The High Instantaneous Current Spec

This very subject came up recently in the Audioholics forum and I would like to expand on this subject further, but before I do I think it's important to go into the history of how this specification became popular.

A Brief History

In the early days of stereo and stereo loudspeaker design some loudspeaker designers did not pay close attention to the impedance of the crossover in the loudspeaker. This resulted in some very low impedances at certain point(s) in the crossover, and the power amplifier would be required to "dump" excessive amounts of current to meet the demand. This practice pretty much fell out of favor by the early 1990's as speaker designers decided that this was not very good design and it also forced users to have huge, heavy, and excessively powerful amplifiers. A majority of the better Home Theater loudspeaker designers have decided, for the better I might add, not to repeat these mistakes.

The High Instantaneous Current test was typically a short burst of either a sine or square wave into a very low resistive load. The amount of current was derived by observing how much voltage could be developed across the load before it clipped, blew a fuse, or in some cases just blew up.

Many manufacturers decided to "enhance" their designs at the time to meet this spec but with great sacrifices in other areas. Typically a manufacturer would use a higher voltage rated transformer but lower steady state current to put a higher potential across the main power supply capacitors giving higher values of instantaneous current. The problem with this was that during moderate sustained energy plateaus the transformer would saturate and radiate excessive amounts of magnetic field, draw excessive current from the AC line and radiate more heat. This gave birth to some of the worst in power supply design in audio history. The staff at Audioholics will soon be monitoring amplifiers under test for this type of poor power supply design and reviews will include monitoring of the AC line with the unit under various load conditions.

There is also an important lesson for audiophiles and HT people here. Typically a manufacturer will give a VA rating for the transformer in a given unit. Note that the VA rating does not give you how many volts and how many amps for the transformer, it gives the combination of the two. For example: If I have a 400VA transformer I could have a transformer that is rated at 40 volts and 10 amps or I could have a transformer rated at 80 volts and 5 amps. The VA rating is the same for both!

The Application

Does this test really apply to either Home Theater or Stereo Loudspeakers in use today and even in the past decade? Lets look for a minute or so at some basic math that will help understand as it applies to amplifiers and loudspeakers. For the entire example I will use 75 amps of current which was the figure bandied about in the Audioholics forums.

How Do We Define Power?

Power is a function of three but very interrelated variables. They are as follows:

• Voltage
• Current
• Resistance/Impedance

The Basic power formulas are as follows:

• Power (watts) = Voltage (volts) times the current (amps)
• Power (watts) = the square of the voltage (volts) divided by the resistance/Impedance (ohms)
• Power (watts) = the square of the current (amps) times the resistance/impedance (ohms)

Now that we are armed with our basic mathematical formulas lets take a look at what happens into actual loads.

For an 8 ohm load with 75 amps:

• Power (watts) = the square of the current (amps) times the resistance/impedance (ohms)
• Power = 75 amps times 75 amps times 8 ohms which = 45,000 Watts!

However, we need a voltage across that 8 ohms to produce that 45,000 watts so to find that voltage we use this next formula.

• Power (watts) = Voltage (volts) times the current (amps) or Algebraically this formula converts to:
• Voltage (volts) = Power (watts) divided by the current (amps)
• Voltage = 45,000 divided by 75 which equals 600 volts!

But Wait a Minute Here: No audio amplifier has power supply rails to that go to 600 volts, the maximum we might find for some of the largest sound reinforcement amplifiers used for live concerts is somewhere between 80 to 95 volts, which a far cry from 600 volts. So we can conclude that our high instantaneous current into 8 ohms is just not useable.

Let's try cutting our load in half which brings us to 4 ohms.

• Power (watts) = the square of the current (amps) times the resistance/impedance (ohms)
• Power (watts) = 75 amps times 75 amps times 4 ohms.
• Power = 22,500 watts!

To find the voltage across the 4 ohm load to produce this power we need the following formula:

• Power (watts) = Voltage (volts) times the current (amps) or Algebraically this formula converts to:
• Voltage (volts) = Power (watts) divided by the current (amps)
• Voltage (volts) = 22,500 divided by 75 amps which equals 300 volts!

AGAIN this is approximately three times higher than our supply rails for even the largest amplifiers. So once again we can conclude that our high instantaneous current into 4 ohms is just not useable.

Let's cut our load in half once again to 2 ohms and let's see what we find.

• Power (watts) = the square of the current (amps) times the resistance/impedance (ohms)
• Power (watts) = 75 amps times 75 amps times 2 ohms.
• Power = 11,250 watts!

To find the voltage across the 2 ohm load to produce this power we need the following formula:

• Power (watts) = Voltage (volts) times the current (amps) or Algebraically this formula converts to:
• Voltage (volts) = Power (watts) divided by the current (amps)
• Voltage (volts) = 11,250 watts divided by 75 amps, which equals 150 volts!

YET AGAIN this is just over 50 volts higher than our power supply rails for even the largest amplifiers. So once again we can conclude out high instantaneous current into 2 ohms is not useable.

Let's try cutting our load to 1 ohm and see what happens.

• Power (watts) = the square of the current (amps) times the resistance/impedance (ohms)
• Power (watts) = 75 amps times 75 amps times 1 ohms.
• Power = 5,625 watts!

Power (watts) = Voltage (volts) times the current (amps) or Algebraically this formula converts to:

• Voltage (volts) = Power (watts) divided by the current (amps)
• Voltage = 75 volts

We now are starting to get close to out largest power supply rails but there is one major problem here:

Loudspeakers Used for Home Theater Don't Go Down to One Ohm!!!

I think the only loudspeaker today that goes down close to one ohm are some of the Martin Logan Electrostatic Loudspeakers and that is only at frequencies above approximately 15 kHz. Since this end of the spectrum is only harmonic content, only a few watts are needed here. Below about 8 kHz the speaker is a relatively benign 4 ohm load.

Power Table Summary for Various Loads under the 75A Current Specification