Amplifiers

Be sure to follow manufacturer's instructions for installation.  Be aware of recommended OHM levels.  This is the amount of resistance a speaker or set of speakers will produce (covered in the speaker section).  Most mobile audio amplifiers are rated for 4 OHM, but some can handle a lower OHM load.  
 
Types:
There are basically 4 types of amplifiers; mono (one channel), two channel, four channel, and multiple channel.  
 
Mono
Theses are amplifiers with one channel output for a speaker.  These are generally used for subwoofers.  
 
Two channel
These amplifiers have two outputs for speakers which are generally marked left and right. Generally used to amplify the signal to a pair of speakers, like a set of 6 1/2s. Some are "bridgeable"  This means that the two outputs can be combined to send a larger amplified signal to a single speaker, or a set of speakers.  This would be used to send a signal to a single subwoofer or multiple subwoofers. Using in 2 channel mode - every output to individual speakers. Using in "bridged" or mono mode - combine both outputs to a single speaker (follow manufacturer's installation instructions for bridging).
 
Four channel
These amplifiers have four outputs for speakers which are generally marked left front, right front, left rear and right rear.  
Generally used to amplify the signal to 4 individual speakers. Some of these are also "bridgeable".  Most you can use in a combination of regular and bridged. Using in 4 channel mode - every output to individual speakers. Using in 3 channel or "mixed bridge" mode - front left and front right outputs to individual speakers.  Combine rear left and rear right outputs to a single speaker (follow manufacturer's installation instructions for bridging). Using in "bridged" mode - combine front left and front right outputs to a single speaker (follow manufacturer's installation instructions for bridging).  Combine rear left and rear right outputs to a single speaker (follow manufacturer's installation instructions for bridging)
 
Multiple channel
These amplifiers (usually) have 5 outputs for speakers which are generally marked left front, right front, left rear, right rear, and sub. Generally used to amplify the signal to 4 individual speakers and a subwoofer
 
 
Power connection:
 
There are three power connections for amplifiers:
 
+12 V or Pos - This is connected (usually directly to the battery) to a constant source of power (positive post of battery).  Be sure to add a fuse inline as per the manufacturer's instructions. -12 V or Neg or Ground - This is connected (usually directly to the battery in a boat) to a ground source (negative post of battery) - rem or switched - This is connected to a switched source wire that has power when the head unit is turned on.  This is usually a blue wire with a white stripe of the head unit.  Some head units have power antenna leads (usually blue - no stripe) AND remote out (usually blue with a white stripe).  Be sure to use the remote out instead of the power antenna lead.  Otherwise, your amplifiers will only turn on when the radio portion of the head unit is on and not the CD portion.
 
RCA connections:
Depending on the manufacturer and type of amplifier, it will have anywhere from 1 to six RCA inputs, and possibly a set or pair of RCA outputs. Generally speaking, the RCA inputs of the amplifier directly correlate to the speaker outputs.  Again follow manufacturer's instructions for specific installations.
 
 
Tuning the amplifier:
 
There are generally two different adjustments to be made for amplifiers; gains and crossover
 
Crossover:
There should be a single crossover control per pair of outputs on the amplifier.  For example, a four channel amplifier will generally have two crossover controls; one for the front channels, and one for the rears.
The crossover is used to send a specific range of frequencies to a certain type of speaker.  There are two types of crossovers; high pass and low pass
 
High pass - This will allow the higher frequencies (treble and mid-range) to pass and block lower frequencies (bass).  This is generally used when amplifying signals to speakers like 5 1/4s, 6 1/2s, 6x9s, as these speakers can not reproduce lower bass frequencies without distorting.  A good starting point to set these is between 80 and 120 Hz.  You want to set these at a point to where the speaker is not distorting because of bass.
 
Low pass - This will allow lower frequencies to pass and block higher frequencies.  This is generally used when amplifying signals to subwoofers like 8s, 10s, 12s, etc.  A good starting point to set these is around 80 Hz.  You want to set these at a point where you are only hearing bass from these speakers, and higher frequencies are not being produced that will make the bass sound "muddy" or garbled
 
Gains:
There should be a single gain control per pair of outputs on the amplifier. For example, a four channel amplifier will generally have two gain controls; one for the front channels, and one for the rears. This is the level in which the amplifier will amplify the signal.  Although this is not entirely true, you can think of this as the "volume" on the amplifier.  To set this, first turn all the way down.  Then set the volume on your head unit to about 3/4.  Then slowly turn up the gain on the amplifier until the speaker begins to distort slightly.  Then turn the gain back down slightly.
 
