The High Instantaneous Current Spec - page 2

The Reactive Load

Marketing specialists over the years have claimed that "high current" is needed to drive a loudspeaker due to the reactive load that loudspeaker represents to the amplifier. But is this true? Let's take a closer look at this.

If we take a look at a typical loudspeaker impedance curve we find that the impedance will vary from +/- 30 degrees to +/- 45 degrees. We know that a 45 degree phase angle is half way between 0 degrees and 90 degrees.

Note the diagrams below:

Note that when the phase angle is + 45 degrees half of our load is inductive and half is resistive. Also note that when our phase angle is - 45 degrees half of our load is capacitive and half is resistive. The resultant of this combination that gives an impedance and that impedance is also measured in ohms.

So will high current help us drive the reactive load? The answer is NO because all the formulas in the previous section still apply. The only time there is a problem is when the reactive load goes very close to either +/- 90 degrees, but since that never happens in a loudspeaker this is not an issue.

In reality there will be less power to the load and this is due to Power Factor. Power Factor states that whenever you have a reactive load the voltage and current are out of phase with each other and the degree of the phase angle tells us how much power is lost. The key here is to remember that maximum power is only applied to a pure resistive load, and that power diminishes as the phase angle of a reactive load becomes greater and greater.

In the past a company named Apogee made ribbon loudspeakers that were very inefficient and some of them had an impedance that went down to about 1.5 ohms. Looking at the impedance plots for these speakers showed the phase angle was pretty small but the resistance was very low. These loudspeakers were the exception, they are not the rule, and they aren't made anymore.

Conclusion

From the math and what we know about the basics of how amplifier and loudspeakers perform we can now conclude that High Instantaneous Current is a non-issue and it has no application to driving contemporary and even most of the not so contemporary loudspeakers. In the past, a number of so-called simulations of what music is supposed to represent have been tried to more accurately gauge amplifier power. They have all been dismal failures due to the nature of classical/symphonic music, which has a stunning ability to throw a wrench into just about any generalization. Since many soundtracks use symphonic music this is just as important to Home Theater enthusiasts as well as classical music lovers.

It is the authors firm conviction and experience in designing, building and selling amplifiers that the watts rms into a given load, (8 ohms, 4 ohms, 2 ohms, etc., etc.) over the audio bandwidth of 20 Hz to 20 kHz, is the only accurate and representative way to measure power for this application. For those of you who prefer peak power measurements just remember this: If we multiply the watts rms figure times two we get peak power. So yes, the good old watts rms measurement is also a measure of peak watts.

Special thanks to Dan Banquer of www.redesignsaudio.com

This article was originally published June 5th, 2005.  We are republishing in the memory of Dan Banquer who passed away at the age of 58 years old last Saturday (October 15th, 2011). Dan was a dear friend of ours who was a no nonsense guy and true Audioholic in mind and spirit.  He will be missed by many...