The Pro's Guide To Selecting A Car Audio Head Unit
Ideally, your head unit would be used to provide a signal to your amplifiers that is line level and you would not use its internal amplifiers (if any). They usually do not have the power and strength to drive speakers both loudly and cleanly. The line level signal is cleaner than the speaker level outputs on the head unit because it is does not go through the internal amplifiers in the head unit. That being said there are situations where you would use the head unit's internal power.
No head unit typically has more than about 60watts of total output power because more would require a real DC-DC power supply (which does not fit in a head unit easily). According to Car Audio and Electronics magazine, most head units use the same chips for the internal amplifiers so they all produce about the same low power.
Things to look for: Usability: Your head unit is the part of the car stereo that you interact with most so it is key to get one that "feels" comfortable to you. Always look at a head unit in a store display and use it for awhile. Try to flip through radio stations and tracks on a CD to see if it is quick and easy. If you have issues with small buttons, imagine what it will be like when you are driving! Since most models in the same price range are similar in features and sound quality, usability is often the deciding factor between models.
Power: Even though I just said not to use the built-in power of a head unit I know sometimes it is unavoidable. Bear in mind that the power specifications given by majority of manufacturers for head units are not accurate. They often use terms like "music power" or "peak power" which have little real meaning because there is no standard definition of those terms. If the power is quoted in "RMS" terms then it is usually valid. However, there is still one other place of misconception. Often manufacturers will quote power as "30watts x 4 RMS". The "RMS" seems to mean it is a true indication of power but they are implying that all 4 channels can produce 30watts rms AT THE SAME TIME. With a head unit, this is almost always not true. Because of the small power supplies in head units they can rarely output more than 15-60 watts TOTAL. This means that the power to each channel at maximum loading would only be 1/4 of that total. Some manufacturers are better than others about giving correct specifications and a few models are available with sophisticated power supplies which have higher power output but they are VERY expensive. If you're paying less than $800 for a head unit (and most of us are!) then your head unit will not put out much power. I have written a more comprehensive explanation of power amplifier specs as well. Speakers which are not producing bass do not draw nearly as much power so you can get away with using the head unit to power them but use passive high pass crossovers (bass blockers) and they will play even louder and cleaner. Bear in mind that the distortion may be higher from the head unit than an external amp however.
Pre-amp outputs: These are a definite for any serious head unit. These outputs enable you to run an amplifier directly without need for any conversion. This is the cleanest output of the head unit. A few units have multiple outputs and sometimes ones that are crossed over. Try to find the amount and type that you need for your system but keep in mind future upgrades. One is sufficient but having two allow you fade, or adjust the levels of multiple amplifiers right from the head unit. Some head units now offer 4 volt outputs instead of the usual 1-2 volts. This can be very beneficial since cars have a lot of electrical noise in them. The 4 volt output is less susceptible to noise, however, you must be certain that the amplifier or crossover being connected to the output can handle 4 volts or you will not be able to use the extra voltage. If your head unit does not have pre-amp level (RCA type) outputs you can buy an adapter which will convert your speaker level outputs to line level.
Car Audio Head Unit Power! Car Stereo head unit power is VERY deceiving! Head units all have HUGE power ratings on the Boxes or right on the front of the units! The highest head unit power I have ever actually tested was 13 watts (at a half way decent THD)… So even if the head unit says 35Watts or 40 watts per channel on it don’t EVER expect for it to REALLY BE more than 15! It is just not possible.
The amplifier can generate much more power per channel than the car head unit. Why can't a head unit generate this kind of power??? The transformer converts the 12 volts from the car battery into a much higher voltage (in this case +36 volts and -36volts)? Since the car head unit doesn't have a transformer to boost the voltage (not enough room in it) the head unit can only limp along trying to make a lot of power with the measly 12 volts from your car. Most high power head units use a bridged output (also called floating ground or push pull), head units use an IC chip that makes both speaker wires to each speaker work together with one pushing while the other pulls (so to speak) just like a real amplifier does when you bridge it. But unlike a real amplifier, most car head units cannot be un bridged. But even still, having no transformer in the head unit limits it severely and your output will be around 12 to 13 watts pr speaker. Don't fall for the high power LIES.
