Description

This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.

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.

Continue to:

• prev: 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]
• Index
• next: 2.9.1 Do I need a capacitor? [MZ]