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Archive-name: car-audio/part2
Rec-audio-car-archive-name: FAQ/part2
Version: 4.54
Last-modified: 05 March 2004
3 Components
*****************
This section describes various components that you can have in a car
audio system, along with common specifications, desirable features,
some of the best and worst brands, and so on.
Be aware that there is no standardized testing mechanism in place for
rating car audio products. As such, manufacturers are open to
exaggerating, "fudging", or just plain lying when it comes to rating
their own products.
3.1 What do all of those specifications on speakers mean? [JSC, CD]
======================================================================
"Input sensitivity" is the SPL the driver will produce given one watt
of power as measured from one meter away given some input frequency
(usually 1kHz unless otherwise noted on the speaker). Typical
sensitivities for car audio speakers are around 90dB/Wm. Some
subwoofers and piezo horns claim over 100dB/Wm. However, some
manufacturers do not use true 1W tests, especially on low impedance
subwoofers. Rather, they use a constant voltage test which produces
more impressive sensitivity ratings.
"Frequency response" in a speaker refers to the range of frequencies
which the speaker can reproduce within a certain power range, usually
+/-3dB.
"Impedance" is the impedance of the driver (see Section 1.1), typically
4 ohms, although some subwoofers are 8 ohms, some stock Delco speakers
are 10 ohms, and some stock Japanese imports are 6 ohms.
"Nominal power handling" is the continuous power handling of the
driver. This figure tells you how much power you can put into the
driver for very long periods of time without having to worry about
breaking the suspension, overheating the voice coil, or other nasty
things.
"Peak power handling" is the maximum power handling of the driver.
This figure tells you how much power you can put into the driver for
very brief periods of time without having to worry about destroying it.
3.2 Are component/separates any better than fullrange or coaxials?
[JSC, DK]
======================================================================
Usually, yes. Using separates allows you to position the drivers
independently and more carefully, which will give you greater control
over your imaging. For best results, try to keep the mid and tweeter
as close together as possible - this will make the two drivers act more
like a single point source (which is ideal).
For rear fill applications, however, coaxial speakers will perform
fine, as imaging is not a primary concern. However, it is very common
to use a low pass crossover with the rear speakers (at 2500 Hz) since
rear-fill is intended to produce "ambiance," and high frequencies (>
2500 Hz) can confuse the soundstage, making it appear that music is
originating behind you.
3.3 What are some good (and bad) brands of speakers? [JSC]
=============================================================
People will emotionally defend their particular brand of speakers, so
asking what the "best" is is not a good idea. Besides, the best
speaker is the one which suits the application the best. In general,
however, various people have claimed excellent experiences with such
brands as Boston Acoustics, MB Quart, a/d/s/, and Polk. Also, most
people agree that you should avoid brands like Sparkomatic and Kraco at
all costs.
3.4 What do all of those specifications on amplifiers mean? [JSC,
BG]
====================================================================
"Frequency response" refers to the range of frequencies which the
amplifier can reproduce within a certain power range, usually +/-3dB.
"Continuous power output" is the power output of the amplifier into one
channel into a certain load (usually four ohms) below a certain
distortion level (usually at most 1%THD) at a certain frequency
(usually 1kHz). A complete power specification should include all of
this information, e.g. "20W/ch into 4 ohms at < 0.03%THD at 1kHz"
although this can also be stated as (and be assumed equivalent to)
"20W/ch at < 0.03%THD". The amplifier should also be able to sustain
this power level for long periods of time without difficulties such as
overheating.
"Peak power output" is the power output of the amplifier into one
channel into a certain load (usually four ohms) below a certain
distortion level (usually much higher than the continuous rating level)
at a certain frequency (usually 1kHz). A complete power specification
should include all of this information, e.g. "35W/ch into 4 ohms at <
10.0%THD at 1kHz" although this can also be stated as (and be assumed
equivalent to) "35Wch at < 10.0%THD". Consumer warning: some
manufacturers will state the "peak power output" rating by including
the amount of power which can be drawn from "headroom", which means
power supply capacitors. They usually will not tell you this in the
specification, however; indeed, they tend to prominently display the
figure in big, bold letters on the front of the box, such as "MAXIMUM
200W PER CHANNEL!!!" when the continuous rating is 15W/ch and the unit
has a 5A fuse.
