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Meditations on Speaker Impedance  ("OHMmmmmmmmm")

One source of confusion we frequently run across is the concept of speaker impedance.  This confusion is also a likely cause of many blown power amplifiers.  This article is intended to explain the meaning of speaker impedance and guide the reader in connecting multiple speakers to an amplifier.

What are ohms, anyway?
Short answer: The ohm is the unit of measure for impedance, which is the property of a speaker that restricts the flow of electrical current through it.  Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms.  The impedance of a speaker is a physical property that (ideally) does not change value, although from an engineering standpoint, there are many complex characteristics that make up speaker impedance    For this reason, the rating of a speaker is called its 'nominal' value, which pretty much means "in name only".   For the average audio user, the nominal impedance is the dominant characteristic and for the purposes of this discussion, we will use the nominal value of the speaker's impedance.

Why are ohms important?
Two reasons:
(1) If you connect your amplifier to the wrong speaker impedance, you risk damaging the amp.  In tube amps, too high a load impedance (or a disconnected load) can result in damage to the output tubes or output transformer,  while in solid state amps, if  the speaker impedance is too low, the amplifier will tend to overheat and more power is used up in the amplifier than is delivered to the speaker.  Too many speakers on a solid state amp can burn up the power output section.
(2) The amplifier will deliver maximum power (volume) to the speaker when the speaker impedance matches (is equal to) the internal impedance (called the OUTPUT IMPEDANCE) of the amplifier.  Too low an impedance will result in weak output and poor tone.  If the speaker impedance is higher than that of the amplifier, its power output will again be less than it is capable of.

Understanding Ohms and Impedance:
In order to understand the reasons for the rules for speaker connection, we need a bit of electrical theory. You probably had this as a lesson in high school at some point, but were more interested in other things at the time.  In order to relate it to something you are more familiar with, let's consider the ordinary garden hose.  Print this off and go outside, hook up the hose (no nozzle) and turn on the water.  Pretty soon, water should start flowing out the end of the hose.  This flow of water through the hose  is similar to electric current, which is usually described as the flow of electrons through the wire and is measured in Amperes. 

Now put your thumb over the end of the hose and try to stop the flow of water.  Feel the pressure? This pressure is similar to Voltage. It is the force of electricity that pushes the electrons through the wire.  Notice that if you succeed in plugging the water flow, (no current) the pressure is still there.  This is like an amplifier with no speakers attached, or an AC outlet with nothing plugged in.  Voltage is present, but there is no current flow.

Finally, move your thumb a bit to allow some water to spray. By varying the position of your thumb, you can control how much water comes out of the hose. Your thumb is restricting the flow of water.  In an electrical circuit, things that restrict or control the flow of current are said to impede current flow, and are described as having impedance.  In a hose, we use a nozzle to restrict the flow.  In an electrical circuit, the device that uses electrical energy and has impedance is called the LOAD.

It should be apparent by now that there is a relationship between pressure (voltage), flow (current) and restriction (impedance).  Since voltage or pressure is what moves the current, increasing the voltage pressure should increase the current, assuming the impedance doesn't change. Decreasing the voltage should decrease the current.  On the other hand, increasing the impedance restricting the flow of current will cause the current to decrease, like turning the nozzle toward OFF. Lowering the impedance is like opening the nozzle to allow more flow.  This relationship was analyzed by a fellow by the name of George Simon Ohm a long time ago, and he identified a simple formula that is extremely important in electricity and electronics which bears his name: Ohm's Law

Ohm's Law states: In an electrical circuit, current flow is directly proportional to voltage and inversely proportional to impedance.  Mathematically, this becomes: Current (in amperes) equals voltage (in volts) divided by impedance (in ohms). 

