Tutorial 6 - Wiring, Capacitors, Door/Sub Box Measurements

I have had several questions from people and noticed several threads asking about box dimensions, factory door speaker location, mounting depths, etc. I also get a lot of queries about power and amp connection so I put together a range of topics in this 6th tutorial.

Here are some answers to some very common questions.


EF/EL Door Measurements

Hole size: 126mm
Width around hole: 160mm ( recessed section around hole)

Allowing 2-3mm gap to clear the window the following is a guide to the max mounting depth allowable
Depth @ top of hole 60mm
Depth @ centre of hole 70mm
Depth @ bottom of hole 72mm

If you use a 12mm MDF spacer ring then the maximum depth in hole centre is 82mm. Below is a 4" midrange mounted on a 6mm MDF spacer with rubber under one side to give it an angle

That means the max depth speaker you could fit into the door and still have the window wind up and the door skin fit on would be 92mm deep in the middle.

An additional spacer ring can be made up to 12mm thick and this gives an additional 12mm mounting depth without fouling on the standard door skin and allow the mounting of 6.5� speakers. Alternatively you can use 9mm MDF and use some rubber under the front edge to give it some backwards angle (which is desirable) Even a few degrees can make a big difference.


SUB BOX Dimensions
I have had heaps of questions regarding PYROLY sub box size etc… Lisa’s sub box shown below is approximately:
760mm W x 360mm H x 460mm D @ bottom x 230mm S @ top
Total internal volume is around 2.33 c/f minus the displacement of the subs assuming you mount them with magnets inwards. If you do that then the displacement of the subvsreduces the internal volume by approx .1c/f per 12" sub



In order for this box design to fit you need to cut a small piece of steel frame under the parcel shelf. A simple handheld hacksaw does the job in a few minutes then use a hammer to fold it back on itself. Alternatively a 4� angle grinder makes very short work of it and because it is so small it does not affect the structural integrity of the car. If you don’t wish to cut anything then you limit your width by another 50mm roughly per side.

This size sub box would typically suit a single 15" sealed or 2 x 12"s but you should refer to your specific subs for recommended enclosure sizes.

Understanding Power, Earth & Remote Wiring etc)
This goes into a bit of depth but should clear things up for some people.
Basically class a/b amps (most amps that are not monoblocks are class a/b) are at best 60% efficient, which means 60% of the power they use is output to your speakers while the remaining 40% is wasted (mainly as heat). Class D monoblocks are up to 85%.
So imagine you have a class A/B amp running your splits @ 2 x 120rms (240rms total) and a sub amp running your 2 x 12"s producing 600rms.
Assuming a normal operating voltage of 13.8 volts you need to work backwards
240rms / 60% = 400watts is used to produce the 240rms output to your speakers
400watts / 13.8v = 28.98amps
Your front amp will be drawing around 30amps @ 13.8v to produce 240rms

Now sub amp is the same but with higher efficiency
600rms / 85% = 705watts is used to produce the 600rms output to your speakers
705watts / 13.8v = 51.15amps

Your sub amp will be drawing around 50amps @ 13.8v to produce 600rms
So in total your system running at maximum power (which it will do very infrequently) is drawing 80amps of current.
Now in reality your amps will only draw their maximum current for very short bursts with peaks in the music. Car audio fuses are quite “slow blow� so in this system a 60amp fuse would hold up fine and is what I would use. A 40amp would not be enough and an 80 or 100amp would not offer any protection until things were already over their maximum which is of no use.
The chart below can be used as a gauge for power and earth cable sizes. It is important to remember that your amplifier uses a circuit from positive to negative so you must have the same size (or larger) earth as your power cable. It’s no good having 0 gauge to all amps and a piece of 8 gauge for earth. Your system will be limited to the amount of current the 8 gauge can flow.

This table is a handy guide, while my rule of thumb is 4 gauge as a minimum as it’s not much more expensive or harder to run than 8 gauge and may not need to upgraded later on. Any more than 2 amps and I use 0 gauge. If the voltage at your amplifiers drops your amps will go into clipping and destroy your speaker’s and/or the amp. Spend a little extra and get the larger cable and never have to worry about insufficient voltage supply.

Grounding (connecting Earth)
The best place to ground your amplifier to is a point that is attached to the chassis of the car and is as close to the amplifier as possible. The ground wire should be at least as large as the power wire and the point to which you make your ground connection should be an unpainted piece of bare metal. In most cases you will need to get a wire brush or piece of sandpaper and rub back the paint before connection.
Some cars (Audi, Porsche) have galvanized bodies, and in these cars, you must find one of the manufacturers' grounding points or else some noise can result.

Below is a picture of 0 gauge cable connected to a good earth. Once you have sanded or rubbed the surface back and connected the earth give it a spray with an anti rust agent (as shown) to avoid that area oxodizing or developing rust.

For multiple amplifier systems you should connect all your amps back to a common ground point and this is best done through a distribution block.