"Damping factor" represents the ratio of the load being driven (that
is, the speaker - usually four ohms) to the output impedance of the
amplifier (that is, the output impedance of the transistors which drive
the speakers). The lower the output impedance, the higher the damping
factor. Higher damping factors indicate a greater ability to help
control the motion of the cone of the speaker which is being driven.
When this motion is tightly controlled, a greater transient response is
evident in the system, which most people refer to as a "tight" or
"crisp" sound. Damping factors above 100 are generally regarded as
good.
"Signal to Noise" or "S/N" is the ratio, usually expressed in decibels,
of the amount of true amplified output of the amplifier to the amount
of extraneous noise injected into the signal. S/N ratios above 90 to
95dB are generally regarded as good.
3.5 What does "bridging an amp" mean? [MHa]
==============================================
"Bridging" refers to taking two channels of an amplifier and combining
them to turn the amplifier into a one channel amplifier.
3.5.1 Why should I bridge my amp?
----------------------------------
For increased power. If your amp can handle the load, it will put out
more power through a bridged channel than it would into through a
non-bridged channel. Theoretically, a "perfect" amplifier that puts
out X watts into Y impedance into each of two channels will put out 4X
watts into Y impedance into one bridged channel. Be aware that some
amps more closely approximate that perfect amp than others, and some
manufacturers build current limiters into their amps to allow them to
remain stable into difficult loads at the expense of power gains.
3.5.2 Why shouldn't I bridge my amp?
-------------------------------------
There are several reasons: you might need those extra channels; your
amp might not be stable into the load your speakers present if the amp
is bridged; you might be a hyper-perfectionist that can't stand the
thought of an small increase in distortion; or perhaps you just don't
need more power. Car audio power is relatively cheap, and if you are
not trying to make a mega-gonzo system, you may not need to double your
power.
3.5.3 What happens when an amp is bridged?
-------------------------------------------
Basically, one channel's signal is inverted, and then the two channels
are combined to form one channel with twice the voltage of either of
the original channels.
Ohm's Law for Alternating Current states that I = V/Z where I is
current, V is voltage, and Z is impedance. We also know that P = IV,
where P is power. If we use Ohm's Law and substitute into the power
equation, we get P = V(V/Z), which can be rewritten as P = (V^2)/Z.
Therefore, power is the square of voltage divided by impedance.
Now, why do we care about all that? Because it explains precisely what
happens when an amp is bridged. I'll give a practical example and
explain the theoretical basis of that example.
Imagine you have a two-channel amp that puts out 50 watts into each
channel when driven into a load of 4 ohms per channel. Since we know P
and Z, we can plug these numbers back into our power equation and find
V. 50 = V^2/4 -> V = sqrt(200). So, we're seeing a voltage of 14.1
volts across each channel.
Now, imagine we bridge this amp, and use it to push just one of those 4
ohms loads. When the amp is bridged, the voltage is doubled. Since we
know the voltage (2*14.1 volts), and the impedance (4 ohms), we can
calculate power. Remember that P = V*V/Z. That means P = (28.2)^2/4,
which is 198.1 watts. It should be clear by now that the new power is
approximately 200 watts - quadruple the power of a single, unbridged
channel!
You can probably see that should be the case, especially if you look
back at the power equation. Since P = V*V/Z, if you double V, you
quadruple power, since V is squared in the power equation.
Now, all this assumes the amp is stable into 4 ohms mono. The mono
channel is putting out four times as much power as a single unbridged
channel, so it must be putting out twice as much as the two single
channels combined. Since the voltage on the supply side of the amp is
dependent on the car's electrical system, it doesn't change (OK, the
increased current might cause a voltage *drop*, but let's not worry
about that now). Looking at the first power equation, at the supply
side of the amp, we see P = IV. Now, when we bridged the amp, we
doubled the power, but the input voltage stayed the same. So, if we
hold V constant, the only way to double the power is to double the
current.
