# volatage, impedence, all that good stuff

Discussion in 'AV Receivers' started by JeremyErwin, Feb 19, 2006.

1. ### JeremyErwin Producer

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I'm a bit curious about the basics of how a amplifier works, and how it relates to speaker impedence, power consumption, decibels, and so on. I'm not in the market for a new amplifier, I just would like to have at least some grounding in electrical engineering, so as not to be lost in any future discussions,
Some questions come to mind...
First, I understand that a speaker-amplifier combination uses AC power, not DC. So, even if one were to assume that a speaker's impedance remains constant (not a wise assumption, perhaps, since everything is "nominal"), shouldn't one use something like the following?

P=IV cos(phi)
?
How does one calculate phi? Is it related to the frequency of the sound being generated? Or is it something else?

How does a speaker's impedance change?

How does a high current amplifier differ from an average run of the mill amp?

What do the impedance switches found on some amplifiers actually do?

Is there a benefit associated with designing a 4 ohm speaker over an 8 ohm device?

And so on. To one group of persons, I may sound like a total novice. To another, I probably sound like someone who's worrying about the wrong things. Nonetheless, I'd appreciate some insight.

Don't be afraid to break out the PDEs, either. I'm not a math novice. I'm just not an electrical engineer.

2. ### Mike_P Agent

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The fist thing most electronic boxes do is convert the 120 volts ac coming in from the power cord to a dc voltage to run the electronics. The power supply, or supplies, are very important in that you need lots of good, clean power to run everything else in the box. One reason people are so interested in the weight of receivers is that a lot of the weight comes from the transformer(s) in the power supply(s). The bigger they are, the better. Lots of receivers measure their power with only 1 channel driven, and say the receiver can do 100 watts in this situation. But if you ran all 5 channels at once, in many cases you would get far fewer watts. This is because the power supply is too small to run 5 channels at once to the same 100 watt level.

In the DC world, volts = current x resistance. So, in this case, a radio in your car that pulls half an amp would have a resistance of 24 ohms.

12 volts = .5 amps x 24 ohms.

This example is a simple one, looking at only dc and resistance. In the ac world, things get more complicated. We use circuit components like capacitors and coils, which have impedance instead of resistance. The impedance of a capacitor equals 1/jwC, where j is the imaginary number whose square equals -1, w (it's really omega, not w, but I don't know how to type it) is 2 x pi x f, where f equals the frequency, and C is the capacitance. So the the impedance of a capacitor varies depending on the frequency of the signal you feed it, decreasing as the frequency increases.

The impedance of a coil, or inductor, equals jwL, where j and w are as described for the capacitor, and L is the inductance. The impedance of an inductor also varies with frequency, this time increasing as the frequency increases.

To an amplifier, a single speaker driver looks basically like a coil. A speaker with a couple of drivers and a crossover network looks like a very complex network of capacitors, inductors, and resistors. We can develop an equation that describes the impedance of this entire network. Because the impedance of the many of the components in this network are frequency-dependant, the impedance of the whole network, or speaker, is also frequency dependent. We could feed a series of tones into the speaker, say 1kilohertz, 2 kilohertz, 3 kilohertz, and so forth. We could measure the current into the speaker at each frequency. By dividing the voltage of our signal by the current at each frequency, we could find the impedance at each frequency. If we graphed our results, we could see that the speaker's impedance varies with the frequency of the signal we send it.

A high current amplifier will have less difficulty driving a speaker with a lower impedance.

The impedance switches on some receivers make the amplifiers match up better to the nominal impedance of the speaker. There are a number of different ways they could do this within the amp circuitry. The most simple might be to just switch different resistors into the final amp stages to "match" input impedance of the speakers.

Typically they say a speaker is easier to drive if it has a higher impedance, because it will demand less current from the amp. I don't think there is any particulat benefit of a 4 ohm speaker over an 8 ohm speaker, as long as the amplifier matches up to it.

3. ### Chu Gai Lead Actor

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I'm not sure where you're going Jeremy.

As you stated, the signal going from the amp to the speaker is sinusoidal (AC) and one cannot under any circumstances consider the speaker's impedance to be constant. Even a single driver's impedance varies with frequency for that matter.

As far as determining the amount of power that an amp is putting out, that's a complex question because of the distribution of energy in your musical program as well as the speaker's impedance curve.

A speaker's impedance changes with frequency. As to why it does so, is a more complex issue.

A high current amplifier may or may not differ from a run of the mill amp. A robust amplifier will provide the current necessary given a certain voltage that's put out by the amp in order to satisfy the impedance. E = iR and all that, right? So you'll find some amps that are spec'd and rated to provide continuous power into impedances as low as 2 or 1 ohms. However, if you don't need it, then you can get by with a more modest amp.

The impedance switch on most amps or receivers acts as a voltage limiter. This then allows the amp or receiver to not exceed its current capability. You'll find some discussions of this on Audiholics where they review receivers with this capability.

There is no inherent benefit in 4, 8 or whatever ohm driver. Designers, competent ones that is, use what's available consistent with a variety of goals like availability, price, consistency, perormance parameters, and god knows what else.

I'm not sure what it is that you're worrying about. If it's about what speaker/amp to choose, my only advice would be to find your speakers and then worry about the amp end of things.

4. ### JeremyErwin Producer

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I'm interested in formulating a "scientific" rationale for my existing prejudices.

Maybe I should seek a hands on eduction. Build a few speakers, fry a few amps.

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