Topic: Audio Capacitors - Stiffening Capacitors

From The Basics To The Benefits Of Audio Capacitors For Your Electrical System
If your headlights dim, your CD skips a beat, and your audio system distorts, your electrical system may not be up to par. It's likely your alternator is having a difficult time maintaining a running voltage of 14 volts, as your battery is not capable of holding the system voltage above 12 volts. This leaves you with a fluctuation of two or more volts, all the while killing your listening experience.

You need help and audio capacitors may just be the answer. The following tutorial will guide you through the importance of capacitors, how they work, and where and when to use them.

What are they?
Stiffening capacitors, often called "load leveling" devices, are becoming more and more common these days in the world of car audio, especially in high-end installations. These are commonly used as buffers between the battery and the amplifiers or electronic hardware. But the value of these is often misunderstood, the how and when to use them misconceived. For this, we will walk you through the ins and outs of capacitor technology.

In order to find our test subjects, we conducted a search on the World Wide Web. We found a plethora of sizes and styles of capacitors available from a variety of audio manufacturers. However, as our objective was to discover different technologies in the capacitance field and the workings behind them, the field narrowed quickly. We decided on the following four leaders in capacitor technology: Alumapro, Lightning Audio, Phoenix Gold, and Xstatic.

So the question remains: do they really have an effect on an audio system? We think you'll be surprised with our results.

What is a capacitor?
Like a battery, a capacitor is an energy storage device. While a battery stores and releases energy through an electro-chemical reaction, a capacitor stores energy via an electrostatic charge. As we will see, this difference in technology is the reason why capacitors can be helpful in an audio system.

How do capacitors work?
First we will start you off with a little tech jargon (repeat this to your friends, they will be amazed). The energy stored in a capacitor is measured in Joules, with the total energy stored = 11/42CV2 (see diagram below). This energy increases with the square of the voltage times the capacitance in farads.

In the simplest form, capacitors have two plates, each connected to a terminal separated by an insulator. When a cap is connected to a battery, the negative plate gains electrons from the battery while the positive plate loses electrons. This charge is called an electrostatic field. When the capacitor is discharged, the reverse happens. The capacitance increases with the size of the plates. As well, the capacitance will increase inversely with the distance between them.

Because the capacitance increases with surface area, large value capacitors must have a large surface area and a very short distance between the plates. In the past, a capacitance of 1 farad seemed huge. These were called aluminum electrolytic and were constructed of large rolls of aluminum foil with an electrolyte between them. The foil was coated with an oxide to increase the surface area. To further increase the surface area, the foil was then rolled. This is why most capacitors are cylindrical.

Electrochemical Double Layer CapacitorsIn recent years we've seen a giant leap in capacitance technology, also known as Electrochemical Double Layer Capacitors, or EDLC for short. EDLC capacitors have energy densities that are typically greater than 300 times that of the largest conventional capacitors.

EDLC capacitors, often termed supercapacitors or ultracapacitors, are based on an old technology first discovered in the 1950s but not developed until recent years. To get this quantum leap in capacitance, the surface area was increased by using porous carbon electrodes. The porous nature of these electrodes results in enormously high surface area in a typical EDLC said to be equivalent to that of a football field.

The theoretical capacitor is missing two degrading parameters: ESR and ESL. ESR stands for Equivalent Series Resistance, while ESL is Equivalent Series Inductance. Usually these two parameters are referred to as just ESR or Equivalent Series Reactance. When high instantaneous power is required from a capacitor, the ESR is the limiting factor.

A capacitor that charges to 14 volts dumping its charge into 11 volts could deliver 3/0.01 or 300 instantaneous amps if it has an ESR of 0.01 ohms.

In conventional capacitors, the long rolled aluminum results in relatively high resistance and inductance. Different construction techniques with multiple electrode connections have reduced the ESR in conventional capacitors. Fortunately the EDLC capacitor's construction results in a relatively low ESR.

Why all the hype about EDLC capacitors?
Well, all but one of the four capacitors we tested use some form of this technology (see page 76). It is because of this EDLC technology that we are now seeing supercapacitors in values of 15-50 farads! We took the opportunity to look at the internal workings of each of these with the exception of the AlumaPro 50 Farad that was sealed. As EDLC capacitors are low voltage devices, they use several banks (usually six to eight) strung together in series to obtain the required voltage. This series arrangement requires parallel resistors or other circuitry to divide the voltage among the series caps. The downside of these resistors is that it causes battery drain. Two of the capacitors, the Alumapro and Phoenix Gold, include a large turn-on relay to prevent this drain.