That means the amp is now drawing twice as much current when it's
running at a given impedance mono than it would be running two stereo
channels at the same impedance. There are only two ways the amp can do
that - it can simply draw more through it's circuits, and dissipate the
extra heat, or it can utilize a current limiter, to prevent the
increase in current. Of course, using the current limiter means you
don't get the power gains, either! So, if the amp can't handle the
extra current, and it doesn't limit the current in some way, kiss it
goodbye. For that reason, an amp is typically considered mono stable
into twice the impedance it is considered stereo stable.
3.5.4 Does bridging an amp would halve the impedance of the speakers?
----------------------------------------------------------------------
Impedance is a characteristic of the speakers. The speakers don't give
a flip how the amp is configured: they have a given impedance curve,
and that's that. It should be clear that when you bridge an amp, you
are changing *the amp*. The speaker's impedance is *not* a function of
the amp, but the amp's tolerance to a given impedance depends
completely on the way the amp is configured. If you'll remember from
section 4, an amp bridged into a given impedance draws twice as much
current as it would if it were driving two separate channels, each at
that impedance. So, a four ohm speaker stays a four ohm speaker, if
it's hooked to one channel, a bridged channel, a toaster, or the wall
socket. But, it is more stressful for the amp to drive any impedance
bridged than unbridged.
So, why do people talk about the impedance halving? Well, it's a
simple model that isn't correct but is easy to explain to people who
don't know what's really going on. It goes like this: When you bridge
the amp, each channel is "seeing" half the load presented to the amp.
So, if you bridge an amp to 4 ohms, each channel "sees" 2 ohms.
Therefore, each channel puts out twice as much power, and the combined
output is quadruple a single channel at 4 ohms.
Why is that still wrong? Because each channel isn't really used as a
single channel. You've used part of one channel, and an inverted part
of another channel to create a totally new channel, the bridged
channel. Also, there's no way for a channel to "see" only part of a
circuit. If it's "seeing" half the speaker, it's "seeing" it all.
Second, it makes it awkward if people believe that the impedance is
really, literally, changing. If you use that model, is it safe to run
a 4 ohm mono stable amp into a 4 ohm speaker? It should be, but we
just said the impedance halves, so that's now a 2 ohm speaker, and you
can't use it. That's wrong, and confusing, and it makes people think
they can't do things they really can.
3.5.5 Can I bridge my 4 channel head unit?
-------------------------------------------
Generally, NO. Unless the manuals that came with your head unit
specifically state that your head unit can be bridged, then do NOT
attempt it - this could destroy the head unit's internal amplifier, and
possibly void your warranty.
3.6 What is "mixed-mono?" Can my amp do it? [JSC, IDB]
=========================================================
Some amplifiers which are both bridgeable and able to drive low
impedance loads also allow you to use "mixed-mono" mode. This involves
driving a pair of speakers in stereo mode as well as simultaneously
driving a single speaker in bridged mono mode off of ONE pair of the
amp's channels.
To do this, you connect the mono speaker (typically a subwoofer) to the
amp as you normally would in bridged mode, and then connect the left
and right stereo speakers to the left and right stereo channels,
respectively.
However, for this to work, the amplifier must actually use both input
channels in bridged mode. Many amplifiers, when placed in bridged
mode, will simply "copy" and invert either the left or the right
channel. This practice ensures high output to the mono speaker, but
eliminates the possibility of mixed mono since you lose one channel.
It is VERY important to use passive crossovers when configuring your
amplifier in mixed-mono mode in order to keep from overloading the amp.
