Adding on to a Head Unit
Car CD Changers
There are two methods of connecting a CD changer, either through direct connection to the head unit’s preamp (DIN input plug) or by FM modulation. Direct connection is accomplished using a special cable that connects between the head unit and a compatible CD changer. This gives the CD changer power and control commands that allow it to work with the rest of the audio system. If an aftermarket CD changer is being direct connected to an OEM head unit, a special adapter will be needed to interface the two devices. Keep in mind that not just any CD changer will work with any CD changer control enabled OEM head units. The aftermarket changer must be made by the same manufacturer that makes the factory OEM changer. Contact a specialty manufacturer such as Precision Interface Electronics (PIE) or SoundGate that manufacture these adapters.
The second method is through an FM modulator and is probably the most common way of connecting a CD changer to an OEM head unit or aftermarket head unit that lacks CD changer controls. In this method the CD changer interfaces with an outboard control unit that accepts the music signal from the changer and converts it to an FM radio signal. This FM radio signal then passes to the FM tuner section of the head unit through the radio antenna plug. The modulator unit receives signals from a wired remote control that is usually placed on the dash or console within easy reach of the driver. This remote controls track, seek, disc, and other functions and displays the CD information on its screen.
The drawbacks to this method over the direct connect method are two fold. First, the addition of a second control unit can be confusing and looks less clean then a one control unit setup. Second, the sound quality will not be as good as with a direct connection. The reason for this has to do with how the signal is passed to the head unit. In a direct connection the music passes as a clean preamp signal that is then amplified by the head unit (or passed the system amplifiers). With an FM modulator the sound quality is limited by the frequency response of your FM radio section. While a direct connection will pass the entire audio band from 20Hz-20kHz, the FM tuner will only cover the spectrum of the FM band. This will lower the high frequency response otherwise possible with a CD system. An FM modulated system will sound like a radio station with excellent reception. Certainly acceptable sound but not true audiophile quality and something to be aware of.
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| CD Changer Image Courtesy of Kenwood |
FM Modulated CD Changer Image Courtesy of Kenwood |
Features to look for when shopping for a CD changer are:
CDR/CDRW Compatibility: While most CD changers can read CD recordable discs, not all can read CD Re-Writable discs. This may be important if you burn your own CDs.
Disc Capacity: The number of CDs that the CD changer can hold. This will range from 6 to 12, though 50 disc changers are available. The more your CD changer can hold, the less often you will have to change the discs.
Disc Changing Time: The time it takes a CD changer to go from CD to CD. Though it has no effect on sound quality, if you are impatient you might want to take this into account.
Frequency Response: This is the portion of the audio spectrum a piece of equipment can produce. The average human can hear sounds in the range of 20Hz-20kHz. Most good CD changers can produce this entire range.
Loading Method: The means by which CDs are loaded into the CD changer. The most common is by a removable CD magazine. Some in-dash CD changers units accept CDs directly.
MP3/WMA capability: Some of the newer CD changers have the ability to play MP3/WMA encoded discs. This can be a real benefit if you have a CD burner and a collection of MP3/WMAs on your computer as it eliminates the need to have additional equipment to play your MP3/WMA collection in your automobile.
Skip Protection: Some CD changers have a built in memory buffer, usually between three and ten seconds, that will continue to play the music skip free even when the player is actually mis-tracking. This is a great feature if you drive on a lot of bumpy roads but is an uncommon feature usually reserved for higher end changers and MiniDisc units.
Size: This is the physical size of the CD changer itself. It's important to know if you have limited space in your chosen mounting location. Some units are even small enough to fit in the dash or glove compartment.
Car MP3 Players and Other Add-Ons
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Other source units that are commonly added on to an audio system are small portable devices such as CD, cassette, MP3, MiniDisc, iPod, etc. These devices are usually added one of three ways. With a cassette head unit they are most often added by use of a cassette adapter. One end of the adapter looks like a cassette tape with a wire attached to it. This end goes into the cassette player and the other end plugs into the line out or headphone jack of the portable unit. |
| Cassette Adapter Image courtesy of Parts Express |
| Common to both CD and tape units is a kind of direct connection through a preamp jack either on the front or back of the unit, usually in the form of a 3.5mm stereo jack (the jack used on portable unit headphone outputs). Though uncommon, they are a better way of connection for the same reason as those described for the CD changer direct connection. A 3.5mm to 3.5mm stereo extension wire, that looks like a headphone wire with two male ends, is connected between the line out or headphone jack on the portable unit and the 3.5mm jack on the head unit. This allows the signal from the portable unit to go directly to the preamp section of the head unit, giving the best signal practical. |  |
| 3.5mm Stereo Wire Image courtesy of Parts Express |
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The third method is to buy a small battery powered FM transmitter for about $20. These work with both tape and CD players and are about the size of a deck of cards. The unit plugs into the line out or headphone jack of the portable unit and transmits a signal to the FM section of the radio at the frequency you choose (an unused section of the FM band). This unit is basically a small FM transmitter and its quality and effectiveness varies. It is often the only solution available when a non-tape head unit is used and a portable source hookup is desired. Keep in mind that these are battery operated and will need frequent battery changes if they are not switched off every time the radio is turned off. |
| FM Transmitter Image courtesy of Parts Express |
| While on the subject of portable units, please know that the mounting of a portable unit in an automobile can be tricky. One of the better methods is to buy a stalk mount for your unit. The stalk mount is a small platform that your portable unit sits on. It connects to a mounting plate through a semi-flexible stalk. The mounting plate is then screwed into the console, dash, floor or other solid part of the interior. Double sided tape is also an option over screws but I have not seen that work successfully for long. Depending on your vehicle, the stalk mount may interfere with other functions or passenger leg room so take this into consideration before purchasing. If you're connecting an iPod you will find many car mounts for these popular units. |  |
| Stalk Mount Image courtesy of Parts Express |
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