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Archive-name: car-audio/part1
Rec-audio-car-archive-name: FAQ/part1
Version: 4.55
Last-modified: 08 August 2005

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rac-faq.texi. Frequently Asked Questions

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August 2005

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Table of Contents

1 Definitions                                              
  1.1 What do all of those acronyms mean?                    
  1.2 What is meant by "frequency response?"                 
  1.3 What is a "soundstage?" What is an "image?"            
  1.4 What is meant by "anechoic?"                           
2 Electrical                                               
  2.1 My speakers make this high-pitched whine which matches the
      engine's RPMs. What is it, and how can I get rid of it?    
      2.1.1 Level 1: Check out the Amplifier(s)                  
      2.1.2 Level 2: Reduce the System                           
      2.1.3 Level 3: Move the Head Unit                          
      2.1.4 Level 4: Testing the Car                             
      2.1.5 Level 5: Adding Signal Processors                    
      2.1.6 Level 6: Processor Isolation Tests                   
  2.2 My system "pops" when I turn it off. What is happening and
      how can I get rid of it?                                   
  2.3 What is the best power wire to use?                    
  2.4 What is the best speaker wire to use?                  
  2.5 I heard that I should run my power wire directly to my car's
      battery. Why should I bother, and how do I do it?          
  2.6 Should I do the same thing with my ground wire, then?  
  2.7 Sometimes when I step out of my car, I get a really bad
      shock. What is wrong with my system?                       
  2.8 When my car is running and I have the music turned up loud,
      my headlights dim with the music. Do I need a new battery or
      a new alternator?                                          
  2.9 2.9 What is a "stiffening capacitor", and how does it work?
      2.9.1 Do I need a capacitor?                               
      2.9.2 Can I just upgrade my headlight wiring instead?      
      2.9.3 Will the dimming go away if I upgrade the amplifier power/ground
      2.9.4 What do I look for when buying a capacitor?          
      2.9.5 How do I install a capacitor?                        
      2.9.6 I have more than one amp in my audio system. Which one
            should I have the capacitor run?                           
      2.9.7 Will my bass response improve by adding a capacitor? 
  2.10  When should I upgrade my battery or add a second battery?
3 Components                                               
  3.1 What do all of those specifications on speakers mean?  
  3.2 Are component/separates any better than fullrange or coaxials?
  3.3 What are some good (and bad) brands of speakers?       
  3.4 What do all of those specifications on amplifiers mean?
  3.5 What does "bridging an amp" mean?                      
      3.5.1 Why should I bridge my amp?                          
      3.5.2 Why shouldn't I bridge my amp?                       
      3.5.3 What happens when an amp is bridged?                 
      3.5.4 Does bridging an amp would halve the impedance of the
      3.5.5 Can I bridge my 4 channel head unit?                 
  3.6 What is "mixed-mono?" Can my amp do it?                
  3.7 What does "two ohm stable" mean? What is a "high-current"
  3.8 Should I buy a two or four (or more) channel amplifier?
  3.9 What are some good (and bad) brands of amplifiers?     
  3.10 What is a crossover? Why would I need one?            
  3.11 Should I get an active or a passive crossover?        
  3.12 Should I buy an equalizer?                            
  3.13 What are some good (and bad) brands of equalizers?    
  3.14 What do all of those specifications on tape deck head units
  3.15 What are features to look for in a tape deck?         
  3.16 What are some good (and bad) brands of tape decks?    
  3.17 What are features to look for in a CD head unit?      
  3.18 Should I buy a detachable faceplate or pullout CD player?
  3.19 What are some good (and bad) brands of CD head units? 
  3.20 Can I use my portable CD player in my car? Won't it skip
       a lot?                                                     
  3.21 What's that weird motor noise I get with my portable CD
  3.22 What are some good (and bad) brands of portable CD players?
  3.23 What's in store for car audio with respect to MD, DAT and
  3.24 Are those FM modulator CD changers any good? What are my
       other options?                                             
  3.25 What kind of changer will work with my factory head unit?
  3.26 What are some good (and bad) brands of CD changers?   
  3.27 Why do I need a center channel in my car, and how do I
       do it?                                                     
  3.28 Should I buy a sound field processor?                 
  3.29 What are some good (and bad) brands of signal processors?
  3.30 I keep hearing that speakers for Company X are made by
       Company Y. What's the deal?                                
  3.31 What is a Line Driver? Do I need one?                 
  3.32 Can I play MP3 files in my car?                       
4 Subwoofers                                               
  4.1 What are "Thiele/Small parameters?"                    
  4.2 How does speaker sensitivity affect real world SPL? Will
      a higher sensitivity give me a larger SPL?                 
  4.3 What are the enclosure types available?                
  4.4 Which enclosure type is right for me?                  
      4.4.1 Infinite Baffle ("free-air")                         
      4.4.2 Sealed Box                                           
      4.4.3 Ported Box                                           
      4.4.4 Bandpass Box                                         
  4.5 How do I build an enclosure?                           
  4.6 MDF for Dummies                                        
      4.6.1 What is MDF?                                         
      4.6.2 Where can I get MDF?                                 
      4.6.3 What type of saw blade works best when cutting MDF?  
      4.6.4 What type of router bits work well with MDF?         
  4.7 What driver should I use?                              
  4.8 Is there any computer software available to help me choose
      an enclosure and a driver?                                 
  4.9 What is an "aperiodic membrane?"                       
  4.10 Can I use my subs in the winter?                      
  4.11 How can I carpet my enclosure?                        
  4.12 Are large magnets always better than small magnets?   
  4.13 I know the box volume required for my subwoofer, but what
       are the best dimensions for my enclosure?                  
5 Installation                                             
  5.1 Where should I buy the components I want?              
  5.2 What mail-order companies are out there?               
  5.3 What tools should I have in order to do a good installation?
  5.4 Where should I mount my speakers?                      
  5.5 What is "rear fill", and how do I effectively use it?  
  5.6 How do I set the gains on my amp?                      
  5.7 How do I select proper crossover points and slopes?    
  5.8 How do I flatten my system's frequency response curve? 
  5.9 How do I wire speakers "in series" and "in parallel?"  
  5.10 Are there any alternatives for Dynamat? It's too expensive!
  5.11 How many devices can I attach to my remote turn-on lead?
  5.12 How do I wire a relay in my system?                   
  5.13 How do I design my own passive crossovers?            
  5.14 How do I build my own passive crossovers?             
  5.15 Can I split the single pre-amp output from my head unit
       to drive two amplifiers with a Y-cable?                    
  5.16 How do I turn a stereo signal into a mono signal      
  5.17 How do I determine a speaker's polarity?              
  5.18 How can I use an oscilloscope to set the gains in my system?
  5.19 Why are kickpanels such a popular location for mounting
  5.20 How can I build custom kickpanels?                    
  5.21 What's worse for a speaker, too much or too little power?
  5.22 Why is distortion harmful to my speakers?             
  5.23 What tools do I need to cut Plexiglas?                
  5.24 Are there any other special requirements for working with
6 Competition                                              
  6.1 What is IASCA, and how do I get involved?              
  6.2 What is USAC, and how do I get involved?               
  6.3 What are the competitions like?                        
  6.4 Should I compete?                                      
  6.5 What class am I in?                                    
  6.6 Where can I find out when these Sound-Offs are?        
  6.7 How do I get sponsored by a manufacturer?              
7 Literature                                               
  7.1 What magazines are good for car audio enthusiasts?     
  7.2 Are there any newsletters I can read?                  
  7.3 What books can I read?                                 
  7.4 Can I contact any manufacturers on-line?               
8 Credits                                                  
9 Changes                                                  

