Speakers and Power Amps: Don’t Blow That Cabinet

Q: I’ve been thinking about getting a new amp head, and I want to make sure that it will work properly with my current cabinets. How do I make sure that the head I’m getting won’t blow a speaker or ruin my shiny new head?

A: While I’m not a super “techy” guy, I have learned about some of this the hard way (blowing up a power amp on a tour).

I would encourage you to Google ohms and power handling if you’re looking for more detailed explanations of what I’ll cover here, as there is a lot of information on the web about this as well.

Basically, you need to know what the power handling is for both your cabinets and your power amp. I’ve never harmed a speaker due to using a head that was too powerful, but I have ruined a poweramp because I’ve run it too hot (by connecting too many cabs to the power amp).

It all comes down to ohms. The math is pretty simple.

1. How many ohms can your power amp run at safely?

The lower the number, the more power will be pulled from the power amp.

2. How many ohms are your cabs, and how many can you chain together?

Here’s how it breaks down:

If your cabs are 8ohm cabs, each cab will draw 8 ohms. If you run two 8 ohm cabs together, they will run your power amp at 4 ohms (twice as much power being pulled from the power amp).

If your head runs at a max of 4 ohms, that means that you can run one 4 ohm cab or two 8 ohm cabs safely.

Some heads will run safely at 2 ohms. This means that you can run two 4ohm cabs or one, two, three or four 8 ohm cabs. You could also run one 4 ohm cab and 2 8 ohm cabs, for example.

A 500 watt power amp will generally put out 250 watts into an 8ohm cab, and will allow a 4 ohm cab to draw the full 500 watts.

Read the manual for the power amp and it should lay out how many watts it’ll put out at “X” ohms.

If I’m chaining cabs together, I generally use 8 ohm cabs, but I also have one 4 ohm 410 cab that I use for my hi-output (loud) gigs, as the head I currently use only goes down to 4 ohms.

Again, a quick search on the web should lead you to a good breakdown of ohms and how they work on a more technical level. I encourage you to learn as much as you can about it to avoid any problems.

Readers, what advice do you have to give? Share your thoughts in the comments.

Have a question for Damian Erskine? Send it to [email protected]. Check out Damian’s instructional books, Right Hand Drive and The Improviser’s Path.

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  1. This is a great explanation of something I’ve never been able to grasp. Thanks!

  2. That should read “if your amp runs at a MINIMUM of 4 ohms”.

  3. there is an important distinction you overlooked. there is a huge difference in if your speakers are hooked up in series or parallel.

    • Who on earth hooks up bass cabinets in series?

    • people who don’t know any better. people with only one functioning speaker output on their amp. people hooking up things other than bass cabs (like the fellow who posted below), the bozo who set up the p.a. on my latin gig last weekend. etc.

    • Robin Kok Some amps have only one speaker output. Someone might link two cabinets in series because there is a chance of that in every cabinet, as far as I know. I’m sorry if I understood something wrong but I’m more used to speak about these things in finnish.

    • I’m talking about cabinets, not individual speakers. If you link two cabinets in series, the nominal impedance goes UP. (Rtot=R1+R2…Rz). This is why it’s useless to hook up cabinets in series. Within a cabinet speakers are (partly) wired in series. But this is irrelevant to the original story, which is about cabs and not speakers.

    • Robin Kok People who have a preamp/poweramp – rig with twice a given ammount of power at 8 Ohm. If you have 2 Cabs with 4 Ohms either and one 8-Ohm Cab, you daisychain them 4-Ohm-cabs in series to get 8 Ohms. You connect this load to one side of the poweramp. On the other side of the poweramp you connect the single 8-Ohm-load. So you are able to use three cabs, two 4-Ohm-loads chained in series to 8 Ohm, and one 8-Ohm-load, used alone on one side.

    • Robin Kok In some configurations it may be necessary or useful. Like I described in the case of using two 4-Ohm-cabs and one 8-Ohm-cab on a poweramp with, lets say, 2x 500 Watt @ 8 Ohms minimum. The whole impedance – thing is much more complex as cabs are reactive loads and change theyr load while playing. A 8-Ohm-cab may get down to 4 Ohms by getting pushed hard with a huge ammount of low end.

    • Uwe Forschner I have never seen a bass cabinet that had speakers inputs connected in series.Maybe a few wonky guitar / PA cabs, but not the mainstream models I’ve seen (and I’ve worked with nearly all mainstream brands and models of cabs). Although I agree with everything you said, coming across a bass cab with jacks wired in series is a very, very rare occasion. For the record, I have a pre/power setup.

    • Glenn Grant The in-and output jacks in common, modern bass-cabs are designed to get a parallel connection when daisy-chained. You have to know exactly by opening the cab and looking at the wiring if you want to hook up two cabs by using the cab jacks. But normally you will built a parallel connection.

  4. Ohms law for parallel cabs: 1/(1/R1)+(1/R2).
    R1 = the resistance or impedance of cab one.
    R2 = the resistance or impedance of cab two.

    So, with two 8 ohm cabs, you get 1/.125+.125.
    or 1/.25
    or 4 ohms total impedance.

    If you have an 8 ohm and a 4 ohm cab you get: 1/.125+.25.
    Or 32/12
    or 2.6666666667 total impedance.

