Originally Posted by Heinrich S
But this is what I'm confused about. How can current be high at a low voltage? You guys knocked it inside my head that high current can only result from high voltage.
So then correct me if I'm wrong, but source load-wise, you can't have a high current from a low voltage. But a speaker could demand a higher current than another speaker with less voltage applied?
I think part of the problem is that the folks that are answering you, also have a point of view here and are careful to keep you from going there
. Since I post with reckless abandon
, let me explain this a different way.
A speaker has both inductive and capacitive elements in it. As soon as you have these, the current and voltage waveforms no longer match. This is a hard concept to understand but it is how the system works. What this means is that you can throw out any explanation of this question that uses resistive loads such as "8 ohms" and such. No speaker acts like that. This is why we use the term "impedance" as opposed to resistance with the former having inductive/capacitive characteristics.
In such loads, the peak current and voltage requirements occur at different points in time. Here is an extreme example where they are 90 degrees "out of phase" which is a fancy way of saying there is a time delay between current and voltage:
Look at the "360" degree mark. At that point if we assume the red is the current, it is at maximum. Now let's assume the blue is the voltage. Note that it is at zero. Yes, zero! Dead short. At this frequency and in this situation, the speaker presents a dead short but yet, demands lots of current.
This presents a problem beyond what the power supply can generate. The output transistors do not like to be shorted out like this. Most amplifiers have a protection circuit that attempts to protect the output transistors by limiting the amplifier output. The amount of current provided may be very little in this case. Some will also shut down completely. A much beefier amplifier not only has better power supply but also many output transistors in parallel. Well designed ones can sit there with such a "dead short" and put out 10 amp or more all the day long. They may not even have to have any protection circuit as the output stage can withstand all the current the power supply provides without being damaged.
If the smaller amp does not go into protection, it will modulate its output on every transient in this frequency. Such "pumping" of the output is not the classical voltage clipping by a dynamic situation where nonlinearities are generated.
So we have two extremes of this situation:
1. Resistive load. No speaker is purely resistive yet almost all the amplifier power testing is done with "dummy loads" which are resistive. This means that the amplifier tests by magazines and such in this regard is the easiest test case, not typical or worst!
2. The 90 degree out of phase example. This is again is an extreme situation and one that is not likely to occur in speakers.
The speaker you own lands somewhere in between these two extremes. Where? You don't know unless you have its measurements. WIthout measurements you are in the dark and can never perform this analysis. What to do? Simple: buy the most powerful amplifier you can buy. The more powerful the amplifier, the less it is impacted by the "difficult loads." Companies building active speakers know the load perfectly and therefore tailor the amplification to that. For the rest of the systems with passive speakers, it is a guessing game.
Since amplifiers do not become obsolete like the rest of the system components, it pays to buy high quality and buy it once. My workhorse amp is a proceed 5 channel amplifier. I have had it more than a decade with never a need to get something better. It has 5 separate amplifiers in the box and weighs something like 120 pounds! I never have to worry about it being the cause of distortion. Of course as powerful as it is, it is not a fit for driving subwoofers and such.
See, I understand that. I know some speakers are difficult to drive. But even then, if you have a low voltage applied, you can't have lots of current. The current will be determined by the load impedance and the voltage applied. If you lower the voltage, current has to go down.
Am I wrong?
The "difficult to drive" speaker is one that has its current and voltage out of phase the most when its impedance is also the lowest. That is the worst of both worlds. These speakers demand the highest power and most robust amplification you can throw at them. But again, without measurement you don't know if yours is such and hence my advice is to buy the beefiest amp you can and be done with it.
Edit: fixed a typo and formatting.Edited by amirm - 2/12/14 at 9:18am