1      Definitions

This section contains background information which defines some of the
acronyms and terminology commonly used in the car audio world.
Understanding these definitions is important in order to understand the
other sections of this document.

1.1    What do all of those acronyms mean? [JSC,MZ]

`A' is for "amperes", which is a measurement of current equal to one
coulomb of charge per second.  You usually speak of positive current -
current which flows from the more positive potential to the more
negative potential, with respect to some reference point (usually
ground, which is designated as zero potential).  The electrons in a
circuit flow in the opposite direction as the current itself.  Ampere
is commonly abbreviated as "amp", not to be confused with amplifiers, of
course, which are also commonly abbreviated "amp".  In computation, the
abbreviation for amps is commonly "I".

`V' is for "volts", which is a measurement of electric potential.
Voltages don't "go" or "move", they simply exist as a measurement (like
saying that there is one mile between you and some other point).

`DC' is for "direct current", which is a type of circuit.  In a DC
circuit, all of the current always flows in one direction, and so it is
important to understand which points are at a high potential and which
points are at a low potential.  For example, cars are typically 12VDC
(twelve volts direct current) systems, and it is important to keep
track of which wires in a circuit are attached to the +12V (positive
twelve volts) lead of the battery, and which wires are attached to the
ground (or "negative") lead of the battery.  In reality, car batteries
tend to have a potential difference of slightly higher than 12V, and
the charging system can produce upwards of 14.5V when the engine is

`AC' is for "alternating current", which is a type of circuit in which
the voltage potential fluctuates so that current can flow in either
direction through the circuit.  In an AC circuit, it is typically not
as important to keep track of which lead is which, which is why you can
plug household appliances into an outlet the "wrong way" and still have
a functioning device.  The speaker portions of an audio system comprise
an AC circuit.  In certain situations, it is indeed important to
understand which lead is "positive" and which lead is "negative"
(although these are just reference terms and not technically correct).
See below for examples.  The voltage of an AC circuit is usually given
as the RMS (root mean square) voltage, which, for sinusoidal waves, is
simply the peak voltage divided by the square root of two.

