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.