Contemporary Music Instruction and Mentoring

Understanding Guitar Amplifiers and PA Systems:
Watts, Volts, Ohms, Amps, & Volume Made Easy

NOTE: Before we begin, let's clear up some terminology.  The word "amp" (and the plural "amps") is used to describe two entirely different things.  That's because the word "amp" is an abbreviation of two different words: "ampere" and "amplifier."  Ampere is a measure of current, commonly abbreviated as "amp."  Amplifier is an electronic device that increases the voltage of an electronic wave form, and it is also commonly abbreviated as "amp."  To avoid confusion, in this article, the word "amp(s)" will always refer to ampere(s).  The word "amplifier" will not be abbreviated.

PA systems and guitar/bass/keyboard amplifiers are a lot like a garden hose.  

Current (amps) is the amount of electrons flowing through a circuit.  It is sort of like the amount of water (gallons per hour) flowing through a garden hose.

Voltage (volts) is the amount of pressure pushing the electrons through the circuit.  It is sort of like the pressure (pounds per square inch) in the city water supply that pushes water through your hose.

Impedance (ohms) is what constricts or restrains the electrons from going through the circuit.  It is sort of like the inside diameter of the hose.  If you change to a hose with a smaller inside diameter, less water will flow, and vice versa if you change the hose to a larger diameter.  Likewise, if you reduce the diameter of a wire, less current will flow through it and the other way around if you use a wire that is larger in diameter.  Also, if you turn down the tap, less water will flow because of the restriction that happens at that one place.  Likewise, a variable resistor (potentiometer) that increases impedance is like the tap where you adjust how much water goes through the hose.  If you turn down an inline potentiometer, less current will flow.  Finally, a sprinkler on the end of the hose that has a small hole to spray the water won't let as much water through as will a sprinkler with a large hole.  So if you change the sprinkler to one with a smaller hole in it, less water will flow.  Likewise, if you use speaker with a higher impedance, less current will flow, and the volume will be reduced.

The goal is current, just like the goal is to move water from one place to another with your hose.  Current charges the speaker’s coil (electromagnet), which reacts with the permanent magnet to cause the diaphragm to move, which causes the speaker to vibrate and make sound.  The higher the amps flowing through a speaker, the stronger the vibrations will be, and the louder the sound it will produce.  Voltage (volts) causes the current to flow, just like the water pressure causes the water to flow.  If you increase the water pressure, more water will flow, and vice versa if you decrease the water pressure.  Similarly, if you increase voltage, more current will flow, and vice versa if you decrease voltage.

The combination of volts divided by ohms determines how much current flows, just like the combination of water pressure divided by hose length/diameter determines how much water flows. As we said already, at any given pressure, the smaller the diameter of the hose, the less water flows through the hose.  But you can make up for a smaller hose with more water pressure; likewise, you can make up for more impedance by having more volts.

Volts divided by ohms equals amps.  If you take an 8 ohm speaker and push it with 80 volts, there will be 10 amps flowing through it, making it vibrate and produce sound.  Just like pressure divided by how small the hose is equals water flow through the hose.

Now, imagine what would happen if you were to hook up two hoses to a water faucet with a splitter, and put a sprinkler on the end of each hose.  Assuming the water pressure is constant, then twice as much water would flow and twice as much lawn would get watered.  That's just like what happens if you take two 8 ohm speakers and hook them up in parallel.  Theoretically, two speakers wired in parallel will result in twice as many amps flowing, and therefore twice as loud of sound, because each of the two speakers will be working just as hard as was the one speaker when it was the only one hooked up.  Hooking up two 8 ohm speakers in parallel reduces the ohms that restrict the flow of electrons from the amplifier from 8 ohms to 4 ohms (8 divided by 2).  This is because to complete the circuit the electrons can go through EITHER of the two speaker coils.  This results in the amplifier working twice as hard.  Just like hooking up two hoses and sprinklers to the tap reduces the resistance to water flow in half, and makes the city water pumps work twice as hard.

That's in theory.  In reality, it depends on the capacity of the amplifier, just like it depends on whether the city water supply can keep the same pressure if two hoses are hooked up instead of one.  If your amplifier has a high enough power rating (Watts RMS) and is rated to not melt down under a 4 ohm load, then yes, hooking up two speakers in parallel will double the current and therefore the volume.  Just like if the city water supply has big enough pipes and big enough pumps, then hooking up two hoses will double the mount of water flow.  Each of the two speakers “sees” the same voltage that the amplifier was previously sending to just one speaker, and therefore each speaker allows the same amount of current to flow through it, but there are now two of them instead of one, so there's twice as much current in total.

