What does Current mean in a power amp???


I need a high (at least that is what I am told) current amp to drive my speakers. What numbers should I be looking for?

I am not a tech person so keep the answers a simple as possible. Thanks to all!
rwd

Showing 3 responses by mburns92

Rwd,
What kind of speakers are you intending to drive?

"High Current" is a relative term. For a load with a given, fixed impedance, a higher powered amp will always deliver higher current INTO THAT LOAD. In other words, for a fixed 8 ohm load, a 200W amp will supply more current than a 100W amp. Therefor, based solely on current requirements, it could appear that using an amp with a higher rated power output would be the answer.

However, the rated power of an amp into an 8 ohm load is not really a good indication of the MAXIMUM current delivery capability of that amp. Most speakers do not have a constant impedance over the entire audio frequency range, and many have less than 8 ohms nominal impedance. As described in the post above, using ohm's law, into an 8 ohm load, a 100 watt amp would deliver 3.54 amps of current (assuming a 0 degree phase angle between the voltage and current waveforms, for you hardcore engineers :). If instead of an 8 ohm speaker, we were driving a 4 ohm speaker, then to deliver the same 100 watts, the amplifier would have to output 5.00 amps of current. If the impedance dropped to one ohm, to provide 100 watts of power to the load, the current delivered would have to be 10 amps, almost 3 times the current at the 8 ohm rated power output. The ability of an amp to deliver higher current into low impedance loads is influenced primarily by the design of the power supply, and the type and number of output devices. The ability to dissipate larger amounts of heat also becomes an issue as current delivery increases.

If you have difficult to drive speakers, such as Martin Logans, where the impedance drops as low as one ohm at 20kHz, it is important to have an amplifier that is designed to drive low impedance loads.

Therefor, it is probably more helpful for you to consider an amplifiers performance into low impedance loads, rather than zero in on the term "high current". As described above, it is possible to have an amplifier with a large power rating into 8 ohms that will have "high current" at 8 ohms, but may not be suitable for driving low impedance speakers. It is also possible to have an amp with a modest power rating into 8 ohms, and thus a modest current delivery at 8 ohms, that is capable of delivering much higher current, or even much higher power, into lower impedance loads.

In general terms, if you have speakers with an unusual characteristic impedance plot, you probably want to look for an amp with the ability to drive a 2 ohm load continuously. If a manufacturer won't put into writing that their design is capable of driving a 2 ohm load, then it probably isn't.

That said, if your speakers sound good to you with the amp that you are using today, and the amp does not appear to be running abnormally hot, don't mess with a good thing.
A minor clarification to my earlier response:

Ohm's law does not provide for a definition of power in an electrical circuit, rather it defines a relationship between the DC voltage, DC current, and DC resistance. When I stated "using ohm's law", I omitted that I was applying ohm's law, along with the basic power equation, to obtain the relationship of P=I^2*R, where P=power, I=current, and R=resistance.

Sorry for any confusion that may have caused,
Mike
This has probably progressed well beyond the original question, but it makes for an interesting topic of discussion.

I disagree with the statements that ohm's law is not at all applicable, because with the substitution of the term "impedance" for "resistance", ohm's law is very much true for a single-phase AC circuit. As stated by both Sean and Bigtee, the AC impedance is made up of resistance, capacitive reactance, and inductive reactance. The resistive component is constant at all frequencies, but the capacitive reactance is inversely proportional to frequency, and inductive reactance is proportional to frequency. These properties, along with the phenomenon of mechanical resonance, described above by Sean, explain why any given speakers impedance plot can vary so wildly, both above and below the nominal impedance.

However, in order to look at a "simple" model of a loudspeaker, one must have some way to express the relationship between voltage, current, and impedance, and ohm's law provides that relationship. The other relationship required to model the circuit is a basic power equation, which is also different for an AC circuit. For a single-phase AC circuit, Power=Voltage*Current*Cosine of the phase angle between the voltage and current waveforms. This difference in phase is, of course, a result of the net reactance at any given frequency. To apply the power equation without accounting for phase relationships, you must assume that the reactive component of the equivalent impedance is zero, yielding a purely resistive load. I assume (but don't know for sure) that this is how published amplifier ratings are derived, with a discrete frequency sinusoidal waveform applied to an 8 ohm resistive load.

If you accept the above, then in a very roundabout way, ohm's law does in fact have an effect on whether an amplifier can drive a given speaker. If we could build a "complex" mathematical model for a given type of speaker paired with a certain amplifier (and don't forget the role of our choice of speaker cables in this model) ohm's law could describe, at any particular frequency, how much current our voltage source (amplifier) could supply. We could then look at the phase relationship between the voltage and current, apply our power equation, and we would have our value of power at clipping for any frequency that we cared to look at. Since I didn't do very well in differential equations, I will leave this modeling process to the wonderful people who design audio electronics for a living.

BTW, I noticed that while I was formulating this response, a couple additional posts were added. Thanks to clueless for thinking on the same line as me (kind of scary, huh?), and Seandtaylor99 hit the nail right on the head, although I think we are still just short of a full fledged pissing contest :-)