As an example, if a (solid state) amplifier is producing 10 volts AC to an 8 ohm speaker, the current in the speaker will be 10 volts / 8 ohms or 1.25 amperes. If the amplifier output is increased to 20 volts to that 8 ohm speaker, the current becomes 20 Volts / 8 ohms or 2.5 amperes. So increasing the voltage increased the current. If the voltage decreases back to 10 volts, the current will decrease back to 1.25 amperes.

Now, if our amplifier with 10 volts output is connected to a 4 ohm speaker, the lower impedance will allow more current to flow. The amount will be found by 10 volts / 4 ohms = 2.5 amperes.  If we use a 2 ohm speaker, even more current flows: 10V/2 ohms = 5 amperes.

Finally, if we can measure or in some other way determine the amount of current being drawn from the amplifier, we can calculate the value of the load impedance using Ohm's Law.  We will use this shortly to figure out what happens when we connect several speakers to the output of an amplifier.  The formula for this is: Impedance (in ohms) equals Voltage  (in volts) divided by Current (in amperes). 

Let's use an amplifier with banana jack terminals and connect the red terminal of the amplifier to the red or '+' terminal of an 8 ohm speaker. Also connect the black terminal of the amp to the black or '-' terminal of the speaker.  If you feed a pure tone through the amp so that it delivers 10 volts to the speaker, the current flow through the speaker (as we saw above) should be 1.25 amperes.


Next, let's connect another 8 ohm speaker to the amplifier terminals in the same way, so you have two wires from the amp's red terminal going to the '+' terminals of the speakers, and two wires from the amp's black  terminal to the speaker '-' terminals.  This is called a PARALLEL connection, because of the way it looks in an electrical schematic diagram.

The first thing to understand is that the voltage output from the amplifier does not change. (In reality, it might drop just a hair, but for this discussion let's assume a perfect amplifier.)  So it's still 10 volts AC.  And since each speaker is connected directly to the amp's output terminals, each speaker will receive 10 volts from the amplifier. As we saw earlier, if 10 volts is applied to an 8 ohm speaker, it will draw a current of 1.25 amperes from the amplifier.  And if each speaker needs 1.25 amperes, then the amplifier must supply a total of 2.5 amperes to the two speakers. If you add a third speaker, it will also draw another 1.25 amperes, (total 3.75 amperes) as will a fourth (which would total 5 amperes).  If you keep adding speakers, at some point the speakers will demand more current than the amplifier can deliver, and it gives up its smoke and dies.  Too many loads is an overload. (See importance #1, above.)

Now, we are ready for impedance.  As we said earlier, if you know the voltage and can figure the total current, you can calculate the total impedance of all the speakers together by dividing the voltage by the total current.  A single speaker is simple: 10 volts divided by 1.25 amperes equals 8 ohms.  Remember that two 8 ohm speakers would draw a total of  2.5 amperes from a 10 volt output. So 10 volts divided by 2.5 amperes equals 4 ohms.  Notice that adding a speaker in parallel DECREASED the total impedance. What about 3 speakers that draw 3.75 amperes? 10 volts divided by 3.75 amperes equals 2.67 ohms.  Four speakers that draw 5 amperes from a 10 volt source have a total impedance of 10 volts divided by 5 amperes which equals 2 ohms.  As more speakers are added, each one draws additional current from the 10 volt source, so there must be less total restriction of current.  So the first thing to conclude is that ADDING SPEAKERS DECREASES THE TOTAL OHMS IMPEDANCE.

Well, what if the speakers have different impedances? Like an 8 ohm cabinet and a 4 ohm cabinet? The same method can be used. To make it simpler, remember that impedance was a physical property that doesn't depend on the voltage. The speaker has the same impedance whether the source is 10 volts or 1 volt. So let's use 1 volt to make it simpler.  The 8 ohm cabinet would draw 1V/8 ohms or 0.125 amperes. The 4 ohm cabinet would draw 1V/4 ohms or 0.250 amperes. Both together draw 0.375 amperes. Total impedance is 1V/0.375 amperes, or 2.67 ohms. (Notice that the total is less than the lowest value speaker.)

