From the rec.car.audio newsgroup faq (part 1 of 5)
http://www.faqs.org/faqs/car-audio/part1/
2.9 What is a "stiffening capacitor", and how does it work? [JSC]
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"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]
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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]
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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]
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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]
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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]
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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]
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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]
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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.
