Car Audio 101

[purecarsound]

Car Audio 101.

The first thing to do when planning an ICE install for your vehicle is to consider a few of the following questions.

Is styling as important as sound?
Am I happy to give up my boot/seats etc or do I want something more discreet?
Do I prefer a sound quality system or would I rather have something loud and proud?
What music do I normally listen to?
What is my budget?
What usage will the system get, and what sort of usage will that be?

With these questions answered you should be able to get at least some of the way down the road of getting a good system designed. Let us now take a look in a bit more detail at getting the system planned.

The FIRST thing to consider is wiring. While it is essential to have enough wire in the vehicle for the system, it is also often seen as fashion led to oversize the wiring in the vehicle. While this does allow for future upgrades, the cost of wire versus the requirements you have for it have to be considered – i.e. a small amplifier (lets say 4 x 100 w RMS) does not need to have 1/0 AWG feeding it – 4AWG or even 8AWG would be ample in this instance.

Moving on from there I would briefly like to pass my thoughts on “Power Caps”. From an electrical point of view, these products put simply do not offer any benefit to the system. In fact the only place they are likely to work is on a badly designed (read cheap) amplifier, in which case the extra money spent getting the better amplifier in the first place is a far more sensible place to invest in, unless looking pretty is the main factor, in which case “fill yer boots” so to speak.

If power supply strengthening is required to give you some idea most Caps hold 1 farad. A 063 model battery (oem on something like an Escort or 306) on the other hand holds approx 10,000 farads of power. The common comment is that a cap will stop “lights dimming”. It won’t. It will stop FLICKERING of the lights as a cap cannot sustain the flow to stop this. (If you want to stop lights dimming it is cheaper and more effective to spend the money on a pair of 0.1 farad caps from maplin and wire them in line with the positive feed to your headlamp bulbs!)

So where to start?

Let’s start at the source – the head unit. When choosing a head unit there are a few things to take into account. First one is the amount of pre amplifier outputs that the unit has. In addition whether the unit has onboard crossovers needs to be considered, as does a sub level control. As prices rise expect more features such as time correction and onboard EQ.

What does the above mean?

Pre amp outputs use “RCA” plugs (or Phono’s – but invented by RCA - Recording
Company of America in the 1950’s as a better solution to the 5 pin mini-din). These output what is normally termed “line level”. Historically this was 1 volt RMS (root-mean-square – or quite literally the root of the mean value of the square wave output – square wave being what an AC amplifier does when it is “clipped” or distorted). Nowadays however the trend, for better or worse has been for 4v to become the key. The reason given usually is that this is because of sound quality (specifically the lowering of the signal to noise ratio) however this is not really the case if the op-amps in the gain stage of the amplifier you are using are of a decent quality.

Anyway 4v preouts usually are a good thing if a) the amplifier can take them, and they have a DC-DC converter supplying them. Higher end Alpine and Pioneer decks usually have these, and it allows the 12v car supply to be boosted to about 40 or more V DC to feed the preouts power supply.

Time alignment/correction is an incredibly powerful tool for the serious sound quality focused listener. When you listen at home you would place yourself in the middle of the 2 speakers for an accurate stereo image and soundstage, giving a realistic impression of the performers. In a car this is not possible, as you sit over to one side, thus killing the image and the impression of space a good stereo system can provide. Time alignment, when set up correctly, delays the sound from the speakers nearest to you which allows the sound to hit you at the same time, as it would if you were sat in the middle. The one drawback with time alignment is that the sound will be atrocious for the person in the passenger seat.

Equalization is a handy feature, but has the potential of causing more problems than it solves. Ideally you should set the system up using what is called an RTA (real time analyzer). What this does is when a special CD is played (which carry’s all frequencies identically) it shows you how the car responds so you can adjust it accordingly. The main key of EQ’ing a vehicle is always CUT, never BOOST.

The other thing on the head unit front is crossovers for the Subwoofer, or all channels. Certainly from the sub side of things this is a feature that is very handy, as it allows you to adjust the relative level of the subwoofer output from the head unit – which means you have a larger range of amplifiers to choose from and also simplifying wiring.

The crossover allows the correct frequencies to be sent to the right speakers. What I like about having this control on the head unit over and above on the amplifier or external active crossover (passive crossovers are different and are dealt with further down the post) is that you can normally easily set up a couple of presets. This allows you to have one setup for sensible listening, and a second for loud “cruise” demonstrations and maybe even a further for competition usage, which allows the system to be used with far less fear of damage to the speakers. It also allows you to make adjustments in “real time” without running to the boot then getting back in.

Moving onto amplifiers, there is more to worry about than the raw power ratings of the amplifier. While the RMS ratings give a good idea of output capability it is at present quite hard to judge the circumstances under which these ratings were achieved, and therefore a certain amount of common sense should be applied. I.e. a £99 “eBay special” purporting to output 2000 watts RMS power is likely to be a tad suspect.

In addition to power ratings, other specifications that should be taken into account are THD figures, slew rates and damping factors. Also it is worth considering whether the amplifier will perform in your system – i.e. things like input sensitivities match your head units and the amp has the correct crossovers and bass eq if applicable to perform the required functions. Efficiency is often discussed in regards to amplifiers and is one of the less understood aspects of the amp. First of all you have to consider that in a car, and looked at in a basic level an amplifier is effectively a DC (direct current) to AC (alternating current) converter. And the efficiency of the amplifier is a result of the effectiveness of the power supply as well as its subsequent conversion to AC through the output side of the amplifier. Let us now look at the electrical side behind this.

Hoffman’s iron law says that Power (Watts) = Voltage (V) x Current (I). It also states that Voltage (V)/ Current (I) = Impedance ®. Now this may seem very pointless and not at all relevant - but bear with me for a moment. Consider the fact that Voltage is over 5 times more efficient to produce than Current. This is why the national grid produces power at 11,000 volts or more. Efficiency.

OK - so what the hell does this have to do with car audio - well quite a lot as it goes?

At 4 ohms if your amp produces 50 volts and 12.5 amps you get 625 watts.
At 2 ohms if your amp produces 25 volts and 25 amps you get 625 watts.
At 1 ohm if your amp produces 12.5 volts and 50 amps you get 625 watts.

As impedance drops efficiency also drops. At 4 ohms and a class a/b amp expect 65% efficiency, 50% at 2 ohms and 40% 1 ohm.

This means the amplifier turns that percentage of battery DC into Speaker AC. So looking at the 2 ohm load - the amplifier needs 1250 watts input from the power source. Using the iron law above 1250 watts @ 11.5 volts (cars float at 14 volts but as load is increased voltage drops considerably - i.e. when the bass hits.) equals 110 amperes current. Which is more than the alternator provides so therefore it is pulling it from the battery.

Now that is a relatively small amplifier, and you can see it can get quite juicy. In addition be aware that the above does not apply to class D amps although I will start another thread for that another day.

Obviously sound quality is something that it is very difficult to quantify to people. However, like most things in life there is a simpler adage – you get what you pay for. Be it features, styling, raw power or whatever, you pay for it somewhere along the line. Take all these factors into account and you should make an informed guess.

Written by Dave Wilkins. (aka Troublemaker on most forums)