On the other hand, if the amplifier is not capable of producing double the amount of current, then when two speakers are hooked up in parallel and the amplifier is turned up loud so that maximum voltage is produced by the amplifier, it will "clip" the signal.  This means the amplifier will try to produce the voltage demanded but will be unable to do so.  Therefore, the peak of each AC (alternating current) voltage "wave" will be cut off or truncated.  This will result in a distortion that sounds terrible, and it can also rattle the speakers in a way that causes them to self destruct.  Furthermore, Class A, B, and AB amplifiers are fairly inefficient, so they generate a lot of heat.  If the impedance is too low (not enough ohms), the amplifier will try to produce too much current, and it can heat up to the point where some component in the amplifier will melt from the heat and the amplifier will die.  (Class D digital amplifiers are much more efficient and less prone to dying from thermal failure but still have a minimum ohm load below which they are likely to fail.)  For this reason, all amplifiers have a minimum ohm load that they are designed to work with.  Higher ohm loads than the rated minimum will not hurt the amplifier but will produce less volume.  But lower ohm loads than the rated minimum can cause expensive equipment failure.  For this reason, it is ideal to run amplifiers at their minimum rated ohm load.  This results in the loudest sound from the fewest number of amplifiers and the most efficient use of 110V power, without melting down.  As you increase the number of speakers hooked up in parallel, the volume increases, until you go below the minimum ohms and therefore exceed the maximum power of the amplifier and start either clipping or overheating the power transistors in the amplifier.

If we hook up two speakers in series instead of parallel, the effect is opposite.  For example, if we wire the hot side of the amplifier output to the + side of one 8 ohm speaker, then hook the - side of that speaker to a wire that goes to the + side of a second speaker, then hook the - side of that second speaker to the ground side of the amplifier, the electrons have to go through both speakers, one after the other, in order to return to the amplifier.  That doubles the impedance of the system from 8 ohms to 16 ohms.  This is because in order for the electrons to complete the circuit, they must pass through BOTH speaker coils.  Doing this will cut the current in half, and thereby it will reduce the volume produced by the PA system by half.  In other words, each speaker will produce 1/4 as much volume as one speaker would have produced by itself, and because there are two speakers, the volume will be in total 1/2 as loud as one speaker would have been.

This makes for some interesting implications for cabinet design.  For example, if you have four speakers in a cabinet, and you want the cabinet to be 4 ohms, you can use four 16 ohm speakers and wire them all in parallel and you'll have 4 ohms.  Or you can wire two 4 ohm speakers in parallel to get 2 ohms, then the other two 4 ohm speakers in parallel to get 2 ohms, then wire the two pairs in series to get 4 ohms.  Or if you want 4 ohms in total but you want to use two different cabinets with 4 speakers each, you can do the same thing with 8 ohm speakers and then wire the two cabinets in parallel to get 4 ohms (two 8 ohm speakers in parallel to get 4 ohms, two more to get another 4 ohms, then wire those two pairs in series to get 8 ohms, do the same thing in the other cabinet, then wire the two cabinets in parallel to get 4 ohms).

For example, in my band we have eight speaker cabinets: two subwoofers, two woofers, two squawkers, and two tweeters.  One group of four speakers is placed on one side of the stage, and the other set is for the other side.  We run our mains signal from the mixer output to an active crossover unit that splits the signal into four parts, one for each frequency band of the signal.  These four signals go to four separate amplifiers.  Each of the subwoofer cabinets has one 18" 8 ohm speaker.  Each of the woofers has one 15" 8 ohm speaker.  Each of the squawkers has two 16 ohm 10" speakers wired in parallel... or two 4 ohm 10" speakers wired in series, I'm not sure which.  Either way, there is an 8 ohm load for each squawker cabinet.  Each of the tweeters has one 8 ohm 1" horn.  Each of our four amplifiers is rated for a minimum 4 ohm load.  So we hook up the two subs in parallel, and therefore the subwoofer amplifier "sees" a 4 ohm load.  Similarly with the woofers, squawkers, and tweeters.  Each has its own dedicated amplifier (or amplifier channel) rated for 4 ohms minimum.  Our subwoofer amplifier is rated for 1850 Watts RMS into 4 ohms.  That means if we run that same amplifier into an 8 ohm load, its maximum power output would be half of 1850 Watts, or 925 Watts.  So we could use two of those amplifiers, one for each sub cabinet, and achieve the desired 1850 Watts (925 for each 8 ohm sub cabinet).  Instead, by hooking the two speakers up in parallel, we achieve exactly the same power output with only one amplifier.  That way, we don't have to purchase, haul around, and provide power to twice as many amplifiers.  This is also true of the other three amplifiers.  So we only use four amplifiers instead of eight for the eight speaker cabinets in our mains system.