Capacitors Explained
A Capacitor (stiffening capacitor) as a generic term, refers to a large capacitor (several thousand microfarads or greater) placed in parallel with an amplifier. The purpose of doing so is to provide a sort of reserve power source from which the amplifier can rapidly draw power when it needs it (such as during a deep bass note). The electrical theory is that when the amplifier attempts to draw a large amount of current, not only will the battery be relatively slow to respond, but the voltage at the amplifier will be a little lower than the voltage at the battery itself (this is called line drop). A capacitor at the amplifier which is charged to the battery voltage will try to stabilize the voltage level at the amplifier, dumping current into the amplifier.


Capacitors - Should I use one
A lot of mixed opinion here so I'll leave my personal feelings aside and give some simple information.
If you have a problem with dimming headlights when you have your music turned up and the bass starts to hit and the engine is running and you don't want to upgrade your alternator, or if the transient response of your amplifier is unacceptable to you, a stiffening capacitor could help you out. The commonly accepted "formula" for determining the proper size capacitor to use is 1F/kW (one farad per kilowatt). For example, a system running at 300W would need a 0.3F (or 300,000uF) capacitor.

Connecting a Capacitor
To connect one you initially use a light bulb or resistor and connect the negative to a battery and the positive to the battery through the light bulb or ½ or 1 watt resistor. The point of the light/resistor is to ensure that current doesn’t rush out of your battery too quickly. Most caps will take about 30seconds to charge. Once this is done caps can hold there charge for YEARS so be careful not to touch the terminals across anything that conducts electricity or you will find out all about the big bang theory.
Once charged you can disconnect it and connect in parallel with the amp. This means the positive from battery to the cap positive (same for negative) then from the cap positive and negative to the amp. If you have more than one amp you can go from the cap positive and negative to a distribution block and then to multiple amps.

What about adding a second or larger battery?
Adding a second battery is great if you want to listen to your stereo with the car turned off, or you have a large constant drain on it (like a large stereo system). As far as upgrading your current battery to a larger model, you may find that this will help because batteries like an Optima / Oddessy / Stinger etc… offer a larger number of cold cranking amps.

If you are always listening to your stereo at a fairly high volume and use the A/C and a few other misc. accessories then a second battery is definitely worth considering. Of course depending on how large your alternator is you may also need to examine wether it needs to be increased in the long run but I have found the Falcon alternators to be quite sufficient in systems I have installed and in my case I am running 4 amps.

Trigger Wire (Remote turn on)
The remote turn-on lead that most head units will not provide very much current (usually 250-300mA), so there is a limit to the number of components you can activate with it. Generally, it is safe to hook up two devices to the lead without having to worry about problems. However, if you'll be activating more components, then you should probably use a relay. Any relay (say a 5amp one) from autobarn will do the job and they are simple to connect. They require
1 x earth connection
1 x constant power (good sizes power connection like 8 - 12gauge)
1 x switching input (this is your headunit trigger wire)
1 x Switched output.

When it gets a signal from your original trigger wire it will connect the permanent power to the switched power output which can handle switch much larger loads (as big as the relay or input power source can handle)

Last edited by pyroay on Fri Apr 27, 2007 10:41 pm, edited 5 times in total.

Very nice Aaron!
Very handy too, especially the Cap and Battery section.
Awesome work.
One question, am I misunderstanding here, or is it a typo?

well done mate! Pro all the way!!!

impressive mate ... [img]distribution%20block[/img] pic didn't work though...

Thanks for pointing those couple of things out, errors have been corrected.
It takes ages to put one of these together and I still miss things.

By the time i write it, check it several times and then link pictures + go out and take actual pictures from my car (or others cars) it takes around 3-4hrs to produce a finished one.

Hope it looks right now.

Yeah, it's a fair effort to put one together!
Didn't mean to sound picky.
I'm sure everyone appreciates the effort.
Having someone of your calibre explain some of this 'sound stuff' has definately widened my understanding of it.
I bet I speak on everyones behalf when I say thanks!

Jared.

Cool i didn't mean to come accross that way either.

I'm LOL, it's just funny for me to go through it so many times and still miss seemingly obvious things...

yeah, thanks veery much for that.

Looks like i'll have to get a capacitor for my system. hopefully stiifen the subs abit and givr a clearer sound.

Ok i'll share my personal thoughts on Capacitors.

I believe they can assist in improving transient response (handy in SQ systems) and are of a limitedvalue in reducing line drop (voltage drop).

I think in most cases a boot mounted battery is a better proposition however they are often more expensive, larger and more difficult to install and connect so they are not always practical.

They also dont look as impressive.

Great work aaron ... i knew eventually these would come you sub box looks s**t hot to man nice goin

just a quick question, was wondering on wire size from amp to sub on a 15inch sub. havnt been able to get a straight answer from shop's. cheers guys!

oh its a monoblock amp 1100 watt!

Amp to sub id use 12gauge or 10gauge if your keen. doesn;t need to be any larger unless your an SPL competitor with hige amps then 8 gauge is oothe way to go...

Amp to sub id use 12gauge or 10gauge if your keen. doesn;t need to be any larger unless your an SPL competitor with hige amps then 8 gauge is oothe way to go...

cheers, thanks mate. at the moment i was only using speaker wire, how much would it be holding it back? becuase it seems to go great! thanks for your help!