The reason almost all new amplifiers are able to run in mixed-mono
mode (even if they are only 2-ohm stable) is that the impedance seen by
each channel of the amplifier is the same across the entire frequency
spectrum when using passive crossovers. Here's how it works: Take a
typical 2-channel amplifier that is stable to 2 ohms (stereo) or 4 ohms
(mono). When the subwoofer is connected with a low-pass crossover (at
100Hz, for example) then the amplifier "sees" a 2 ohm load on each of
its channels (see 3.5) from 100Hz and down. When the full range
speakers are connected with a high-pass crossover (at 125Hz, for
example), the amplifier "sees" a 4 ohm load on each of its channels from
125Hz and up. The passive crossovers prevent the amplifier from seeing
more than one speaker on either channel at any given frequency. Of
course, between the two crossover points the amp DOES see more than one
speaker (and therefore the load on the amp dips to 1.33 ohms when using
4 ohms speakers).
A graph of impedance vs frequency for ONE channel of an amplifier would
look similar to this when using 3 4-ohm speakers and crossover points
at 100Hz(LP) and 200Hz (HP):
+-----------------------------------------------------------+
| ****************************************| 4
| * |
| * |
|************* * | 2
| * * |
| * |
| | 1
| |
| |
+^-----^-----^-----^-----^-----^-----^-----^-----^-----^---^+ 0
25 50 100 200 400 800 1.6K 3.15K 6.3K 12.5K 20K
3.7 What does "two ohm stable" mean? What is a "high-current" amplifier?
[JSC]
===========================================================================
An X-ohm stable amplifier is an amp which is able to continuously power
loads of X ohms per channel without encountering difficulties such as
overheating. Almost all car amplifiers are at least four ohm stable.
Some are two ohm stable, which means that you could run a pair of four
ohm speakers in parallel on each channel of the amplifier, and each
channel of the amp would "see" two ohms. Some amps are referred to as
"high-current", which is a buzzword which indicates that the amp is
able to deliver very large (relatively) amounts of current, which
usually means that it is stable at very low load impedances, such as
1/4 or 1/2 of an ohm. Note that the minimum load rating (such as "two
ohm stable") is a stereo (per channel) rating. In bridged mode, the
total stability is the sum of the individual channels' stability *Note
Bridging::.
3.8 Should I buy a two or four (or more) channel amplifier? [JSC]
===========================================================================
If you only have one line-level set of outputs available, and wish to
power two sets of speakers from a single amplifier, you may be able to
save money by purchasing a two channel amplifier which is stable to two
ohms rather than spending the extra money for a four channel amp. If
you do this, however, you will be unable to fade between the two sets
of speakers (without additional hardware), and the damping factor of the
amplifier will effectively be cut in half. Also, the amp may run hot
and require fans to prevent overheating. If you have the money, a four
channel amp would be a better choice. You would need to add a dual-amp
balancer in order to maintain fader capability, however, but it is more
efficient than building a fader for a two channel amp. If you wish to
power a subwoofer or additional speakers as well, you may want to
purchase a five or six channel amp.
3.9 What are some good (and bad) brands of amplifiers? [JSC, IDB]
===========================================================================
As with speakers, people emotionally defend their amplifier, so
choosing the best is difficult. However, some brands stand out as
being consistently good while others are consistently bad. Among the
good are HiFonics, Phoenix Gold, a/d/s/, and Precision Power.
Generally, "good" amplifiers tend to cost more (in money/watt) than
"bad" amplifiers. So when you see an amp advertising 300W for only
$100, and are comparing an amp with 50W for $300, you will usually find
that the 50W/$300 amp will be of much higher quality than the 300W/$100
amp.
3.10 What is a crossover? Why would I need one? [JSC]
=======================================================
A crossover is a device which filters signals based on frequency. A
"high pass" crossover is a filter which allows frequencies above a
certain point to pass unfiltered; those below that same point still get
through, but are attenuated according to the crossover slope. A "low
pass" crossover is just the opposite: the lows pass through, but the
highs are attenuated. A "band pass" crossover is a filter that allows a
certain range of frequencies to pass through while attenuating those
above and below that range.