How Capacitors HelpWhile a battery can store much more energy for its size than a capacitor, it has an ESR that is substantially higher than a capacitor. This ESR limits a battery's ability to provide huge amounts of instantaneous power. Because capacitors absorb some of the transient power, a supercapacitor can extend battery life. Our charts and explanations following will help you understand when they help and when they have little effect.

Maximum SPL Competition Test
We measured power and captured waveforms using a test to simulate maximum SPL for competition. We did not test with thousands of watts as in typical SPL competition, but the results would be the same.

In this case an amplifier was driven into heavy clipping with a 50Hz sine wave. We measured the waveform without a charging system both with and without a 50-farad capacitor. The voltage at the amplifier power terminals dropped about 1.4 volts in both cases. The energy stored in the capacitor kept the battery voltage up for a short time, but not enough to make a significant difference in the power output. The addition of the 50-farad capacitor only gains three watts, less than one percent.

Next, we measured the ripple voltage and steady state power at 20 percent THD. While some smoothing of the battery voltage occurs, we again gain only a few watts.

The formula for a capacitor that relates voltage, current, time, and capacitance is:

So, a 50-farad cap discharging at the rate of 50 amps will drop one volt in one second. Increase this current to 500 amps and the cap would drop one volt in just 1/10 of a second. So, even these huge supercapacitors can't provide much more than short transient power. Remember, the amplifier also has some internal energy storage: input filter, output filters, and in the case of regulated amplifiers, output chokes. Because the energy stored in a capacitor increases with the square of the voltage,(By the way, this square law relationship is just one reason why I personally prefer 2-4 ohm systems - see, then, when are these caps most beneficial?

Technically Speaking
Capacitance: a) the property of an electric nonconductor that permits the storage of energy as a result of the separation of charge that occurs when opposite surfaces of the nonconductor are maintained at a difference of potential b) the measure of this property that is equal to the ratio of the charge on either surface to the potential difference between the surfaces

Dielectric: a nonconductor of direct electric current; an insulator

Electrolyte: a chemical compound that separates into ions in a solution that is able to conduct electricity

Electrolytic: relating to, containing, or consisting of electrolytes

Electrons: a negatively charged particle found in an atom. Each electron carries a negative electric charge of -1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31 kg.

Electrostatic Field: An electrostatic field is created when there are two surfaces in close proximity to each other and one surface has more electrons than the other. This imbalance is called an electrical charge. Electrons are those little guys in the orbits of atoms and electricity is the flow of these electrons. When an electrical charge exists, energy is stored as on the plates of a capacitor. When a conducting path exists, the electrons flow from one plate to the other, releasing the stored energy into this conducting path. A simple example is a lightning bolt: as electrons build up on the surface of the earth, a strong positive charge exists with respect to the clouds. When this charge finds a conductive path up to the clouds, the stored energy is released in the form of a lightning strike.

Farad: the unit of capacitance

Inductance: the property of an electric circuit or device whereby an electromotive force is created by a change of current in it or in a circuit near it

Insulator: a device made of an electrical insulating material and used for separating or supporting conductors

Joules: a unit of work or energy equal to the work done by a force of one newton acting through a distance of one meter

Resistance: the opposition offered by a body or substance to the passage through it of a steady electric current

ESR and Current Capacity
A perfect capacitor has no resistance. If you shorted a perfect cap, it would deliver an infinite amount of current for an infinitely short period of time. The ESR limits the current during this time to what Ohm's law predicts: I = E/R (I = Current, E = Voltage, R = Resistance). In simplified terms, ESR relates to the total instant power the capacitor is capable of producing.

For example, if a cap is charged to 14 volts with an ESR of 0.01 ohms of ESR, the instantaneous current is 14/0.01 = 1400 amps. But we are not shorting the cap - the amount of current it could deliver depends on two voltages, the voltage before the transient, and the battery voltage under heavy load, as well as other parameters such as the rise time of the transient. An ESR of zero is best and 0.01 is good.

Relating a Farad to a Capacitor
Capacitance, or the number of farads, relates to time and total energy stored. For example, the instantaneous current or power is determined by the ESR. How long the cap can deliver power (or total energy) is determined by its capacitance.