`W' is for "watts", a measurement of electrical power.  One watt is
equal to one volt times one amp, or one joule of energy per second.  In
a DC circuit, the power is calculated as the voltage times the current
(P=V x I).  In an AC circuit, the average power is calculated as the
RMS voltage times the RMS current (Prms=Vrms x Irms).

`Hz' is for "hertz", a measurement of frequency.  One hertz is equal to
one inverse second (1/s); that is, one cycle per second, where a cycle
is the duration between similar portions of a wave (between two peaks,
for instance).  Frequency can describe both electrical circuits and
sound waves, and sometimes both.  For example, if an electrical signal
in a speaker circuit is going through one thousand cycles per second
(1000Hz, or 1kHz), the speaker will resonate at 1kHz, producing a 1kHz
sound wave.  The standard range of human hearing is "twenty to twenty",
or 20Hz-20kHz, which is three decades (three tenfold changes in
frequency) or a little under ten octaves (ten twofold changes in

`dB' is for "decibel", and is a measurement for power ratios.  To
measure dB, you must always measure with respect to something else.
The formula for determining these ratios is P=10^(dB/10), which can be
rewritten as dB=10log(P).  For example, to gain 3dB of output compared
to your current output, you must change your current power by a factor
of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power).  The other
way around, if you triple your power (say, from 20W to 60W) and want to
know the corresponding change in dB, it is dB=10log(60/20)=4.77 (that
is, an increase of 4.77dB).  If you know your logarithms, you know that
a negative number simply inverts your answer, so that 3dB corresponding
to double power is the same as -3dB corresponding to half power.  There
are several other dB formulas; for instance, the voltage measurement is
dB=20log(V).  For example, a doubling of voltage produces 20log2 =
6.0dB more output, which makes sense since power is proportional to the
square of voltage, so a doubling in voltage produces a quadrupling in

`SPL' is for "sound pressure level" and is similar to dB.  SPL
measurements are also ratios, but are always measured relative to a
constant.  This constant is 0dB which is defined as the smallest level
of sound pressure that the human ear can detect.  0dB is equal to
10^-12 (ten to the negative twelfth power) W/m^2 (watts per square
meter).  As such, when a speaker is rated to produce 92dB at 1m when
given 1W (92dB/Wm), you know that they mean that it is 92dB louder than
10^-12W/m^2.  You also know than if you double the power (from 1W to
2W), you add 3dB, so it will produce 95dB at 1m with 2W, 98dB at 1m with
4W, 101dB at 1m with 8W, etc.

`THD' is for "total harmonic distortion", and is a measure of the how
much a certain device may distort a signal.  These figures are usually
given as percentages.  It is believed that THD figures below
approximately 0.1% are inaudible.  However, it should be realized that
distortion adds, so that if a head unit, equalizer, signal processor,
crossover, amplifier and speaker are all rated at "no greater than
0.1%THD", together, they could produce 0.6%THD, which could be
noticeable in the output.

An "Ohm" is a measure of resistance and impedance, which tells you how
much a device will resist the flow of current in a circuit.  For
example, if the same signal at the same voltage is sent into two
speakers - one of which is nominally rated at 4 ohms of impedance, the
other at 8 ohms impedance - twice as much current will flow through the
4 ohm speaker as the 8 ohm speaker, which requires twice as much power,
since power is proportional to current.

`PSRR' is the "Power supply rejection ratio".  This is a spec sometimes
provided with amplifiers, but is not exclusive to amplifiers.  It
refers to the propensity for an AC signal present at the output of the
power supply to appear somewhere in the signal path.  A poor PSRR often
leads to an increase in noise, distortion, and crosstalk.

`BJT' is short for "Bipolar junction transistor".  It is a very common
type of transistor that is found in a multitude of circuits.  Often
times, amplifier manufacturers will specify that a certain amplifier
utilizes a BJT output stage.  This simply means that the major
current-carrying output devices (the output transistors) are of the BJT
variety rather than FET.  Regardless of output device type, most
discrete amplifiers will incorporate many BJTs throughout the entire

Many people do not realize that `MOSFET' is an acronym, but it stands
for "Metal oxide semiconductor field-effect transistor".  It is another
common type of transistor, but of the FET class.  This type of
transistor operates in a different manner than BJTs.  There's much
debate about which type of transistor is more suitable for car audio
amplifiers, but in the end it becomes a matter of personal preference
for the designer.  It's important to note that neither design has an
inherent benefit in terms of sound quality.  However, properly
constructed MOSFET output stages are sometimes more durable and immune
to damage produced by device failure or poor output protection
circuitry, whereas BJT output stages can sometimes be slightly more
efficient and cheaper.  But both of these statements depend even moreso
on the circuit design and the transistor used.

