OHMS & IMPEDANCE from the OUTPUT (speaker) perspective

...and additional speaker related topics in the FAQ below.

We've broken the explanations down into two FAQ articles: Input devices such as instruments and preamps, and output devices, usually speaker cabinets or headphones. This article is about the OUTPUT side, what you plug into the SPEAKER OUTPUT of an amp. There is another FAQ that talks about ohms and impedance from the instrument, or input perspective.

What the heck are impedance and ohms?
When you plug speakers into the output jack of your amp, those speakers become part of the circuit. When your amp was built, it was designed for a specific minimum impedance load (expressed in ohms), and what you plug into it affects the outcome. And if it isn't compatible with what is expected, strange and/or very damaging things can happen. This article is a brief overview to help you understand what works with what, what doesn't, and why.

Why should I care?
Because you can blow up your amplifier, or it could mean that you might buy speaker cabinets you won't be able to use with gear you already own.

Impedance is measured in ohms, typically a measure of resistance, but impedance involves more than a simple resistance measurement. You'll see the value expressed as "ohms" or represented by this symbol: . Speakers (drivers, the actual device in the cabinet) and Speaker cabinets are typically 4 or 8.

Drivers (a term used to describe the various speakers inside a cabinet) usually have coils that are wound to be four, eight, or sixteen ohms; there are other, less common values, but they are relatively rare. The drivers are then wired in different configurations inside the cabinet, and that design usually results in a total impedance "load" of four or eight ohms. We are not going to address the guts of the speaker cabinet right now (see below in the FAQ section), all we're discussing here is the use of one or more cabinets with an amplifier.

The importance of Amplifier Specifications: the minimum load in ohms
Solid State (non-tube, and we're including those amps with tubes in the preamplifier stage) amplifiers will have a minimum impedance specification, for example "minimum 4 load. (True tube amplifiers are different, see the paragraph below.) This value is usually printed or stamped next to the speaker output jack on the amp.
Tube amplifiers (again, not those amps with a tube - like a 12AX7 - in preamp section, we're talking a heavy monster with a bank of big momma tubes in the power output section) have specific outputs that are designed for specific impedance loads. Tube-powered amplifiers CANNOT safely operate into an impedance load that is not properly matched to the amplifier's rated load. If your speaker(s) represent an eight ohm load, you plug them into the eight ohm speaker jack. If you have two eight ohm cabinets connected to each other, the resulting load is four ohms, so you'd plug the daisy-chained pair into the four ohm speaker jack. True tube amplifiers have a large output transformer to match the tubes' output to speaker level impedance, and there are tap(s) off the transformer's secondary to match specific impedances.

Here's why it's important: you'll find that the load (impedance) plugged into the unit affects the amount of power delivered to the speaker cabinet(s). For example, you may see a power rating of 350 watts into 8, 500 watts into 4 for a given amp. If you want and/or need the maximum available power the amp can potentially produce, you want to match the lowest impedance for which the amp is rated. But don't obsess about it, the difference in actual volume is typically not that great. Most solid-state amps will handle a 4 load, though those that will handle as low as a 2 load are becoming more common.

Calculating speaker cabinet impedance loads:
If two cabinets are the same impedance, it's very simple:


If it is a mix, the math makes it more complicated. To arrive at the numbers one must use "Ohms Law", which is a mathematical formula.
The formula for figuring the total impedance in Parallel is:
The multiplication of the two loads, divided by the sum of the two loads. 
So, for two 8 ohm speakers: (8 x 8) / (8 + 8) simplifies to 64 / 16, which equals 4 ohms.
Likewise, two 4 ohm speakers: (4 x 4) / (4 + 4) simplifies to 16 / 8, which equals 2 ohms.

For your convenience, here's the ohm load for another common configuration:
Using the formula from above: (8 x 4) / (8 + 4) simplifies to 32 / 12, which equals 2.66 ohms.
Want to know more? Google "Ohms Law" for more information and some on-line calculators, though you'll also run into more complex voltage, current, etc., calculations that are not pertinent to this discussion. There's a good discussion of impedance calculations here, and an impedance calculator on Roger Sadowsky's site.
You'll have to be careful to interpret the specification for combo amps (those with a built-in speaker) as they may express them in different ways. Some indicate total minimum impedance, so the internal speaker's impedance has to be factored in. Others will indicate the minimum impedance of the extension speaker that can be added.

