Good luck with trying to fill the Mississippi using a fish-tank pump.
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Yup – there are a million-billion analogies about impedance. I’m going to do one more anyway. Maybe this will be the one somebody reads – you never know.
The electrical underpinnings of this business range from simple concepts to alien geometries. Looking at the “EE” side of audio is a great way to realize just how much you don’t know. It’s particularly tough on non-technical folk and newbies, for whom the “alien geometries” bar is very low. They’re trying to get things all plugged in and playing nicely, but they’re also faced with these mysterious pronouncements on rear panels: “Minimum Load: 4 Ohms.”
Seriously…what do you mean, “minimum?” A load is something you can carry, and you want to avoid having too much, right? WHAT INSANITY IS THIS?
Impedance is an important topic for those of us who use electricity to create various noises. I personally think that the basics ARE fairly intuitive, but are rarely presented in an intuitive way. So, I’m going to do my best.
Up A Creek
Water and electricity are very analogous to one another. Voltage is like pressure, and current is like, well, current. Flow, you might say. The amount that’s “going by” per unit of time. Impedance is opposition to flow in an AC (Alternating Current) circuit. This opposition to current varies with frequency. Some circuits have very high impedance at low frequencies, but pass high frequency material readily. Other circuits do the reverse. Other circuits easily pass a specific range of frequencies, and offer higher impedance on both the high and low sides of that range. (This is how you make analog EQ, by the way.)
Rivers and streams are imperfect analogies, because they are really examples of DC (Direct Current). Opposition to DC flow is resistance, and it’s much simpler than impedance overall. Nevertheless, simple is a good way to start.
Consider two waterways. One is a creek. The creek is seven feet wide and five feet deep. The other is a major river that’s 500 feet across and 50 feet from the surface to the bottom. Here’s an SVG to help you visualize the difference in scale:
What if you could get 25,000 cubic feet of water to flow down both waterways each second. Which one would be likely to knock you off your feet and slam you into a rock?
The creek, of course.
The creek has a very small cross-section when compared to the river. In order to get 25,000 cubic feet of water down the creek every second, the fluid would have to flow at a speed of 714 feet per second. That’s about 487 MPH. (!) The big river, on the other hand, is going less than 3/4 of a mile per hour. The narrow, little creek offers a proportionally high opposition to flow, so getting the same amount of current as the river requires a lot of pressure – or voltage, if electricity is our thing.
What you can begin to see here is the relationship described by Ohm’s law. If the impedance of a circuit rises, maintaining the same flow requires greater “motive force/” pressure/ voltage. If the impedance drops, maintaining the same voltage creates more flow. (If you could run a main sewer pipe at the same pressure as a power-washer, you would have a LOT of water going down that pipe.)
What This Means To You
Let’s say you have a pump. It’s built for a home aquarium. It has no trouble at all pressurizing a 1/8″ tube and keeping water flowing along.
Now…connect that pump to a large sewer pipe, and try to pressurize THAT.
Even if you did something rather dangerous (do not try this at home, or AT ALL) and found a way to make the pump run harder, all you’d do is burn out the poor thing. The unit simply wasn’t designed to put that kind of flow down a pipe.
This basic principle is why amplifiers have “minimum” load ratings. Loudspeakers connected in parallel are effectively being attached as a larger and larger “pipe.” The overall circuit impedance goes DOWN, because there are more possible paths for the electricity to take. It becomes easier and easier for electricity to flow somewhere. The problem is that your amplifier is a pump that attempts to create a constant pressure. That is, if you have one load attached, and you send an input signal that should result in, say, 50 VRMS (Volts RMS) at the amplifier outputs, the amp will attempt to swing 50 VRMS at the outputs if you change the load. To fill the larger pipe, the amplifier has to supply proportionately more energy to maintain the voltage.
At some point of decreasing load impedance, the amp just can’t keep up. It can’t deliver that much energy on a continuous basis. It’s running hotter and hotter, with greater stress on everything from the power supply to the output devices. Eventually, depending on the amp’s sophistication, it might “thermal” and shut itself off, current-limit by throwing a resettable breaker, or drop its output to keep giving you something whilst recovering.
This also connects to the whole issue of impedance bridging, which is what enables maximum voltage transfer. Maximum voltage transfer is what we want in pro-audio, and it happens when low output impedances drive high INPUT impedances. To keep the water analogy going, it’s like connecting a city water line to a house. The city water is a big pipe (low impedance), which feeds the rather smaller house inlet (high impedance). As long as everything is working correctly, the city line has no trouble keeping the house inlet fully supplied. There’s plenty of pressure available to the house, because of good pressure (voltage) transfer.
The opposite of this is when you connect something like a piezo pickup to a “vanilla,” passive DI box. The piezo transducer actually makes a good bit of pressure, but its output impedance is very high. It’s like one of those tiny little capillary tubes that the folks at the doctor’s office use when drawing blood from a finger stick. The input impedance of the passive DI is actually pretty high, but it’s proportionally low compared to the piezo output impedance. The piezo drives the passive DI poorly, resulting in very low level, and the circuit configuration causes a noticeable loss of low-frequency material.
Solve the impedance bridging problem by connecting an active DI with very high input impedance, and your problems go away.
The overall point is that you can’t fill an infinitely large conduit with a finite supply. That’s why audio devices have appropriate loads for their outputs, and why you have to be mindful of those loads.