Speakers

There are two types of speakers; separates and co-axial
 
Separates
There are several different types of separates, but they can pretty much be categorized into three categories; tweeters, mid-range, and woofer (subwoofers)
 
Tweeters - These are usually small speakers that reproduce the higher frequency tones (treble)
Mid-Range (mids) - Pretty self explanatory
 
Most good manufactures sell tweeter and mid-range separates as package that includes tweeter, mid-range and a passive crossover.  The purpose of the crossover is to send the high frequencies to the tweeters and the mid-range to the mids.  These sets are generally more expensive than co-axials and reproduce sound more clearly.  However, they usually require an amplifier as they take more power (wattage) than co-axials.
 
Subwoofers
Subwoofers are generally larger speakers (8s, 10s, 12s, etc) and are capable of reproducing lower frequency tones (bass).  There are two types of subwoofers; free-air and sealed.  All subwoofers require some type of enclosure to work properly.
 
Free-air
These subwoofers are designed to work in two ways; infinite baffle and ported enclosures

Infinite baffle - This is where the subwoofer uses a part of the car (or boat) as the enclosure.  An example of this would be installing the sub along the wall of the walk-though to the open bow and using the storage area under the observer's seat as an enclosure.

Ported enclosure - This is where the subwoofer in installed in an enclosure (or box) with a port.  You can tune the frequency of the bass response by changing the length or diameter of the port.  Be sure to follow manufacturer's instructions for box size and port recommendations. Notes - Free-air installations generally produce more of a "boomy" and "carrying" bass.  Ported enclosure are generally much larger than sealed enclosures and do not require as much power (wattage)

Sealed
These are subwoofers that are installed in a completely air-tight enclosure.  Be sure to follow manufacturer's instructions for box size. Notes - Sealed enclosures generally reproduce more of a "tight" bass and can reproduce a larger frequency range than infinite baffle or ported enclosures.  They do, however, generally require more power.
 
Notes about subwoofer enclosures - Ported enclosures are less tolerant of changes in enclosure size and port length and size. Sealed enclosures are much more forgiving.  For boats where space can be an issue, a sealed enclosure is usually the way to go.
Notes about separate sets - Most tower speakers are sold in sets that include a tweeter, a mid and a crossover enclosed in housings that attach to the tower.  They have one input.  Check the ohm level of these to be sure to wire them according the amplifier manufacturer's instructions (see Ohm's law section)
 
 
Co-axial
These are usually a mid-range speaker with a tweeter (or multiple tweeters) attached to them on a pole going through the center of the mid-range's cone.  These are usually less expensive than a set of separate, and do not require an amplifier, but generally do not reproduce sound as clearly as a set of separates
 
 
Ohm's law.
 
An Ohm is the measure of resistance a speaker produces.  Most car or boat audio speakers are 4 Ohms.  You can check this by using an Ohm meter, or it is usually on the manufacturer's specification sheet for the speaker
There are three ways to wire speakers; direct, parallel, or series.
 
Direct - This is where you wire the + and - terminals directly to the amplifier or head unit.  The head unit or amplifier will see the ohm level of the speaker
 
Parallel - This is where you wire the + and - output of the head unit or amplifier and wire it to the + and - input terminals of multiple speakers.  
Rt = resistance total
R1 = resistance of speaker #1
R2 = resistance of speaker #2
R3 = resistance of speaker #3
 
The equation for this is 1/Rt = 1/R1 + 1/R2 +1/R3.
For example if you wire two 4 ohm speakers in parallel:
1/Rt = 1/4 + 1/4
1/Rt = 1/2
Rt = 2
So the total ohm load the amplifier or head unit will see is 2 ohms
 
Another example is three 4 ohm speaker in parallel:
1/Rt = 1/4 + 1/4 + 1/4
1/Rt = 3/4
Rt = 4/3
So the total ohm load the amplifier or head unit will see is 1.33 ohms
 
Series - This is where you wire the + of the head unit or the amplifier to the + input terminal of speaker 1.  Then the - input terminal of speaker 1 to the + input terminal of speaker 2.  Then the - input terminal of speaker 2 to the - output of the head unit or amplifier.
The equation for this is Rt = R1 + R2 + R3.
For example if you wire two 4 ohm speakers in series:
Rt = 4 + 4
Rt = 8
So the total ohm load the amplifier or head unit will see is 8 ohms
 
Another example is three 4 ohm speaker in series:
Rt = 4 + 4 + 4
Rt = 12
So the total ohm load the amplifier or head unit will see is 12 ohms
 
Again - Be sure to follow recommendations on the manufacturer's instructions for ohm levels as too low of an ohm load could damage the head unit or amplifier.

Direct Speaker Wiring


Parallel Speaker Wiring


Four Channel Amp

Four Channel Amp With Tower & Fade