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Subject: rec.audio.car FAQ (Part 1/5)

This article was archived around: Tue, 9 Aug 2005 06:30:26 +0000 (UTC)

All FAQs in Directory: car-audio
All FAQs posted in: rec.audio.car, uk.rec.audio.car
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Archive-name: car-audio/part1 Rec-audio-car-archive-name: FAQ/part1 Version: 4.55 Last-modified: 08 August 2005
This is rac-faq.info, produced by makeinfo version 4.7 from rac-faq.texi. rec.audio.car Frequently Asked Questions **************************************** $Revision: 4.55 $ August 2005 This is the FAQ list for the Usenet newsgroup rec.audio.car, maintained by Ian D. Bjorhovde <ianbjor@mobileaudio.com>, with contributions from many other people (see Section 8). 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 as it remains wholly unaltered and includes this notice. If you do redistribute this document, especially on a commercial basis, please contact the FAQ maintainer before doing so. 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. This document is posted once per month to the Usenet newsgroups rec.audio.car, rec.answers, and news.answers, and is available via anonymous FTP from: `ftp://rtfm.mit.edu/pub/usenet/rec.audio.car/FAQ/' It is also available on the World Wide Web (WWW) and can be accessed using the URL `http://www.mobileaudio.com/rac-faq/' If you do not have access to FTP or a WWW client (such as Mozilla or Internet Explorer) and cannot find the FAQ on any of the aforementioned newsgroups, you can obtain the FAQ via email by sending email to <mail-server@rtfm.mit.edu> with the following three lines in the message body: `send usenet/rec.audio.car/FAQ/part1' `send usenet/rec.audio.car/FAQ/part2' `send usenet/rec.audio.car/FAQ/part3' `send usenet/rec.audio.car/FAQ/part4' `send usenet/rec.audio.car/FAQ/part5' Note that the rtfm.mit.edu server is a very popular machine, and accordingly, responses to requests may be slow, especially via the mail-server. Please be patient when using this service. Changes to this document are listed in the last section. *Note Changes::. 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 wiring? 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 speakers? 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" 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 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 mean? 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 player? 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 DCC? 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 speakers? 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 Plexiglas? 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 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 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 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. 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 amplifier. 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 standard. `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, 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. 2.7 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 requires. 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 -> ianbjor@mobileaudio.com internet mobile audio -> http://mobileaudio.com