About the Author
Professor Apex is an Expert In Car Audio and Mobile Video. He has more than 13 years of experience. You can find him here: Wholesale Car Audio
The first step is to determine what your needs are. If you’re unsure, we will first look at the individual components that make up a complete system.
The Head Unit
AKA Source Unite. This is the most important part of your audio system. The decision you make here will either make or break your system, never mind you hearing.
What to look for in a head unit:
Manufacturer: Look for a well know manufacturer i.e. Sony, Alpine, Clarion, Blaupunkt, JVC etc.
Sound Re-production: Accurate sound re-production is a must, forget looks.
Warranty: Good warranty is also a must, avoid Grey-Market units.
Build Quality: Make sure its robust.
Low budget features:
Forget Fascination: Forget colorful displays, flip down panels, inbuilt equalizers, bass boost etc.
Higher budget features:
CD: If your music collection is on CD, then consider a CD head unit or one with a CD changer.
MP3 & WMA: If you have a vast music library then you may want to look for units that can read MP3 and/or WMA music formats.
VCD & DVD: Long journeys will benefit from this feature and keep the kids out of trouble too.
Amplifier: If you planning to add an amp to your car choose a head unit with RCA outputs.
Speakers
This is the second most important part of your audio system. Speakers convert electronic signals into sound waves.
What to look for in speakers:
Spending: Speakers come in several types, sizes and power ratings. Be careful before spending more money then you need to.
Power: Speaker power is one of the most over-rated features in head units and speakers.
Numbers: Don’t get pulled in by numbers like 300Watts or 450Watts etc. Even the big company’s use this.
Ratings: Don’t buy speakers based upon the company’s ratings like PMPO, Peak Power, Max Power etc.
RMS: Root Mean Square ratings are equivalent of the actual power rating delivered.
Installation: This is one of the most important points of the speaker, “proper installation”. Direction, Height and Placement is key to the sound and if not installed properly can make the sound feel lifeless.
Manufacturer Locations: Usually speaker locations provided by the car manufacturer are not the best, but you can install the speakers in the provided spaces.
Speaker Types:
Coaxial: They are two (or more) speakers’ built-in the same unit. They usually consist of a mid-bass and tweeters. They are cheaper than separate component speakers and easier to install.
Component 2-Way: They consist of a mid-bass and tweeter. They are harder to install then coaxial.
Component 3-Way: They consist of a separate woofer, mid-bass or mid-range and tweeters. They are harder to install then coaxial.
Mid-Bases: Usually 5” to 8” speakers that are designed for low frequency sound and fill the frequency gap between the woofer and the tweeter. Because of there size they are hard to fit but do make the sound more lifelike.
Horns: These are funnel shaped speakers that are for staging, they are particularly good for vocal frequencies but are the hardest to install.
Tweeters: Usually 1” to 3” in size and great for high frequency sounds. They can be hard to install in some cars as there mounting is usually done around or in the dashboard.
Sub-Woofers: The largest speakers raging in size from 8” to 24” and sometimes bigger. They are the most important if you want to feel the beats and not just hear them. The woofer is usually found in the trunk in an enclosure but installations may vary.
Speaker Locations:
Front Speakers: Commonly located in or under the dashboard or in door panels.
Rear Speakers: Located behind the rear seats or in door panels.
Woofer: Depending on size, behind the rear seats or in the trunk.
Tweeters: Usually on the front pillars, front doors or dashboard.
Note: Try out your speaker positions before installing them so that you get the best sound.
Amplifiers
Amps amplify or multiply the input signals from the head unit to the speakers. The Amp not only make the sound louder they also make it clearer.
What to look for:
Class: Amps are classified according to their amplification, Class-A, B, AB, and D.
Channel: They can also be classified by their channel, Monobloc 1-Channel, 2-Channel, 4-Channel etc.
Crossovers
They are a combination of series and parallel placed induction coils and capacitors, or either of them. They are given the job of frequency distribution among different types of speaker for distortion free sound reproduction. Inductors stop the high frequencies from passing through.
A combination of these together make a band pass which cut through a certain portion of both the high as well as the low frequencies and are rightly called band-pass crossovers.
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About the Author
About The Author: Carl Malley is a in car entertainment enthusiast. His website at www.incardiscounts.com offers the most simple solution for in car equipment.
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