Variable impedence inputs: good or evil?

[johnS]

So the lower the impedence of an input, the more current/voltage it requires from the output of the previous device? I must say, that's not very intuitive. You would think the less a thing "impedes the flow," the less current it would require to be driven.

Does the same hold true for amps and speakers? For example, does it take more current to drive a 8 ohm speaker than it does to drive a 16 ohm speaker?

[puffpastry]

Yes--the same is true for amps and speakers. An 8 ohm load requires more current than a 16 ohm load. In fact, this theoretical output stage we've been discussing is exactly the same as one in an amp, but on a smaller scale. The difference is largely going to be the amount of current they are designed to put out.

I agree--it's sort of counter intuitive. When you think about something being a heavy load (in the electronics sense), you automatically expect a large number. The opposite is the case.

[dagosto]

A simple way to remember that impedance has an inverse relationship with current is to keep in mind Ohm's law for AC current:

I = V/Z

I is current, V is voltage, and Z is total system impedance.

[clineaudio]

logically, you can also think of all electronics stuff as water down a river. Or water in a pipe. An 8 ohm load is less impedance, therefore a larger pipe, more "flow" than a 16ohm "pipe" which would be more flow being impeded.

This is also a great analogy to tie in with speaker wire guages, which is sometimes counterintuitive. 14 guage cable is a smaller "pipe", than a 12 AWG cable; a 12 AWG cable will allow more voltage, more current than the "smaller pipe" of the 14 AWG cable.

[johnS]

Yep, it all makes sense, except the analogy is exactly backward. A pipe with a bigger diameter allows more water to flow, but from what I've gathered here, an electronic input with a bigger "Ohm" allows less current to flow.

This is unecessarily convoluted and if they ever invent a time machine I'm going back and whacking Dr. Ohm upside the head for inventing such a confusing law.

[clineaudio]

Originally Posted by johnS

Yep, it all makes sense, except the analogy is exactly backward. A pipe with a bigger diameter allows more water to flow, but from what I've gathered here, an electronic input with a bigger "Ohm" allows less current to flow.

This is unecessarily convoluted and if they ever invent a time machine I'm going back and whacking Dr. Ohm upside the head for inventing such a confusing law.

The analogy isn't backward, the analogy makes perfect sense, it's simply numbering system that is counterintuitive. 16ohms means more impedance, a smaller hole, less "water" will be allowed to go through. 8 ohms is less impedance, therefore a larger hole, which means more water will be allowed to go through. You are correct though, in that that it takes more "work" to move the same amount of water through a smaller sized holes than the larger.

Here's an amplifier spec, so that you can think about it numerically:
an RMX2450 by QSC
8 Ohms: 500 W

4 Ohms: 750 W

2 Ohms: 1200 W

as you can see, less impedance means an easier flow of current, hence 1200W, and more impedance of flow means that less current will flow for the same amount of original "work"


Wow, I hope that as at least somewhere near clear.

[johnS]

It's getting clearer . . .

So all else being constant, amps get louder when you switch to lower impedence speakers?

[puffpastry]

Output wattage will increase, provided the necessary current is there. But audio power amplifiers always-without exception-give more distortion with heavier loading. The lower the load impedance, the worse the non-linearity. There is no way around this.

[johnS]

OK this is making more sense. (I hope.)

To bring it back to the topic that started this thread. . . If a 200 Ohm mic output is feeding say, a 1200 Ohm preamp input, does this difference mean that, as the AC current alternates, it flows in one direction more easily than the other?