`RMS' is the abbreviation for "root mean-square".  It is usually
associated with power measurements, and refers to a calculation that
consists of the following procedure: 1) square the waveform; 2) take
the mean of the result; 3) take the square root of this number.  If the
waveform is a sine wave, the RMS value is .707 times the peak value.
If the waveform is a square wave, the RMS value is equal to the peak
value.  Importantly, the RMS voltage of a signal multiplied by the RMS
current of a signal will yield the signal's average power.  Sometimes,
average power is erroneously referred to by manufacturers and hobbyists
alike as "RMS power".  "RMS power" would require calculating the root
mean-square value of a power waveform, which ends up yielding a
different result than average power, but it's become almost an industry

`DCR' is the "DC resistance" of a speaker driver (it is sometimes
referred to as `RE').  This number is typically lower than the nominal
impedance provided by the manufacturer.  It specifies the resistive
component of the speaker's impedance characteristic at a given
temperature.  Sometimes it's useful to estimate a speaker's impedance
by measuring its DCR value with a simple ohmmeter.  Typical DCR values
for 4 ohm speakers are on the order of 3.2 to 3.5 ohms, and for 8 ohm
speakers, usually over 5.5 ohms.

`Q' is typically known as "quality factor", and tends to refer to the
rolloff behavior of a filter or group of filters.  Values of Q are
intimately related to the breadth of bandpass rolloff (eg. in
equalizers), degree of overlap between filters, filter alignment, or
loudspeaker/enclosure interaction.  In general, the higher the Q, the
narrower the passband, greater the overlap, or steeper the slope
(depending on what the Q is referring to).

1.2    What is meant by "frequency response?" [JSC]

The frequency response of a device is the range of frequencies over
which that device can perform in some fashion.  The action is specific
to the device in question.  For example, the frequency response of the
human ear is around 20Hz-20kHz, which is the range of frequencies which
can be resolved by the eardrum.  The frequency response of an amplifier
may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz.
In the car audio world, frequency responses should usually be given
with a power ratio range as well, such as (in the case of the speaker)
120Hz-17kHz +/-3dB.  What this means is that given an input signal
anywhere from 120Hz to 17kHz, the output signal is guaranteed to be
within an "envelope" that is 6dB tall.  Typically the extreme ends of
the frequency range are the hardest to reproduce, so in this example,
the 120Hz and 17kHz points may be referred to as the "-3dB points" of
the amplifier.  When no dB range is given with a frequency response
specification, it can sometimes be assumed to be +/-3dB.

1.3    What is a "soundstage?" What is an "image?" [CD]

The "soundstage" is the position (front/back and high/low) that the
music appears to be coming from, as well as the depth of the stage.  A
car with speakers only in the front will likely have a forward
soundstage, but may not have enough rear fill to make the music seem
live.  A car with both front and rear speakers may have anything from a
forward to a rear soundstage, with an accompanying fill from the softer
drivers depending on the relative power levels and the frequencies
reproduced.  The high/low position of the soundstage is generally only
obvious in a car with a forward soundstage.  The music may seem to be
originating in the footwells, the dash, or out on the hood, depending
on how the drivers interact with the environment.

The "stereo image" is the width and definition of the soundstage.
Instruments should appear to be coming from their correct positions,
relative to the recording.  The position of the instruments should be
solid and easily identifiable, not changing with varying frequencies.
A car can image perfectly with only a center-mounted mono speaker, but
the stereo placement of the music will be absent.

1.4    What is meant by "anechoic?" [JSC]

"Anechoic" means not echoing.  It usually refers to a style of
measuring a speaker's output which attempts to eliminate echoes (or
"reflections") of the speaker's output back to the measurement area,
which could alter the measurement (positively or negatively).

2      Electrical

This section describes various problems and concepts which are closely
related to electronics.

2.1    My speakers make this high-pitched whine which matches the
       engine's RPMs. What is it, and how can I get rid of it? [IDB]

The answer to this section was generously provided by David Navone of
Autosound 2000.  The material in these instructions was adapted from
the Autosound 2000 Troubleshooting Flow Chart by Ian Bjorhovde with the
permission of Autosound 2000.  For more information about Autosound
2000, (see Section 7).

This is a set of instructions to debug a stereo installation if there
is any noise present after it is completed.  Follow each step
carefully!  If you have more than one amplifier, repeat level one for
each amp to be sure that none of them are responsible for the noise.

2.1.1  Level 1: Check out the Amplifier(s)

After you have determined that there is noise in the system, determine
if the amplifier is causing the noise.  To do this, mute the signal at
the inputs to the amp by using shorting plugs.  If there is no noise,
then the amp is fine, and you can proceed to level 2.  However, if
there is noise, then use a test speaker at the amp's output.  If this
stops the noise, then the problem is originating in the speaker wiring,
or the passive crossovers.  Check to make sure that none of these are
shorting with the body of the car, and start again at level 1.       If
noise is still present when using the test speaker, then there may be a
problem with the power supply on the amp.  Try connecting an isolated
power supply - if this does not get rid of the noise,  then there is
something seriously wrong with the amp, and it should be replaced.  If
the noise goes away, then there may be a problem with power supply
filtering or isolation.  This can be fixed by changing the amp's ground
point or b adding external supply filtering.

