Amplifiers: High Current? High Voltage?

I've seen alot of mention about current-based and voltage-based amplifiers, and I am confused. I thought all amplifiers recreated the wave form by varying voltage, and their interaction with the speaker (and the impedence characteristics between the two) dictate what current is drawn. I understand that the power supply of some amplifiers is less current restricted than others, but my (current) understanding also says that all amplifiers work by controlling voltage. Is there really such a thing as an amplifier that controls current rather than voltage?
They don`t control current. They should dump it into the load when needed. All amps can`t do this.
Then what is the best way to describe a non current restricted amp from a current restriced amp? High current vs. low current? I assume a non-current restricted amp is one that will put out nearly twice the power at 4 ohms than at 8 ohms.
Ballsy amps, as far as current delivery is concerned, double down (they deliver 2x the current into 4 ohms that they deliver into 8 ohms & 4x the current into 2 ohms). For example 300W into 8 ohms, 600W into 4 ohms & 1200W into 2 ohms.
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.
Peter_s, I think your basic understanding is correct. Amplifiers are designed to be voltage sources, i.e., they will attempt to hold the voltage constant regardless of the current demand. The output voltage does vary as it is the input voltage that modulates the output.

For any given input voltage, the output voltage will be determined by the amp's voltage gain. For that same input voltage and derived output voltage, you want the output voltage to stay the same as the load decreases. Many solid state amps can maintain their output voltage when going from an 8 ohm load to a 4 ohm load; thus, doubling the power dissipated. However, as the load is halved again to 2 ohms, almost all amps will be unable to maintain the same output voltage.

You may find the PowerCube graphs of interest...
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.
Thanks Herman. That's what I was getting at with my question about capacitors. I assume all prior discussion about transformers has been input transformers.
the law is current times voltage equals power. in the case of speakers, all a voice coil is, is an inductor used as a motor. for inductors, the larger the current, the stronger the magnectic field. voltage isn't really that important when it comes to pushing a speakers. the higher the current, the more powerfull the electromagnetic field is. high voltage has its advantage, it saves a lot of power, but for subwoofers, high current wins hands down. when looking at amplifier specs, people don't know what to look for, power equals watts, but peak doesn't equal rms, the difference between regular amplifiers and high current amplifiers, is that, regular amplifiers us more voltage in the equation then current (which is like 50volts times 2amps= 100watts when you could use 10volts times 10amps to get the same power but create a stronger magnetic feild), when it is current that is pushing the relitive motion of the induction (voice coil). but another thing is, voltage is what pushes your current through, and with out that voltage, no current would flow, voltage in equals voltage out, and all voltage is, is electronic pressure.
the difference between regular amplifiers and high current amplifiers, is that, regular amplifiers us more voltage in the equation then current (which is like 50volts times 2amps= 100watts when you could use 10volts times 10amps to get the same power

"Regular" amplifiers don't use more voltage in the equation. All voltage amplifiers, which is what almost all amps are, whether or not they are capable of producing high currents , try to maintain a constant voltage out for a given voltage in. The amount of current that flows will be the voltage divided by the impedance, and the impedance is whatever it is. So called "high current" amps are the ones that have the ability to deliver more current if the impedance dips real low, but don't somehow magically push more current through the speaker than one with less reserves.

Another simple way to look at it to compare a much of AA batteries connected in series to get 12V and compare that to a car 12V battery. Both will put out 12V but the AA doesn't have much current capacity.