Archive-name: car-audio/part1
Rec-audio-car-archive-name: FAQ/part1
Version: 3.1
Last-modified: 5 September 95


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+ Frequently Asked Questions with answers for rec.audio.car +
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      0 Introduction and Table of Contents

	This is the FAQ list for the Usenet newsgroup rec.audio.car,
	maintained by Ian D. Bjorhovde (ianbjor@nsma.arizona.edu), 
	with contributions from many other people (see the credits
	section).  The contents of this document are based on the
	contributors' opinions; neither the contributors nor the FAQ
	maintainer accept any responsibility or liability for any
	damages brought about by the information contained herein.

	This document may be freely distributed and reproduced as long
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	If you have suggestions for improvements to this document, or
	if you fail to understand any part of it, please feel free to
	send a note to the FAQ maintainer or to the author of the
	relevant section.  The initials of the author(s) of each
	section can be found in brackets following each question.

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	Changes to the FAQ
	======= == === ===	

        Additions since the last posting (12/6/95)
        -------------------------------------------
	
        Changes since the last posting (12/6/95)
        -----------------------------------------


	Table of Contents
	===== == ========

	1 Definitions
	   1.1 What do all of those acronyms mean (A, V, DC, AC, W, Hz,
	       dB, SPL, THD, ohm)?
	   1.2 What is meant by "frequency response"?
	   1.3 What is a "sound stage"? 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?
	   2.2 What is the best power wire to use?
	   2.3 What is the best speaker wire to use?
	   2.4 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?
	   2.5 Should I do the same thing with my ground wire, then?
	   2.6 Sometimes when I step out of my car, I get a really bad
	       shock.  What is wrong with my system?
	   2.7 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?
	   2.8 What is a "stiffening capacitor", and how does it work?
	   2.9 Should I install one in my car? If so, how big should it
	       be, and where do I get one?
	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 is "bridging"? Can my amp do it?
	   3.6 What is "mixed-mono"? Can my amp do it?
	   3.7 What does "two ohm stable" mean? What is a "high-current"
	       amplifier?
	   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 How do I design my own passive crossovers?
	  3.13 Should I buy an equalizer?
	  3.14 What are some good (and bad) brands of equalizers?
	  3.15 What do all of those specifications on tape deck head
	       units mean?
	  3.16 What are features to look for in a tape deck?
	  3.17 What are some good (and bad) brands of tape decks?
	  3.18 What are features to look for in a CD head unit?
	  3.19 Should I buy a detachable faceplate or pullout CD player?
	  3.20 What are some good (and bad) brands of CD head units?
	  3.21 Can I use my portable CD player in my car? Won't it skip
	       a lot?
	  3.22 What's that weird motor noise I get with my portable CD
	       player?
	  3.23 What are some good (and bad) brands of portable CD
	       players?
	  3.24 What's in store for car audio with respect to MD, DAT and
	       DCC?
	  3.25 Are those FM modulator CD changers any good? What are my
	       other options?
	  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 How do I build my own passive crossovers?
	4 Subwoofers
	   4.1 What are "Thiele/Small parameters"?
	   4.2 What are the enclosure types available?
	   4.3 Which enclosure type is right for me?
	   4.4 How do I build an enclosure?
	   4.5 What driver should I use?
	   4.6 Is there any computer software available to help me choose
	       an enclosure and a driver?
	   4.7 What is an "aperiodic membrane"?
	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!
	6 Competition
	   6.1 What is IASCA, and how do I get involved?
	   6.2 What is USCA, 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?
	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


      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 (A, V, DC, AC, W, Hz, dB,
	SPL, THD, ohm)? [JSC]

	"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 running.

	"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 RMS
	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 frequency).

	"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 power.

	"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.

	"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.

    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.
       

        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 pwoer supply on the amp.  Try 
	connecting an isolated power supply - if this gets rid of the noise, 
	then there is something seriously wrong with the amp, and it should
        be replaced.  However, if the noise is still present, then there may 
	be a problem with power supply filtering or isolation.  This can be 
	fixed by changing the amp's gound point or b adding external supply
        filtering.
       

        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.
       

        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.


        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 definately quiet, so continue with level 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.
       
       
        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 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 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.

    2.3 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.

    2.4 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.

    2.5 Should I do the same thing with my ground wire, then? [JSC, IDB]

	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.

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

	Probably nothing.  This is usually caused by static buildup by
	rubbing against the seats, floormats, etc., just like walking
	across a carpet in a home.  Shocks which can be felt are
	usually in the kilovolt range, so touching a 12VDC wire isn't
	going to do much to you.

    2.7 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.

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

	"Stiffening Capacitor" (note capitals) is a trademark of
	Autosound 2000 (see 7.2).  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 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 requires.

    2.9 Should I install one in my car? If so, how big should it be,
	and where do I get one? [JSC]

	If you have a problem with dimming headlights when you have
	your music turned up and the bass starts to hit and the engine
	is running and you don't want to upgrade your alternator, or if
	the transient response of your amplifier is unacceptable to
	you, a stiffening capacitor could help you out.  The commonly
	accepted "formula" for determining the proper size capacitor to
	use is 1F/kW (one farad per kilowatt).  For example, a system
	running at 300W would need a 0.3F (or 300,000uF) capacitor.  To
	install the capacitor, you should not simply attach it to your
	power and ground wires near your amplifier, as it will draw
	very large amounts of current from your battery and could blow
	fuses (or overcharge).  Instead, you should insert a small-value 
	power resistor (25 ohm, 1/2 watt) or a 12VDC test lamp in between 
	the power lead and the capacitor, and then charge it.  If you use 
	a lamp in series with the cap, when the lamp goes out, the capacitor 
	is done charging.  When it is done charging, carefully remove the 
	capacitor's leads from the charging circuit, being certain not to 
	touch the two leads together.  You may then permanently install the 
	capacitor by wiring it in parallel with your amplifier (be careful 
	not to short the leads!).  Large caps are currently available from
	some audio dealers, such as Phoenix Gold.  You could also try
	electronics shops or mail-order houses.


--
Ian D. Bjorhovde                  UofA Div. of Neural Systems, Memory & Aging
ianbjor@NSMA.Arizona.EDU                           UNIX Systems Administrator
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