There are passive crossovers, which are collections of purely passive
(non-powered) devices - mainly capacitors and inductors and sometimes
resistors. There are also active crossovers which are powered
electrical devices. Passive crossovers are typically placed between
the amplifier and the speakers, while active crossovers are typically
placed between the head unit and the amplifier. There are a few
passive crossovers on the market which are intended for pre-amp use
(between the head unit and the amplifier), but the cutoff frequencies
(also known as the "crossover point", defined below) of these devices
are not typically well-defined since they depend on the input impedance
of the amplifier, which varies from amplifier to amplifier.
There are many reasons for using crossovers. One is to filter out deep
bass from relatively small drivers. Another is to split the signal in
a multi-driver speaker so that the woofer gets the bass, the midrange
gets the mids, and the tweeter gets the highs.
Crossovers are categorized by their order and their crossover point.
The "order" of the crossover indicates how steep the attenuation slope
is. A first order crossover "rolls off" the signal at -6dB/octave
(that is, quarter power per doubling or halving in frequency). A
second order crossover has a slope of -12dB/octave; third order is
-18dB/octave; etc. The "crossover point" is generally the frequency at
which the -3dB point of the attenuation slope occurs. Thus, a first
order high pass crossover at 200Hz is -3dB down at 200Hz, -9dB down at
100Hz, -15dB down at 50Hz, etc.
It should be noted that the slope (rolloff) of a crossover, as defined
above, is only an approximation. This issue will be clarified in
future revisions of this document.
The expected impedance of a passive crossover is important as well. A
crossover which is designed as -6dB/octave at 200Hz high pass with a 4
ohm driver will not have the same crossover frequency with a driver
which is not 4 ohms. With crossovers of order higher than one, using
the wrong impedance driver will wreak havoc with the frequency
response. Don't do it.
3.11 Should I get an active or a passive crossover? [JSC, JR]
===============================================================
Active crossovers are more efficient than passive crossovers. A
typical "insertion loss" (power loss due to use) of a passive crossover
is around 0.5dB. Active crossovers have much lower insertion losses,
if they have any loss at all, since the losses can effectively be
negated by adjusting the amplifier gain. Also, with some active
crossovers, you can continuously vary not only the crossover point, but
also the slope. Thus, if you wanted to, with some active crossovers
you could create a high pass filter at 112.3Hz at -18dB/octave, or
other such things.
However, active crossovers have their disadvantages as well. An active
crossover may very well cost more than an equivalent number of passive
crossovers. Also, since the active crossover has separate outputs for
each frequency band that you desire, you will need to have separate
amplifiers for each frequency range. Furthermore, since an active
crossover is by definition a powered device, the use of one will raise
a system's noise floor, while passive crossovers do not insert any
additional noise into a system.
Many people find it advantageous to use both active and passive
crossovers. Often, a separate amp is dedicated to the subwoofers, to
give them as much power as possible. The other amplifier is used to
power the mids and tweeters. In this scheme, an active crossover is
used to send only the sub-bass frequencies to the sub amp, and the
other frequencies to the other amp. The passive crossovers are used to
send the correct frequencies to the individual speakers (e.g., mids and
tweeters).
Thus, if you have extra money to spend on an active crossover and
separate amplifiers, and are willing to deal with the slightly more
complex installation and possible noise problems, an active crossover
is probably the way to go. However, if you are on a budget and can
find a passive crossover with the characteristics you desire, go with a
passive.
3.12 Should I buy an equalizer? [JSC]
=======================================
Equalizers are normally used to fine-tune a system, and should be
treated as such. Equalizers should not be purchased to boost one band
12dB and to cut another band 12dB and so on - excessive equalization is
indicative of more serious system problems that should not simply be
masked with an EQ. However, if you need to do some minor tweaking, an
EQ can be a valuable tool. Additionally, some EQs have spectrum
analyzers built in, which makes for some extra flash in a system.
There are two main kinds of EQs available today: dash and trunk. Dash
EQs are designed to be installed in the passenger compartment of a car,
near the head unit. They typically have the adjustments for anywhere
from five to eleven (sometimes more) bands on the front panel. Trunk
EQs are designed to be adjusted once and then stashed away. These
types of EQs usually have many bands (sometimes as many as thirty).