2.1.2  Level 2: Reduce the System

The amps have been determined to be noise free.  If you have any
processors between the head unit and the amps, disconnect them and
connect the head unit directly to the amp.  If this gets rid of the
noise, then one (or more) of the processors must be at fault, so
proceed to level 5.  Otherwise, try running the signal cables over a
number of different routes.  If you are able to find one that does not
produce any noise, permanently route the cables in the same manner, and
proceed to level 5.  If not, then you must isolate the head unit from
the car's chassis (except for its ground!) - don't forget to disconnect
the antenna, since it is also grounded to the car.       If isolating
the head unit does not solve the problem, the move the grounding point
of the head unit.  Hopefully the noise will be gone, and you can
install the head unit with a quiet ground and proceed to level 5,
otherwise go on to level 3.

2.1.3  Level 3: Move the Head Unit

The amplifiers are fine, but moving both the ground for the head unit
and the signal cables does not solve the noise problem.  Take the unit
completely out of the dash, and put it on either the seat or carpet,
and run new signal cables to the input of the amp.  If this solves the
problem, re-install the head unit, one step at a time and skip to level
5.  But if the noise persists, then move the head unit as close to the
amp as possible and use the shortest possible signal cables.  This will
verify that the original signal cables are not causing the problem -
assuming the noise is gone, reinstall the head unit one step at a time
and go to level 5.  Otherwise, there may be a problem with the power
filtering for the head unit.  As with the amps, power the head unit
with an isolated power supply (again making sure that the head unit
isn't touching the car's chassis at all).  If the noise goes away, you
can add power supply filtering or an isolated power supply;  go to
level 2.  But if the isolated power supply does not solve the problem,
then you can either replace the head unit and go to level 2, or check
the car's electrical system in level 4.

2.1.4  Level 4: Testing the Car

There does not seem to be a problem with either the head unit or the
amplifier, and the car's charging system is suspect.  To see if this is
the case, we can use a system in a car that is already known to be
"quiet."  Bring both cars together as if you were going to jump one,
and use jumper cables to connect the two batteries.  Start the engine
of the car with the noise problem, and listen to the "quiet" car's
system.  If the noise does not go away, there is a SERIOUS problem with
your car's electrical system (possibly a bad alternator).  Have a
qualified mechanic check the charging system out.  If there is no noise
in the "quiet" car, then the "noisy" car's charging system is
definitely quiet, so continue with level 5.

2.1.5  Level 5:  Adding Signal Processors

We have proven that the amplifiers are good, the head unit is good, and
the car's electrical system is good.  Now we need to reconnect each
signal processor.  Repeat this level for each signal processor used in
your system;  if you have added all of your signal processors, and
there is no longer any noise, CONGRATULATIONS!  You've removed the
noise from your system!       Connect the signal processor.  If there
isn't any noise, then go on to the next signal processor.  Otherwise,
try re-routing the signal cables.  If this cures the problem, the route
them permanently over the quiet path, and install the next processor.
If not, then isolate the processor from the car's chassis except for a
single grounding point.  If this works, then permanently isolate the
processor, and move on to the next processor.  If isolation does not
help, then advance to level 6.

2.1.6  Level 6:  Processor Isolation Tests

Now, noise enters the system when one particular processor is
installed, but regrounding it does not help.  Move the processor very
close to the amp, and check for noise again.  If there isn't any, then
re-install the processor, carefully routing the cables to ensure no
noise, and continue at level 5 with the next processor.  Otherwise, use
an isolated power supply to power the processor, making sure that no
part of the processor is touching the car's chassis.  If this solves
the problem, the consider permanently installing an isolated power
supply or possibly a 1:1 transformer, and go to level 5 with the next
processor.  Otherwise, separate the processor and isolated power supply
from the car by many feet and re- test.  If there is still noise, then
there is a serious problem with the processor's design.  Get a
different processor, and continue at level 5 with it.  If separating
the power supply and processor from the car does solve the noise
problem, then either the processor is damaged, or your tests were
inaccurate.  Repeat level 5.

2.2    My system "pops" when I turn it off. What is happening and
       how can I get rid of it? [JD]                               

This kind of problem is often caused by transients in the signal
processor as it powers down finding their way into the signal path,
which the amplifier then transmits to the speakers.

Usually this can be solved by adding a little turn-off delay to the
processor.  This allows the processor to stay powered on for a short
time after the amplifiers have powered down, thus preventing the pop.

Many components sold today (such as crossovers, equalizers, etc) have
delays built-in.  Read your manual to see if it is possible to set this
delay on your piece of equipment or be sure to look for this feature
during your next car audio purchase.

If your processor does not have this feature, you can build your own
delay circuit with a diode and a capacitor.  Add a 1N4004 diode in
series with the processor's turn-on lead, striped side towards the EQ.
Then add a capacitor in parallel, the (+) side of the cap connects to
the striped (processor) side of the diode, the (-) side of the cap goes
to ground (not the radio or EQ chassis - connect to the car chassis).

