The second advantage is that now that the subwoofer is neutrally damped, it can actually increase the sound quality of the bass in your system so long as the subwoofer is not overdriven or overpowered.
The third and most interesting advantage of an isboaric arrangement is that you are now driving more power into the same box which combines the effects of both subwoofers resulting in a ’super sub’. This means that if you have a tight box space, that you can build a smaller box to give you the same operating characteristic as you need for a single box.

For example you purchase a 15″ subwoofer that requires a minimum 3.5 cubic feet of enclosure space. However, in the spare tire well in your car you measure out an available 2.0 cubic feet of space which is not enough for this sub. Setting up an isobaric setup with two identical 15″ woofers lower your space requirements by a factor of 2 down to only 1.75 cubic feet. What this also means is that any extra space you get over that 1.75 cubic feet will show up as bass extension as your isobaric subwoofer network will be able to hit harder and reach a LOWER corner frequency, as low as half an octave lower if you use an isobaric network in a 3.5 cubic foot box as recommended.

Here is an example of isobaric loading using two 15 inch subwoofers in a tuned vented box in the trunk of a Golf GTI. This box has been tested and proven to hit as low as 16hz!

So how do we use this information ?

1- If you (like i did) have a regular box equipped with two undivided subwoofers sharing the same space, then reversing the polarity of one of those subs will result in a stronger bass hit, and a lower overall frequency response. Have a box loaded with two 15 inch subwoofers ? Simply reverse the polarity on one of them and gain more power around the 16hz region simulating a big 18″. Have two 8 inch subs but want more of a 12″ sound ? Again do the same.

2- Sometimes you are limited for space, such as on a work truck. Work trucks need pretty much every last inch of space to haul tools and carry people around. So some people resort to installing midbass drivers (such as 6.5 inch “subs”) in their door panels. Mid bass drivers are great for 250 to 600hz mid-bass but they don’t really reach that far down into the bass frequencies on the frequency spectrum, especially if they are NOT installed in a ported enclosure with the right resonant frequency. One smart way to increase the frequency dynamic range of your truck would be to use a 6.5″ spacer ring to isobarically load your door mounted midbass drivers with two identical drivers mounted in plain sight. This will not only double your usage of your tiny in-door enclosure (as explained earlier about cutting your space requirements in half), but it will also allow you to gain more true bass from your midbass drivers and increase the overall feel that you get from such a small and minimally invasive install. (To pull this off correctly the speakers have to be wired out of phase (opposite polarity) so that they move in the same direction with the distance between them constant to maintain the isobaric region between them).

Note: Click here to start building your own isobaric network using undivided isobaric sub enclosures for two 12″ drivers and save $20 off retail.

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Technorati Tags: Car Subs, sub enclosures, Subwoofer design

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