Both types sometimes also have crossovers built in.
3.13 What are some good (and bad) brands of equalizers? [HK]
==============================================================
Generally, companies that produce 1/3 octave (30 band) and 2/3 octave
(15 band) equalizers are good. These include AudioControl, USD, Rane,
Phoenix Gold. Most people try to stay away from equalizers that
contain a "booster;" these are made by Kraco, Urban Audio Works and
others.
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? [IDB]
======================================================================
It is getting difficult to find pullout CD players any more, since
detachable faceplates are much more convenient to carry around.
However, there is the obvious trade off - it is still possible to steal
the chassis for the detachable face unit, when that is not possible
with a pullout. Although some companies will advertise that it is very
difficult to get replacement faces without the original receipt,
thieves can still get the faceplates.
Some companies, such as Eclipse, are starting to offer alternative
methods for preventing theft. Some Eclipse decks now offer ESN
(Eclipse Security Network), where the owner chooses a "key" CD that
must be inserted to "revive" the deck should it lose power. The entire
deck stays in the dash, with nothing to carry around; this expands on
the trend towards convenience while offering the owner peace of mind.
3.19 What are some good (and bad) brands of CD head units? [HK]
=================================================================
Generally, Alpine, Clarion, Eclipse, McIntosh, Phillips and Pioneer are
considered to produce good quality CD head units. They all have their
problems, but these brands seem to be common and relatively
problem-free.
Bad brands include Kraco, Radio Shack, Rockwood and other "bargain"
brands.
3.20 Can I use my portable CD player in my car? Won't it skip a lot?
[JSC]
======================================================================
You can use any portable CD player in a car provided that you have
either an amplifier with line level inputs (preferred) or a tape deck.
If you have the former, you can simply buy a 1/8" headphone jack to RCA
jack adapter and plug your CD player directly into your amplifier. If
you have the latter, you can purchase a 1/8" headphone jack to cassette
adapter and play CDs through your tape deck. The cassette adapters
tend to be far more convenient; however, there is a significant
tradeoff: by using cassette adapters, you limit your sound to the
frequency response of the tape head, which is sometimes as much as an
entire order of magnitude worse than the raw digital material encoded
onto the CD itself.
Portable CD players which were not designed for automotive use will
tend to skip frequently when used in a car (relatively). CD players
that are specially designed for automotive use, such as the Sony Car
Discman, tend to include extra dampening to allow the laser to "float"
across the bumps and jolts of a road. Some people have indicated
success with using regular portable CD players in a car when they place
the CD player on a cushion, such as a thick shirt or even on their
thighs.
3.21 What's that weird motor noise I get with my portable CD player?
[JSC]
======================================================================
Many people report problems while playing CDs from a portable CD player
into their car audio systems. The problem, stated very simply, has to
do with the stepping of the motor requiring a varying amount of current
and non-isolated power and audio signal grounds. Using a liberal
application of capacitors and inductors, this voltage variance can be
restricted to a window of 8.990 to 9.005V for a 9V CD player, yet even
the swing between these two levels is enough to cause annoyingly loud
noise on the outputs. It has been reported that this entire problem
can be solved by using a true DC-DC inverter at the power input to the
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?
[HK]
=====================================================================
MiniDisc (MD) seems to have a better future than Digital Audio Tape
(DAT) or Digital Compact Cassette (DCC) which don't seem to have appeal
to the public. Ease of use seems to be an important factor and the CD
formats allows direct access to musical tracks at an instant. Although
MD doesn't match the sound quality of the standard CDs it will probably
be popular since the players have a buffer to eliminate skipping. DAT
will remain as a media for ProAudio for recording purposes before
pressing CDs.
3.24 Are those FM modulator CD changers any good? What are my other
options? [PW, JGr]
=====================================================================
Almost all manufacturers offer an FM modulator for their changers. As
with all equipment, some are good and some are not. A person thinking
about using an FM modulator must take into consideration that the sound
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