Experimenting with the cap value will give you the right amount of delay
before the EQ shuts off. You don't want it too long, just long enough to
make sure the amp is off before the EQ powers down.  220 - 1000 uF is
about right, and make sure the cap is a polarized electrolytic, 16V or
higher.  Also keep in mind that the diode will introduce a 0.7V drop on
the remote wire, which can cause the processor to power down before the
rest of the system.

2.3    What is the best power wire to use? [JSC]

There is much debate over the benefit of certain wiring schemes
(oxygen-free, multistranded, braided, twisted, air core, you name it).
However, most people do agree that the most important factor in
selecting power wire is to use the proper size.  Wire is generally
rated in size by American Wire Gauge, abbreviated AWG, or commonly just
"gauge".  To determine the correct wire size for your application, you
should first determine the maximum current flow through the cable
(looking at the amplifier's fuse is a relatively simple and conservative
way to do this).  Then determine the length of the cable that your will
use, and consult the following chart, taken from the IASCA handbook
(see Section 6.1),

                               Length of run (in feet)
     Current     0-4  4-7 7-10  10-13  13-16  16-19  19-22  22-28

        0-20A     14   12   12     10     10      8      8      8
       20-35A     12   10    8      8      6      6      6      4
       35-50A     10    8    8      6      6      4      4      4
       50-65A      8    8    6      4      4      4      4      2
       65-85A      6    6    4      4      2      2      2      0
      85-105A      6    6    4      2      2      2      2      0
     105-125A      4    4    4      2      2      0      0      0
     125-150A      2    2    2      2      0      0      0     00

If aluminum wire is used instead of copper wire, the next larger size
(smaller number) should be used.  You should also consider the
installation demands: will you need to run the wire around corners or
through doors or into the engine compartment? These sorts of problems
in the car audio application require some special care in cable
selection.  You will want to have cable that is flexible; it should
have thick insulation as well, and not melt at low temperatures.  You
don't want to install wire that is rigid and prone to cracks and cuts,
or else the results could literally be explosive.

2.4    What is the best speaker wire to use? [JSC, JW]

Again, there is much debate over the benefit of the various schemes
that are being used by different manufacturers.  In general, however,
you will probably want to upgrade your speaker wire from the factory
~20 gauge to something bigger when you upgrade your amplifiers and
speakers.  In most cases, 16 or 18 gauge should be sufficient, with the
possible exception of high-power subwoofers.  According to an example by
Jerry Williamson, using 18 gauge instead of 12 gauge would only result
in a power loss of 0.1dB, which is essentially undetectable by humans.
Thus, other factors play more important roles in the selection of
speaker wire.  One issue is that different wires will have different
line capacitances, which could cause the wire to act as a low pass
filter.  Generally, however, the capacitances involved are so small that
this is not a significant problem.  Be sure to heed the warnings above
regarding cable flexibility and insulation, especially when running
wire into doors and other areas with an abundance of sharp metal.

2.5    I heard that I should run my power wire directly to my car's
       battery. Why should I bother, and how do I do it? [JSC]     

For some components, like head units and equalizers, it's acceptable to
use the stock wiring for power.  However, amplifiers generally require
large amounts of power, and accordingly will draw large amounts of
current.  The factory wiring in most cars is not designed to handle
large amounts of current, and most wires have 10-20A fuses on them.
Thus, you will almost always want to run the power line for your
amplifier directly to the positive terminal of the battery.  This could
require drilling a hole through the car's firewall, or at least
spending time hunting for an existing hole (the steering column is a
good place to start looking).  Always remember to place a fuse on your
wire as near to the battery as possible! For various reasons, such as
an accident or simple wear and tear, your wire's insulation may
eventually crack, which could allow the conducting wire to make contact
with the chassis of the car and short the battery through this wire,
which could lead to a serious fire.  The closer you place a fuse to the
battery, the more protected you are.  Also, when running wire through
areas with sharp metal corners, it is a good idea to use rubber
grommets to provide extra protection against tearing through your
wire's insulation.

2.6    Should I do the same thing with my ground wire, then? [JSC,

No.  In almost every case, the best thing to do is to ground your
amplifier to a point that is attached to the chassis of the car and is
as close to the amplifier as possible.  The ground wire should not need
to be more than about eighteen inches long, and should be at least as
large as the power wire.  The point to which you make your ground
connection should be an unpainted piece of bare metal.

Some cars (Audi, Porsche) have galvanized bodies, and in these cars,
you must find one of the manufacturers' grounding points or else some
noise can result.

2.7    Sometimes when I step out of my car, I get a really bad shock.
       What is wrong with my system? [IDB]                         

Nothing.  This is caused by static buildup by rubbing against the seats,
floor mats, etc., just like walking across a carpet in a home.  You can
avoid this shock by touching something metal on your car _before_ you
put your foot on the ground.