Power is measured in watts and does not factor in time. For example, a 100-watt light bulb is consuming 100 watts at any point in time. This is called power. A 100-watt bulb that is on for one hour has consumed 100 watt-hours of energy.

A farad could be compared to a battery's Ampere-hour measurement. This specifies the capacitor's ability to store energy.

Short term, transient power
In this test, we used a burst test of just 300 milliseconds. As can be seen, the battery voltage holds up much better during this short duration.

Stabilizing the System Voltage
The above measurements were taken without a charging system, with the battery voltage at about 12 volts. But what about the affect of the stiffening capacitor when the system is charging, with a battery voltage greater than 14 volts?

The ESR of a battery is much higher when it's being charged. A battery's natural voltage is about 12 volts. It is at this voltage that it can deliver real current. When it's being charged, its voltage is raised above 12, but it will quickly drop to 12 volts under a load. This is what we refer to as "surface charge."

When the amplifier draws a lot of power the battery can't deliver much current until it drops to 12 volts. That's why the lights flicker much more when the engine is running. The alternator is normally charging at 14.4 volts but when the amp draws a lot of current, the voltage briefly drops to 12. This is because the alternator's regulator is very slow to respond. I guess you could say that the battery isn't in the circuit until the voltage drops to its natural cell voltage.

The system resistance is primarily the resistance of the alternator and wiring when you are above 13 volts. The graphs bear this out:

As can be seen above, there's a big improvement in the battery voltage with the stiffening capacitor.

These supercapacitors help stabilize the system voltage. If your headlights flicker with the bass, the other audio components have to deal with these fluctuations as well. Depending on the head unit's internal regulation, voltage fluctuations can introduce distortion into the audio.

Installing a supercap near the amplifier greatly reduces the current pulses flowing from the front of the vehicle to the rear. These current pulses create huge magnetic fields that can easily introduce low frequency harmonic distortion into unbalanced RCA cables. I have measured low frequency distortion introduced into RCA cables by the currents in the power cable. In some instances it increased the distortion to 0.5 percent.

Capacitor Testing
Your garden-variety cap checker isn't going to work on these huge caps, so we'll describe our test methods as we go. In our lab, we used an Agilent constant current power supply to measure capacitance while viewing the result on a Tektronix digital storage scope. We used this method because it generates a straight line on the scope that improves our accuracy. To measure ESR, we discharged the cap through a 0.23-ohm resistor, again viewing the result with the Tektronix storage scope.

XStatic Supercap 100 Alumapro C.A.P. 50 Phoenix Gold PowerCore 20 Lightning Audio Strike LSD10DT XStatic Supercap 100 Alumapro C.A.P. 50 Phoenix Gold PowerCore 20 Lightning Audio Strike LSD10DT XStatic Supe

XStatic Supercap 100
The Supercap 100 capacitor is rated at 35 farads/16 volts. Xstatic packaged this in a relatively small chassis made of injection molded plastic. Considering this capacitor's weight registers the scales at slightly less than two pounds, it is very impressive!

Connections for this unit are simple. At the top are two gold-plated binding posts, positive and negative. With the exception of those there are no other connections.

This capacitor is constructed using twelve 100-farad, 2.7-volt EDLC capacitors in a series-parallel arrangement. This doesn't give us 1200 farads! When you series capacitors, the total value is equal to the total capacitance divided by the number of capacitor banks. Conversely, when you parallel them the capacitance is multiplied by the number of capacitors. This series-parallel arrangement is typical construction for supercapacitors as EDLC caps can individually withstand only about two to three volts each. Resistors must be used to properly divide the voltage among the caps. We measured about 60mA of leakage current due to this arrangement. Because of this "standby" current, you should disconnect this cap if your car is going to sit idle for more than a few weeks.

What about discharge and ESR (Equivalent Series Resistance), isn't that what really matters? To measure the ESR, we discharged the cap through a very low value resistor, 0.23 ohms. This value reflects the resistor value plus the wiring and switching resistance. The sudden drop in voltage, circled in the graph above, is caused by this ESR. During this initial pulse, the discharge current is about 60 amps. Using Ohm's law, we measured an ESR of 37 milliohms, or 0.037 ohms, for this capacitor.

