by Paul Bergman

It seems that for an amplifier to be more powerful is not enough…it must also be able to deliver lots of current. But doesn’t high power imply high current?

Not very long ago, it was in fashion for power amplifiers to be able to deliver very large amounts of power. This is hardly surprising, since small but inefficient speakers were also in fashion at the time. A "mere" 100 watts per channel was considered hardly adequate, and many a manufacturer sought artificial means of boosting published power specifications: IHF power, peak dynamic power, music power, etc.
     Since then the pendulum has swung the other way, and a number of breathtakingly expensive amplifiers offer very low power: 18 watts, 14 watts, or even 6 watts. Loudspeakers, of course, are more efficient than they used to be. It is claimed that these low-powered amplifiers offer greater clarity than large ones, as well as--this is the obscure part-- high current.
     To anyone not versed in electricity this is simply gobbledygook, but even those with engineering or physics diplomas may frown. Since power is simply voltage multiplied by current, doesn't high power imply high current? If it is true that flea-powered amplifiers can deliver high current, why do they promise so few watts?
     In order to understand this odd situation, it is necessary to understand that in fact power is not simply voltage times current. This simple relationship holds for direct current (the form of electricity in a flashlight, or in the amplifier's power supply), and even for alternating current providing the circuit contains only resistance. In real-life AC circuits, which have capacitance and inductance as well as resistance, voltage and current will wind up misphased--a little bit out of step. If peak current does not occur at the same time as peak voltage, the power will be less than the two multiplied together. This has considerable implications for certain high-powered amplifiers, capable of swinging high voltages and also delivering high current, but unable to deliver all of the power you might expect. In actual fact they may deliver their rated power on the test bench, when they are run into a "dummy load," which is a pure resistance, but not into an actual loudspeaker, which is usually inductive but may also be capacitive.
     At this point you may be even more confused than when I began, because you have undoubtedly read the claims for some small amplifiers, implying--if not explicitly stating--that amperes (current) are in some way a substitute for watts (power). To understand the relationship between the two, let us observe what happens inside the amplifier as it attempts to send a musical signal to a loudspeaker.
     The amplifier's output transistors or tubes produce a voltage (which you may think of as electrical pressure, analogous to water pressure in a garden hose) varying with the signal level. That voltage is, at most, a few volts below the voltage of the power supply from which it obtains electricity, but of course most of the time it is well below that maximum. The greater the electrical pressure (the voltage), the greater the current flow into the loudspeaker. The total power delivered is equal to the two multiplied together if the speaker is a pure resistance, somewhat less with a real loudspeaker.
     It is intuitively obvious that, if you continue to turn up the volume, at some point the amplifier will no longer be able to reproduce the signal correctly. Sooner or later, one of two things will happen...
 

Would you like to read the full text, with illustrations, of Paul Bergman's article on Power and Current? Click here to order the print edition of UHF No. 56.