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?
peter_s

Showing 1 response by gs5556

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.