You can find the Supercap 100 at:

Alumapro C.A.P. 50
The Alumapro C.A.P. 50 is the largest of the group, rated at 50 farads. Visually, the C.A.P. 50 is a stunner. The exterior is constructed of aluminum that has been anodized silver. An aluminum plate with the Alumapro logo is provided and has an adhesive backing so that it can be placed in a desired direction.

To one end, heavy duty machined metal blocks, plated in gold, make up the terminals. The terminals allow for up to three amplifiers to be connected. Just above the terminals are "Charging" and "Ready" indicators. Upon turn-on, the yellow "Charging" light indicates that the unit is indeed charging. Once the capacitor reaches about three-quarters charge, the green "Ready" light indicates that the C.A.P. 50 is fully charged and operational.

Between the power terminals is a trigger terminal that can be connected to the remote turn on lead from the radio or a +12Volt ignition source. Upon shut-down, both lights turn off and the terminals become internally isolated from the capacitor bank. This unique feature controls a huge internal relay with turn-on circuitry to eliminate the leakage current, so you won't have to worry about leaving your car for long periods and draining the battery. Very nice indeed!

Unfortunately, we couldn't examine the actual capacitor as it is a sealed unit. However, in speaking with Alumapro's tech support, we learned that the construction utilizes 21, 1000-farad flat layer, carbon-alloy EDLC capacitors. This configuration is said to lower inductance and is optimized for low impedance. Using the same measurements as prior, this capacitor measured:

This comes within 10 percent of Alumapro's specification of 50 farads. Note that our measurement technique may have five to 10 percent of error.

From the Discharge Time graph above, we measured 0.009 ohm ESR, a very respectable value.

You can visit the C.A.P. 50 at:

Lightning Audio Strike LSD10DT XStatic Supercap 100 Alumapro C.A.P. 50 Phoenix Gold PowerCore 20 XStatic Supercap 100

Phoenix Gold PowerCore 20
Phoenix Gold designed the PowerCore 20 to maintain continuity with much of the Titanium lineup. The exterior is made of bent sheet metal with a diamond-shape mesh insert, which allows for a peek at the internals and largely mimics the look of the amplifiers. The finish has a brushed-titanium look that is very pleasing to the eye.

On one end of the amplifier are heavy-duty, gold-plated, brass terminals. The terminals accept #0 gauge power wire for charging and have an additional four #4 gauge for each of the power and ground output terminals. A third terminal is for remote turn-on. The remote turn-on powers the three triangulated blue LEDs on the top of the unit.

Inside, the PowerCore 20-farad capacitor is constructed with 288 four-farad, 2.5-volt capacitors wired in eight series strings of 36 parallel caps. We measured 16.3 farads using our constant current charging method.

This unit appears to still be in the prototype stage. Once the company gets it ready for production, we think the leakage current will be minimal or zero.

From the Discharge Time graph above, we measured 0.008 ohm ESR. Again, a very respectable value. Also, note that our method above can result in five to 10 percent measurement error.

We could not find this cap on Phoenix Gold's web site ( but it should be up by the time you red this.

Lightning Audio Strike LSD10DT XStatic Supercap 100 Alumapro C.A.P. 50

Lightning Audio Strike LSD10DT
It wouldn't be a capacitor test without Lightning Audio. The company started back in 1992 and was the first major player in the capacitor game. For years following, it was the largest supplier of capacitors to the industry. To not include one of the leaders in the technology would be considered a sin.

The Strike LSD10DT is your typical cylindrical capacitor. We chose this particular unit because it is an offspring of the original design. This unit is rated at 1 farad and includes a voltage readout LED display. Although the Strike may be an ordinary electrolytic design, it does have a very low ESR.

We verified that this cap is indeed a 1-farad capacitor. As the ESR was very low, it was difficult to measure with our test setup. We got 0.005 ohms, but this measurement could be off as well because the value is so low.

Our only gripe with the Strike capacitor was with the voltmeter and associated electronics. While at idle, the capacitor had a current leakage of 70mA that could kill a battery if the vehicle was not used for extended periods of time.

Check out this and the rest of Lightning's lineup at

Whatever you want to call them, EDLC, stiffening, supercap, or ultracap, it appears that they aren't of much help for the SPLer, except to stabilize the system voltage for the head unit and signal processors. However, for clean audio and improved transient power, these capacitors can extend the battery and improve the audio performance, including reducing harmonic distortion in the low frequencies.