This brings us to a discussion of power.  Power (watts) is the total energy of the speaker circuit.  Back to our garden hose analogy, imagine you are using the water to spray against a paddle wheel to force it to spin a mill to grind wheat.  How fast the wheel would turn would be a product of how much water is spraying and how hard the water is spraying.  A big stream of low pressure water, or a smaller amount of water at a high pressure would produce the same speed.  Similarly, power is the measure of how hard the amplifier is working and how much volume the speakers produces.  You can have a small amount of amps going through a speaker at high voltage or a large amount of amps going through it at a low voltage and get the same amount of magnetism in the coil and therefore the same amount of music volume.  Watts is measured by multiplying Volts times Amps.

As a practical matter, increasing volts causes more current to flow, so both volts and amps go up and down together.  The more voltage the amplifier produces, the more amps flow through the wires.

Volts (voltage) is pressure, ohms (impedence) is restriction, amps (current) is flow, and power (watts) is volts times amps, which is a measure of how hard the pump (amplifier) is working, and it is also an approximate measure of how loud the sound will be.

It is important to understand that impedance and resistance are not the same thing in an audio circuit.  They are both measured in ohms, but they don't measure the same thing.  If you hook up an ohm meter to an 8 ohm speaker, the meter will not read 8 ohms.  The reading in ohms will always be a little lower than the rated ohms of the speaker.  For example, an 8 ohm speaker will typically register something like 5 or 6 ohms, not 8.  The reason is because impedance is the sum of three types of opposition to current flow: resistance, capacitive reactance, and inductive reactance.  Ohm meters only measure resistance.  Ohm meters do accurately measure impedance for a DC circuit because, in DC, capacitive reactance and inductive reactance are both zero.  But audio circuits are AC and, in AC, capacitive reactance and inductive reactance always have a value.  So the ohm meter is only measuring PART of the total impedance (i.e., the resistance part).

For this reason, care must be exercised when making and wiring speaker cabinets.  You need to know the impedance of the speaker, which is on the label of the speaker.  It’s always 4, or 8, or 16 ohms.  That’s how you compute impedance.  Not with an ohm meter.

What matters is the total impedance as "seen by" the amplifier.  To compute that impedance, you must calculate the total impedance of all the speakers, as wired in series and/or in parallel, based on the stated impedance as marked on the label of the speakers.  To this number you should add the resistance of the wires in the cables going to the speakers.  (If you are using heavy gauge copper wires and good connectors, and don't have long cables, this should be very close to zero.)  Add the calculated impedance of the speakers to the measured resistance of the wires, and that's very close to the actual impedance that will be "seen" by the amplifier.

Two more factors affect the volume produced by the PA system: speaker efficiency and cabinet design.  If you take two different brands and types of 8 ohm speakers, and pump a musical signal into them at a certain voltage, they will both have the same amps flowing through them and therefore they will use the same watts, but they will definitely NOT produce the same volume.  This is because some speakers are more efficient than others.  An efficient speaker may produce 97dB of sound with 1 watt of power, while an inefficient speaker might produce only 87dB of sound with that same 1 watt of power.  10dB is 10 times as much volume (which to the human ear sounds like twice as much volume).  When shopping for speakers, efficiency is one of the most important considerations, in addition to quality of tone, power handling capability, and reliability.  When you see an ad for a speaker that says "500W," that is meaningless!  All that is saying is that it (allegedly) won't blow up if you hook it up to 500 watts (volts times amps).  But if it's an inefficient speaker, you'll need 500 watts just to create a medium volume; whereas you could produce the same amount of sound with only 50 watts of power with a more efficient speaker, which means you could purchase 1/10 as big of an amplifier, which would be much less expensive and much lighter to carry!  For example, if speaker A is 3db more efficient than speaker B, then speaker B will need TWICE AS MUCH POWER (watts) to be as loud as speaker A.  In other words if you put 250W into speaker A and 500W into speaker B, they will be the same volume.  I'll never forget how much sound I could create with my old Klipsch La Scala speakers, with only a tiny 10 watt amplifier.  It was insane!  They were rated 104db at 1 watt.  You could do a gig with a 10W amplifier! 

Cabinet design also has a significant impact on the volume and quality of sound.  For high frequencies, horns are always more efficient and they also project better over farther distance.  Depending on the quality of the driver and the shape of the horn, they may sound awesome or they may sound tinny.  Depending on the quality of the speaker and the design of the cabinet, direct radiator speakers may sound more natural but they are far less efficient and don't project as well.  There is an entire science to designing speaker cabinets that is outside the scope of this short article but is readily available on the web, but just be aware that cabinet shape, cabinet size (volume in cubic feet), and port size/shape all have a huge effect.