2.8    When my car is running and I have the music turned up loud,
       my headlights dim with the music. Do I need a new battery or
       a new alternator? [CD, MO]                                  

The headlights will dim because of a momentary drop in the voltage
level that is available to power the vehicle's accessories, including
the headlights, amplifiers, the engine, etc.  This voltage drop can be
caused by a very large current demand by an accessory, such as an
amplifier trying to reproduce a loud bass note.

The first thing to do is to get your battery and alternator checked for
proper functioning.  A failing battery can place undesirable loads on
the alternator, leaving less power for your system.

If the power system appears to be working correctly, an improved
alternator may be required for the large current demands of the audio
system.  When upgrading an alternator, be careful in your purchase, for
there are some potential problems.  An alternator which advertises a
certain output level may only achieve that output at very high engine
RPM ranges, for instance.  Also, the new alternator must be adjusted to
provide an output voltage within a reasonable range in terms of the
voltage regulator.

If you find your car will not start after playing the stereo for long
periods of time with the engine off, and the present battery is in good
working order, then another, paralleled battery could prevent this
embarrassing problem.

2.9    What is a "stiffening capacitor", and how does it work? [JSC]

"Stiffening Capacitor" (note capitals) is a trademark of Autosound
2000.  However, "stiffening capacitor" (note lowercase), as a generic
term, refers to a large capacitor (several thousand microfarads or
greater) placed in parallel with an amplifier.  The purpose of doing so
is to provide a sort of reserve power source from which the amplifier
can rapidly draw power when it needs it (such as during a deep bass
note).  The electrical theory is that when the amplifier attempts to
draw a large amount of current, not only will the battery be relatively
slow to respond, but the voltage at the amplifier will be a little lower
than the voltage at the battery itself (this is called "line drop").  A
capacitor at the amplifier which is charged to the battery voltage will
try to stabilize the voltage level at the amplifier, dumping current
into the amplifier.  Another way to think about it is that a capacitor
in parallel with a load acts as a low pass filter (see Section 3.10),
and the voltage level dropping at the amplifier will appear as an AC
waveform superimposed upon a DC "wave".  The capacitor, then, will try
to filter out this AC wave, leaving the pure DC which the amplifier

The following sections provide more detail about when and why to use a
stiffening capacitor.

2.9.1  Do I need a capacitor?  [MZ]

Before installation, it's often difficult to predict whether or not a
capacitor will be beneficial to you.  It's generally best to install
the audio equipment prior to making the determination, so that you can
address which symptoms need to be remedied and assess the severity of
the symptoms.  This will not only help you decide whether or not you
need a capacitor, but also how much capacitance would be beneficial.

The most common symptom in need of added capacitance is headlight
dimming (and sometimes dimming of the interior/dash lights).  It's
caused by a drop in system voltage associated with excessive current
draw.  While there may indeed be several loads drawing substantial
amounts of current from the electrical system (eg. heat, AC, and so
forth), it's usually the transient draws that best manifest themselves
in noticeable dimming.  This is partly because our visual systems are
most sensitive to detecting rapidly changing intensity levels rather
than steady absolute differences.

Once you've assessed whether or not the dimming is noticeable (and
sufficiently annoying), you must decide whether a capacitor is
warranted or if you'd be better served by upgrading the alternator.
After initially having your alternator and battery checked out (some
places will do this for free), the choice should be based on the
severity of the dimming.

A commonly-used estimate for determining the appropriate size capacitor
is 1F/kW (one farad per kilowatt).  For example, a system running at
300W would need a 0.3F (or 300,000uF) capacitor.  However, there are
several variables at play here, including the capabilities of the
vehicle's electrical system (which generally varies from idle to higher
RPMs), the efficiency of the amplifiers, and the listening habits of
the user (ie. the tone controls and the type of music).  These factors
should all be considered when making the determination.  Moreover, the
voltage drop can be so severe that added capacitance is nothing more
than a band-aid.  That is, even several Farads of capacitance would not
be able to sustain the voltage for as long as the drop persists.  This
is when an alternator upgrade may be in order.

2.9.2  Can I just upgrade my headlight wiring instead?  [MZ]

Although headlight wiring upgrades can often be beneficial for
achieving a higher steady-state illumination, it will not improve the
dimming situation.  Since the headlights are not the cause of the
voltage fluctuations that are producing the dimming, upgrading the
wiring will not fix the problem.  The voltage fluctuation is present at
the battery terminals, so it will be transmitted to the headlights
regardless of how the headlights are wired.  If you think of the
fluctuation as an AC signal, then it becomes readily apparent that this
circuit can be represented by an AC signal in a voltage divider.
Decreasing the resistance in series with the load by upgrading the
headlight wiring actually serves to slightly enhance the AC signal at
the headlight's terminals.  In other words, the dimming effect could
become even worse by upgrading the headlight wiring!