A 4 ohm, an 8 ohm and a 16 ohm cabinet all connected to the same amplifier (1V out) would draw currents of 1/4, 1/8 and  1/16 amperes, for a total current of 0.4375 amperes. Impedance is 1/0.4375, or 2.286 ohms. (Using a calculator with a 1/x key makes this really simple. Key in: 4 (1/x) + 8 (1/x) + 16 (1/x), =, (1/x) and read the answer.)

While the calculations may seem complicated,  examination of the results above reveals some patterns that make things much easier. 

First, if all speakers (or cabinets) have the same impedance ratings, the total impedance can be found by using the impedance value of one speaker and dividing that by the total number of speakers. If you go back to our example of 8 ohm speakers, we found that a single speaker had a total impedance of 8 ohms (duh....), two 8-ohm speakers had a total impedance of 4 ohms (8/2); three speakers had a total impedance of 8/3 ohms, or 2.67 ohms, and 4 speakers totaled 8/4 or 2 ohms. (What happens with 5, 6, or more speakers?)*

Second, the 2:1 relationship between typical speaker impedance ratings allows for some equivalents when mixing different ratings. A single 4 ohm speaker is the equivalent of two 8 ohm speakers in parallel. So a 4 ohm speaker combined with an 8 ohm speaker would have the same total impedance as three 8 ohm speakers in parallel.  (See if you can figure out the equivalents for a 4, 8 and 16 ohm speaker combination.)*

So, if you see a speaker jack labeled "Minimum Load 4 ohms", that means you can connect up to two 8 ohm speakers or a single 4 ohm speaker to that jack. (What if the jack label says "Minimum 2 ohms"?)* 

If you are mixing speakers with different impedance ratings, be sure to check the total impedance using the rules above to be certain the total is within the limits of the amplifier.  Solid state amps typically have a 'minimum load impedance' indicated near the speaker terminals, and the total speaker impedance must be equal to or greater than that value.  Tube amplifiers typically have a switch on the back to adjust for the speaker load impedance.  Tube amps  have different output characteristics than solid state amplifiers, and too low a load impedance will not normally damage them, but the total output will become weaker and muddy.  So too little load impedance is still undesirable.  Too high a load impedance on a tube amp can cause high voltages inside the amp that can damage power output tubes or the output transformer. 

So, how do you tell what the impedance of a speaker is?  On most cabinets, it should be printed on a label next to the jack.  If the speaker is visible, it may be printed on the speaker label or stamped on the frame or magnet. To measure the true impedance of a speaker or cabinet requires a rather complex procedure involving signal generators, power amplifiers and high frequency AC voltmeters.  However, with raw speakers and many cabinets,  the ohmmeter function of a digital multimeter can help you identify what the impedance of the speaker should be.  Generally, the reading given by an ohmmeter will be about 2/3  to 3/4 of the impedance of the speaker. So, a 4 ohm speaker will typically measure about 2.5 - 3 ohms, and an 8 ohm speaker will typically read about 5-6 ohms, while a 16 ohm speaker will measure around 12 ohms.

Another thing.... As a general rule, all speaker jack connections are considered parallel connections and will follow the above rules. So if you run a cable from the amp to a speaker that has two jacks, and run another cable from the second jack on the first speaker to a second speaker, it is still a parallel connection.

Final note: The rules above are only intended for speaker cabinets connected to an amplifier output. If you are working inside a cabinet, there's a lot more to know, including series and series-parallel connections.  This article doesn't equip you for cabinet wiring.  If you need information on complex speaker circuits, I suggest this site.

 

*Once you have figured out the answers, check them here.

Answers to reader questions.

More information on matching speaker power ratings to your amplifier can be found here.

Copyright 2007-2011 Neil R. Preston, Preston Electronics LLC. All rights reserved.   Please give us your feedback on this article by using the 'Contact Us' link in the menu at left.


 

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