Amplifiers: High Current? High Voltage?

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.

Peter, you are correct. Most amps are designed to act as voltage sources. An amp designed to be a current source is called a transconductance amp and their use is limited because they don't work well with most conventional 2 and 3 way speaker designs due to the design of most crossover networks. Nelson Pass does make one that is intended for single speaker systems such as Lowther based systems.

First Watt article

Gs5556, I don't follow your logic in explaining output power in terms of transfromer ratings. Yes, there is some relationship there, but not as you described. The VA rating on a transformer tells you how much power it can consistently deliver without overheating. The transformer does not choke off the current as you described in your second example (only 8 watts because only one amp can flow to the speaker). It is true that the amp has a maximum voltage that it can produce that is limited by the secondary voltage of the power transformer, but there is no "current resevoir" determined by the transformer.

The current resevoir consists of capacitors in the power supply. A power amp with a large capacitor bank can deliver huge amounts of current for brief periods of time that far exceed the current rating of the transformer secondary. The amp will try to maintain the requested voltage and only starts to dip when it can't deliver the current demanded by the load at that voltage, and even though the dcr of the transformer secondary does play into the complete analysis, the limiting factor is not the VA rating of the transformer.

That is a bit simplified but the basics of the situation.