2.9.3  Will the dimming go away if I upgrade the amplifier power/ground
       wiring? [MZ]                                                

A common myth in the car audio community is that upgrading the power or
ground wire to the amplifier will result in the amplifier drawing less
current and therefore decreasing the voltage fluctuation.  While the
logic is sound, the premise is not.  Most amplifiers on the market have
semi-regulated supplies which don't maintain a steady power output at a
range of supply voltages.   This is reflected in the power ratings
provided by many manufacturers; some provide ratings for their
amplifiers at two different voltages, and the lower voltage almost
always causes the amp to deliver less power.  In general, the
difference in power output tends to correspond well with the supply
voltage such that the current draw remains roughly constant (assuming
somewhat similar efficiency).  Consequently, upgrading the power/ground
wiring, which serves to increase the voltage at the amplifier's
terminals, will not reduce headlight dimming.

2.9.4  What do I look for when buying a capacitor?  [MZ]

The single most important attribute is the capacitance value (expressed
in Farads).  Put simply, more is better.  Another important
consideration is to make sure the maximum voltage rating of the
capacitor safely exceeds the operating voltage of your vehicle's
electrical system.  In addition, ESR and ESL values may be provided
with some capacitors to essentially indicate the amount of voltage drop
that occurs when a capacitor is delivering current.  Smaller values are
better in this regard.

2.9.5  How do I install a capacitor?  [MZ]

If you conclude that your best course of action is to install a
capacitor, it should be installed in parallel with the amplifier and,
generally speaking, should be wired with approximately the same gauge
wire used for a single amplifier (usually 8 ga. is sufficient even for
rather large capacitors).

Before permanently installing it, it must be charged.  Failure to do so
could lead to blown fuses and lots of sparks!  Some capacitors come
with charging resistors.  If yours does not, you can simply buy an
automotive bulb and wire it in series with the capacitor's + lead while
the capacitor is grounded.  The bulb will continue to dim until the
capacitor is fully charged.  Once the capacitor is charged, it should
be treated as you would a car battery; caution must be used to be sure
not to short the terminals.

The final step is to permanently install it into the car.  There's been
much debate about where to install the capacitor.  It's been argued
that the placement is important because it requires shorter wire
lengths.  While this is true, there has never been any evidence
supporting the notion that it should be installed as close
(electrically) to the amplifier as possible.  In fact, electrical
theory demonstrates that it's more effective at quenching the dimming
effects by installing it as close to the device exhibiting the symptom
(ie. the headlights) rather than the device that's drawing the bulk of
the current (ie. the amplifiers).  However, the benefit to doing so is
negligible.  Therefore, hooking it directly to the battery, the
amplifier terminals, or the distribution block are equally valid
solutions as long as the mounting location is safe, the wire lengths
are reasonably short, and there's an adequate ground present.

2.9.6  I have more than one amp in my audio system. Which one
       should I have the capacitor run? [MZ]                       

The amplifiers are all connected in one way or another to the battery.
In fact, unless you're running separate power wires to each amplifier
all the way from the battery, they're usually connected at a more
proximal site (a distribution block, for example).  The effects of the
capacitor are felt by the entire electrical system, including the
amplifiers.  Therefore, you cannot selectively dedicate a capacitor to
a specific amplifier.

2.9.7  Will my bass response improve by adding a capacitor? [MZ]

A capacitor serves to smooth the voltage fluctuations associated with
transient current draw.  As a result, the supply voltage presented to
the amp during peak demands tends to be slightly higher than without
the capacitor.  For most amplifiers, this will increase the power
output of the amplifier during transients.  The degree to which it
increases, however, typically leads to an inaudible improvement.

To illustrate, if you consider an amplifier that delivers 100 watts at
14v and 80 watts at 12v (these numbers are somewhat typical), the
difference in output from the speaker will be at best 1 dB when the
supply voltage fluctuates from 14v to 12v.  However, when you take into
account the fact that no practical amount of capacitance can completely
eliminate this voltage drop during transients, the difference in output
becomes even less pronounced.  Further, if you take into account other
factors such as loudspeaker power compression (discussed elsewhere in
the FAQ), the equivalent series impedance of the capacitor, the length
of the transient, and the human's decreased ability to perceive
differences in intensity for shorter intervals, this difference in
output becomes negligible.

2.10   When should I upgrade my battery or add a second battery? [IDB]                                                       

The battery is most important when the engine is turned off, because it
supplies all of power to the audio system.  The stock battery in your
car may not be up to the task of running a stereo with multiple (or
large) amplifiers if it can't supply enough current to the amplifiers.
Upgrading your current battery to a larger model may help solve the
problem because batteries like the Optima 800 offer a larger number of
cold cranking amps.

Generally, adding a second battery is great if you want to listen to
your stereo with the car turned off (and be able to start the car again
later!).  This is accomplished using a dual-battery isolator: a device
which allows the second battery to be charged by the alternator, but
prevents the amplifiers that are connected to the second battery from
drawing any power from the main battery.  Installing a second battery
may be done instead of upgrading the main battery.

        ian d bjorhovde           ->
     internet mobile audio     ->

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