The key thing to remember:
The impedance load of the combination of the connected speaker cabinets must never be below the minimum impedance load specified by the amp manufacturer.

What happens if there is a mismatch?
If the speaker cabinet impedance load is higher than the amplifier rating, e.g., an eight ohm cabinet plugged into an amplifier with a minimum impedance load of four ohms, you're cool. You're not getting the amp's maximum output, but don't panic over it. (Again, this applies only to solid-state amps - if you have a tube power amp, the impedance needs to match!)

If the speaker cabinet impedance load is lower than the amplifier it is now time to panic! Don't try it!
Amplifiers can react in different ways, most contemporary good quality amps have circuitry that detects the current flow, and if it is higher than specified (the result of a speaker load below specifications) the amp will temporarily shut itself down. However, not all amps have this protection circuitry, so you may be playing with fire... literally!! Another distinct possibility is that you'll see curls of smoke escaping from the amp as you fry the output devices.

How can I avoid a mismatch?
Choose your speaker cabinets carefully and know their impedance loads, know your amp's minimum impedance specifications, and think of the consequences before just plugging in cabinets.

Series and Parallel, what the hell?!?
When you discuss multiple drivers or cabinets, you'll invariably get into the subject of series and parallel connections. If you care to know, here's the lowdown:

Parallel speaker connections are the most common.
  • When you plug two speaker cabinets into the two speaker jacks on the back of a typical amplifier, the overwhelming odds are that the cabinets are connected in parallel. If there is a commonly available amp out there wired with output jacks that results in series connections, I've never seen or heard of it.
  • When you plug one speaker cabinet into an amplifier, and then run a second cable from the first speaker cabinet's second speaker jack to the second speaker, the cabinets are connected in parallel. Again, if there are popularly available cabs out there wired with dual jacks that results in series connections, I'm not aware of any.
  • When you plug a speaker cabinet into a combo amp's (amp with internal speaker) extension speaker jack, the internal speaker and external cabinet are connected in parallel. Again... you know what I'm going to say.
Parallel is when speakers or speaker cabinets are all connected postive (+) to positive (+) and negative (-) to negative (-) to the amplifier's positive (+) and negative (-) terminals.

Series is when speakers or speaker cabinets are connected in a loop, from Amplifier positive (+), to Speaker (cabinet) #1 positive (+), from Speaker (cabinet) #1 negative (-) to Speaker (cabinet) #2 positive (+), from Speaker (cabinet) #2 negative (-) to Amplifier negative (-). You could make custom cables or a box to connect two speakers or more in series to an amplifier.

The diagram to the right demonstrates wiring two drivers in parallel and series configurations.

Common Questions regarding amplifier outputs and speakers:

Can I change a cabinet's impedance from eight to four ohms, or four to eight ohms?
Probably not, but it will depend on what is inside. Most two driver (we're speaking of two main bass drivers, like a 2x10, etc.) cabinets don't have that flexibility. For example, if you have a four ohm 2x10, it is likely that the two drivers are eight ohms each, and they are wired in parallel, which results in four ohms at the output jack. The only other choice would be to wire them in series, which would result in a sixteen ohm load. The other drivers (tweeters, horns, midrange drivers, etc.), if applicable, are wired to a crossover circuit that filters out lower frequencies going to a midrange, tweeter, etc., that would otherwise damage the device. They can also factor into impedance loads, which, by the way, vary over the frequency band.

My speaker cabinet/full range driver/bass driver is not marked. How can I find out its impedance?
While impedance is not exactly the same as resistance, you can use a VOM (Volt Ohm Milliamp meter) to measure the resistance of the device to get a fairly accurate measurement. Set your meter to the lowest resistance scale and measure across the hot and ground (for accurate measurement the driver or cabinet should not be wired or plugged into any circuitry). It will likely settle around four or eight ohms, which will give you the answer.

My amp can go down to 4, but my speaker cabinet is 8. Should I try to get it lower, like buy a different cabinet or try to modify mine?
Don't get your panties in a twist. The audible difference in volume between, say 350w/8 and 500w/4, if using otherwise identical speaker cabinets, is not really that great.

The back of my amp says "450 watts"; is that the power?
Possibly, but only if it is near the speaker output jacks or has language indicating so. If it is near the power cable and along with "110-120VAC" or similar language, it refers to the AC power consumption of the amplifer, not the amp's output wattage, as shown in the graphic on the right.