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Head Units Overview
In a perfect world, 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 clearer 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 occasions where you would use the head unit's internal power. One situation is to save money and the other situation is when you are building a system where the benefits of an amp are not important to you.
No head unit typicallyhas 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). Using the head unit power can be a temporary solution till a separate amplifier can be bought, just make sure you do not try to power any subwoofers or insensitive component sets with the head unit's built in power. 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. The best they have measured is about 14watts into 4 channels at 1% distortion. Their power level at a better lower distortion figure (like 0.1%) is significantly lower.
Note about using factory head units: A lot of people ask me about using the factory head unit that came with their car with external aftermarket amps. Typically you cannot get a clean signal from the head unit because factory heads do not have line level (RCA) pre-amp outputs to drive an amp. You can use a speaker level to line level converter but the sound is still going through the factory head's internal amps. Some are willing to sacrifice some sound quality in order to keep their factory head. Also, if your factory system uses an external amp you may be able to find an adapter so you can use an aftermarket amp instead.
Things to look for:
Usability: Your head unit is the part of the car stereo that you interact with most so it is vital to get one that "feels" good 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! Models fall in the same price range, so usability is key deciding factor.
Power: Even though I just said not to use the built-in power of a head unit I know sometimes it is necessary. 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 no real meaning because there is no standard definition of those terms. If the power is quoted in "RMS" terms then it is usually correct. 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 significant 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.
Cassette vs. CD: This choice is mostly a matter of preference. If you do not have many cassettes then an in-dash CD player is probably right for you. If you need the capability to listen to cassettes and CDs then a cassette head unit with changer controls should be sufficient. Be aware that many in-dash CD head units can control a CD changer as well so you can use both. An in-dash CD is good for changing discs quickly while on the road. Because of size of most CD changers they are usually mounted in the trunk or under the seats although there are some newer models which are small enough to fit in glove compartments. Under the seats or in the trunk are not easy places to get to while you're driving! (Who has cassettes anymore?)
Theft Protection: Detachable faces are the most general theft prevention scheme in head units today. There are two types, fully detachable and partially detachable. With a fully detachable face all the controls on the front come off leaving behind a blank panel, whereas a partially detachable face leaves some features on the head unit but the head unit is still useless without the face. Fully detachable faces are larger and bulkier to carry around than partially detachable ones but leave nothing behind to be seen. Another option is Eclipse's ESN system. With these head units when you first apply power to them you must supply a CD which the unit remembers as the "reference" CD. Thereafter if the unit ever loses power you must insert the "reference" CD before it will work again. Only you know what the "reference CD" is so the head unit is useless to a thief. Eclipse also tracks the units they repair. More than once a stolen head unit was returned to them for service because it was not working. Upon verifying the head unit was stolen they can apprehend the criminal as the person who returned the stolen head unit for service. A new innovation from Kenwood flips the face around when you turn off the power so the thief can't see the head unit. I think it would work even better if the face then went back into the head unit, giving the feel that the unit is a detachable face head unit with its face removed.
Pre-amp outputs: These are a definite for any serious head unit. These outputs enable you to run an amplifier directly without theneed for any conversion. This is the cleanest output of the head unit. Some units have multiple outputs and sometimes ones that are crossed over. Look for 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 useful 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.
Other features: There are many other small differences in features between head units. Choose the one that you like the most. Switch able illumination is nice if you want the head unit's display to match the other instrumentation in your car. Dolby Noise reduction and full logic tape controls are nice touches. Finally, a remote control can be useful or can be a waste depending on whether you use it. A remote control mounted in the steering wheel can be very convenient though. Some CD heads come with a buffer to reduce effects from bumps. This can be useful but in my experience if you mount the head unit tightly it will not skip much anyway and using the anti-skip buffer can have a slight negative effect on sound quality because of the way the buffer is implemented
Pre-Amp and output voltage
Pre amp is typically the control center. Your volume control, tone controls, EQ (if any) and source switching is usually at your pre-amp. The Pre-Amp is designed to be a very clean low signal voltage and take care of any signal modifications you might want to make to the signal (music) BEFORE the signal goes to the amplifier.
Pre-amp signal is usually LOW, it was created to be low so there is less distortion at this stage. Home pre-amps usually operate with about 1 to 1.5 volts of output, the aim was to keep the voltage down as low as possible to keep the distortion down as low as possible in the pre-amp stage and then after the signal leaves the pre-amp we let the amplifier do what it does well.
In car audio we have used this very low level for years with pretty good success. But in many cases in car systems there might be a little noise. Some of the car stereo manufacturers have stepped up the output voltage of their headunits in an attempt to aid us have a noise free system.
Installers have always felt that certain types of noises in a car system can be reduced by turning down the amplifier gain controls. But far too often the customer gets his system fairly noise free but with no improvement!! The system just may not play loud like it should if the gain controls are turned down too far. Turn the headunit all the way up and it still wont get loud. This is where the manufacturers come in with the higher level headunits! Now you can turn them higher, even when your gains are very low (and hopefully any bad noises are also very low)...
Some old fashioned find this added amplification (and distortion) on the headunits a turn off. But the majority of us cant really hear the added distortion and find it much nicer than the noise !!!
Some rookies find the added headunit voltage an advantage because now they think their amp will play louder! NOT TRUE! The amp will still have its max output power and it will not change in most cases of the amps max drive can be achieved. Many rookies compare a 1or 2 volt headunit with a 4 to 8 volt headunit and they only notice that with the higher voltage the system gets loud at 10 (on the volume scale) and you used to have to turn it up to 30..!! Some rookies are fooling themselves into thinking this is GOOD! This is better!! But it may not be.
Ideally, a system has a good swing to the volume control... Too much signal voltage into the amp the system will get loud too soon on the volume swing. Not enough signal voltage to the amp and the system volume control can go all the way up and not get loud like you might like it. Setting the amp gain controls correctly will produce this proper swing!
And the rookies may not know it, but if they set the gain controls correctly with a low signal output headunit and compared it to a high signal output headunit then there would be NO volume difference anywhere in the system volume swing...
I just shake my head in amazement...
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! Just cant happen!
REAL amplifiers use LARGE power supplies in them with transformers to STEP UP the voltage So that large amounts of power can be made! Head units just don’t have room in them For transformers or large power supplies!!!! So NEVER expect more than 15watts per Speaker out of head units! Some of the more reputable head units might LIE on the boxes but If you read the small print in the spec sheets in the owners manual they might admit to the 15watt figure!
The amplifier can generate greater 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.
Professor Apex is an Expert In Car Stereo and Car Video. He has more than 13 years of experience. You can find him here: Wholesale Car Audio
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