Amplifiers: High Current? High Voltage?

First, I don't know that "controlling" is the best word to use in this situation. Many, Many high quality amplifiers use little to zero feedback, espcially in the current gain sections.

A better understanding may be had by thinking about the speaker itself. A voice-coil (or any electro-magnetic mechanical device) is driven by changes in the magenetic field. Electro-magnetics have a direct relationship with current, not voltage. In a voice-coil, the only reason to have high voltage is to overcome secondary effects, such as "back-emf" (an electric motor terms, but it applies here too).

Many amplifiers are designed in the voltage domain, from the perspective of wave form replication, and have devices (tubes, transistors) that are capable of high current. This is valid because of Ohm's Law or V=IR. The resistance is set by the speaker coils, and the current in then a fucntion of the voltage output.

This does not apply for electro-stats, as the rely on HIGH voltages to create the static field.

It has nothing to do with "controlling" current or voltage. An amplifier/speaker interface is bound by the laws of physics. In order to determine the power output, you have to establish a baseline power capability into a given resistance. Then you work backwards to determine the voltage required. The current output will be determined by the voltage at the resistance.

For example. Let's say we want a 100 watt amplifier. To establish 100 watts, we need a transformer that is rated 100 VA (volt-ampere) minimum. We know that the input voltage will be 120 volts. But what do we set the output voltage? That will be determined by the speaker loading.

Use a speaker with an 8-ohm nominal impedance. So the amp spec is 100 watts into 8-ohms. The output voltage required is, by Ohms law (100W=V*V/8), 28 volts. The current output will be determined by the speaker impedance (100W=I*I*R) to be 3.5 amps. The transformer is 100VA so, as a check, the current times voltage should equal 100, (28*3.5=100), which it does. So the current is not controlled by the amp, but by the speaker/amp combo.

Now, say we take that 100VA transformer and wind the secondary to 100 volts. What happens? If we use Ohms law to say the power has to be (100*100/8) or 1,250 watts, do we get more power by simply changing the windings of the xfmr? No. Because the xfmr is still 100 VA, so at 100 volt secondary, the current "reservoir" is only 1 amp (100V * 1A = 100VA). So the speaker power is (1*1*8) only 8 watts because only one amp can flow to the speaker.

What if we turn down the xfmr to a secondary of 1 volt? The current "reservoir" is now 100 amps. Does that mean the speaker power delivery is now (100*100*8) 80,000 watts? No, because the output voltage of 1 volt determines the power, or (1V * 1V/8) 0.125 watt.

That's why it doesn't make sense to call amplifers high current, voltage controlling or current controlling. You only get what the power supply and connected load give you. You had the right idea in your post.

Gs5556, thanks for your detailed explanation. Dare I ask another question and understand the answer? How do large storage caps play into that current/voltage/power equation?

Gs5556- Nice to see your post. The seemingly 'simple' fact that you outlined seems to be missed by many in the audio community- the current in a speaker is the same for a given amount of power regardless of the technology that created the power- Ohm's Law cannot be denied.

In reality all power amps are in fact that: *power* amplifiers, and are incapable of producing current without voltage, or voltage without current. I wish the industry would get this fact straight (and get right with the Law :)

Peter_s, Additional filter caps with help smooth out a power supply of noise created by the operation of the amp. The less noise, to a certain extent, the less distortion. The result is smoother sound with greater authority.