Speaker Impedance: How to Properly Match Your Amp Head with Cabinets

Amplification can be confusing to a newbie coming from a combo amp. If you are new to amps and cabs, this article should be helpful in explaining what speaker impedance is, and how it works with your amp head. You may also want to read my previous article, “Decoding Graphic Equalizers: Get Past ‘Scooping’ Your Tone”, for some more helpful information on amps.

When we first make this big step to a new rig, there are many questions floating through our mind, because we are presented with countless options. One of many options is how to run an amp through cabinets, and with this impedance, or “?” thing means. Furthermore, we run into impedance mis-matching issues when expanding our rigs with new cabinets. This article will explore the relationship between amps and speaker cabinets in regards to impedance, and some things to keep in mind when expanding your rig to multiple cabinets.

Speaker Impedance

Typically, bas cabinets are either 4 ? or 8 ? (ohms). When selecting a cabinet to pair with your amp head, always check your amp to figure-out how many watts of power your amp will put-out in different impedance modes. Amps nowadays, generally speaking, are pretty slick and know what impedance to run at when you plug-in your cables, connecting them to the cabinets. In the old-days, there was a switch you would have to use to select which impedance to run your head. Some nostalgic amps still have this feature, but most amps automatically adjust.

When selecting a cabinet, be sure your amp will run at the impedance your cabinet is wired for, and that you will have enough power to achieve the volume you desire. Since different amps put out different power levels for different impedances, also check the cabinet to be sure it can handle the load coming out of your amp at that impedance. This is crucial in that it will ensure you do not over-drive your speakers and cause damage. Many heads will have a “clip” light indicator to let you know when you are pushing too much volume through the speakers, thus in danger of causing the speakers unwanted damage. Many amps will also have a db pad to cut the output by 20 or 15 db. Again, the amps nowadays are pretty slick, and have many features to make our lives easier.

Multi-Speaker Rigs

When playing through multiple speaker cabinets, one must remember that this will change the impedance that your amp must compensate for. It’s best to match like cabinets: two 4 ? or two 8 ? cabinets. Running cabinets in this fashion from one power amp will give you a 2 ? or 4 ? load, respectively. Make sure your amp can handle running at these impedances. When you mix impedances, say an 4 ? and 8 ? cabinet together, and you’re running one power amp, this will create an over-all impedance of 2.667 ?. If you remember how savvy the amps are these days in compensating, the amps do not like this. It sort of “confuses” the amp, and it doesn’t know what impedance it is running through, and will cause your amp to draw power funny. This can not only potentially damage your amp, but also your speakers (your 8 ? cab may have too much power going through it, since the amp is “running at a quasi-2 ? load”). This is possible to do, but I don’t know anyone who would ever recommend it.

The exceptions to the above paragraph are those amps that have two power amp sections. These amps aren’t all that common, as they are relatively high-end and are typically purchased by people playing larger venues (when I was playing, I was running a head with two power amps, running through two 8×10 cabinets). You can also “bridge” the two power sections and run the amp as if it were one power amp, or run them in stereo. I’m sure I will explore amp heads in more detail in another article. Until then, remember that you must check your amps AND the cabinets to ensure that they will be compatible in respects to impedance and power levels. Although it is possible, try not to mix impedances when running one power amp. If you follow these simple steps, your amp will live a long, healthy life and give you the best performance it can.

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  1. Joe Curatolo

    Thanks Jeff,and the great people at No Treble.I am one of those people coming from a combo to an amp,cab situation.I am dying for an Epifani,and since they no longer make combo’s,I have to go for the later situation, Thanks again, You guys rule.

  2. If you’re hooking up speakers to amps this is all you really need to remember… if you’re running a valve amp, like an Ampeg SVT, then you MUST match the amplifiers’s output impedance to the speaker’s impedance (aka the load). If it’s a 4 ohm box, then switch the output to 4 ohms etc. It’s really important that you also hook up the speaker before you turn on the amp because valve amps don’t like not having a load attached. If you’re running a solid state amplifier then there are a few more variables. We generally talk about the minimum impedance or load the amp can handle. Contrary to what this article says, amps aren’t really smart enough to “compensate” and they don’t get “confused”. They just indiscriminately push current into a load. As long as the combined speaker loads attached to your amp don’t go below the minimum specified output impedance of the amp then you won’t hurt the amplifier. It’s basically all to do with power output. Solid state amplifiers will have a given power output when attached to a certain load… for example, your amp might be 200W into 4 ohms (min). So, this means your amps max output before distortion will be 200W when attached to a 4 ohm load. A rule of thumb is that if you attach an 8 ohm load the power will decrease to about 100W (give or take) due to the higher load on the amp. If you attached a 2 ohm load then the amp would try to output approx 400W and overheat, possibly damaging the amp but most likely the amp’s thermal switch will turn the power section off to protect itself… no sound, but no damage. So, attaching multiple speakers and cabinets… there’s two ways of doing it… series and parallel. I’m not going to get into it too much here but suffice it to say… if you connect in series, impedance will increase and if you connect in parallel it will decrease. If you use a standard speaker cable to attach two cabinets then your impedance will decrease because this is a parallel connection. Just remember that the combined impedance should not go below the minimum your amp can handle.

  3. im still confused, been in the combo amp world for too long.i use em for guitar, but on bass we have a 400 watt amp head at 4 ohms or 8 ohms (has the option), and a 4×10 cab thats at 4 ohms at 1000 watts. is that fine? wont light up on fire or start clipping? the amp head should power the cab right?

  4. Cristian espinoza

    So if I bought a 2×12 and it says the impedance is 8 ohms mono and the head outputs are 1×8 ohms 1×16ohms and 2×16 I whould plug it into the 1×8 ohm output right?

  5. mark warner

    So if I’m running two 4 ohm cabinets that are rated at 500 watts and my amp head is 900 watts at 2 ohms, i will likely fry the speakers, correct? What I really need is two 4 ohm cabs rated at 900-1000 watts…to run at 2 ohms and 900?