    You can extend this equation for as many cabs as you like. If you’re running three 8 ohm cabs you get: 1/.125+.125+.125.
    or 2.666666667

    If you’re running speakers in series, you just add the two impedance ratings together (two 8 ohm speakers running in series is 16 ohms).

    Hope this helps.

    • Sorry about the 32/12 I was using a capacitance equation and remembered the impedance equation and had to substitute my work… forgot to change that step. It should be 1/.375.

    • Thanks for that I have a Bose system with 10 cube speakers and I wanted to know how the load thing worked they are 8 ohms each.

    • I know its just a home system but I know the loads are the same.

    • Thanks! This is great too.

    • be careful, although this formula theoretically works out, if you want to run your amp at 2 ohms and you have mixed components, I would advise that you talk to your bass tech or engineer before going into the 2 ohm range. (I don’t worry with mfg matching amps and cabs)

    • Chris Petroulias – use caution – home systems are not like pro systems – often they are only good for 8 ohms – period. However, as is pointed out in this article, check your power amp’s documentation carefully. Especially before adding more than a single pair of speakers to a home system.

  5. be careful when doing the math for bass amps/cabinets. The formula is written for guitar frequencies and when applied to bass it gets funky and you probably can’t assume the math is correct. Add an 8ohm to 2-4 ohm cabinets and you are in dangerous territory, pushing that amp below the 2 ohm limit…..due to the errors inherent in the calculations. (I found out the hard way and my guru Steve Weiss ‘showed me the light’.

    • There is nothing wrong with this formula, and it most certainly isn’t ‘written for guitar frequencies’. Physics doesn’t change for “guitar frequencies”. Some guitarists would like to believe this to be true, though ;)
      The ohm rating on cabinets is a NOMINAL impedance. What you’re referring to is that the impedance the speaker(s) present to the amp differs over the frequency spectrum. Thus, a speaker that has a 30 ohm impedance at 4KHz may have a 4 ohm impedance at some other frequency (most good manufacturers have a graph of this relation between frequency and impedance). Good amps are able to handle loads under 2 ohms, but only for a *very* short time. If you run an amp below its rated impedance for too long, it will either (a) go into protection mode (well-engineered amps) or (b) give you a nice cloud of magic smoke (crappy amps).
      There are NO errors in this formula. The math is irrefutable. It’s just that bass AND guitar present varying loads to the amp so we need some working numbers (i.e., nominal impedances) to do quick & dirty calculations with this formula.

    • Last sentence should read “bass AND guitar *cabinets*”.

    • like most folks, you have some misinformation.
      c’est la vie. best of luck.

    • a common mistake, be careful or you will learn by experience

    • Enlighten me. What are the ‘inherent errors’ in the calculations? What, according to you, is incorrect about my information? I’m eager to learn.

  6. Another important consideration is whether you are using a valve (tube) amp or solid state (transistor). If you are using a valve amp, it will normally have a method of setting the output impedance (a switch or a configurable link on the back panel). It is pretty important to match this to the total impedance of your speaker setup.

    Most solid state amps will tolerate anything down to their rated output impedance with no requirements to change settings.

  7. There are some threads about this point on talkbass http://www.talkbass.com/forum/
    for the german readers on Musikerboard http://www.musiker-board.de
    or bassic http://www.bassic.ch

    Just use the search function to get to the right thread. The sites I posted are musicians boards or bass boards. You can find a lot of important stuff – and it’s fun to read and discuss with other bassmen or musicians.Give it a try!

  8. This is a very simple explanation for something that can be confusing. good stuff :-)

  9. It’s important to remember the rated power of a speaker cabinet is the maximum amount of power the cabinet will accept continuously before the speaker coils burn, they will distort like crazy much before this. If you want a great sound it’s worth over specifying your speakers to allow headroom before they fart out on you and you end up sounding like a tuba :)

  10. I ran my SVTII-PRO head through a 4 ohm ampeg 8X10 AND 2-8 ohm Hartke 4X10 for months on end in rehearsal. Very seismic. Worked perfectly, no problems. I was also using some decent compression as well, and hearing protection…

    • Helloooo !!! Wtf … Went to see Pop Cherry @ Southsides on Friday nite & u guyzz not there ?!!! Sooo disappointed :( so u guyzz are done with them ?

  11. FYI: My Ashdowns Silk-screened Impedence Rating Reads As Follows: Minimum 4 Ohms. I post this for you because “If your head runs at a max of 4 ohms, that means that you can run one 4 ohm cab or two 8 ohm cabs safely.” Could also be interpreted as follows: “If your head runs at a MIN of 4 ohms, that means you can run one 4 ohm cab alone or it may mean that you can run two 8 ohm cabs simultaneously causing the amp to perform at 4 ohms.” If you used two 4-ohm cabs simultaneously, your amp would split down to 2 ohms and cause excessive heat expressed as acrid smoke and damaging flames!” Be well, – Ernie Leblanc.

  12. If anyone can help me here, it would be appreciated, I’m looking for a 2×10 cab to go with my Ashdown ABM C115 300 combo, which runs at 4ohm. A GK cab’s looking like the best value, but it’s 600w, would that damage the amp or the cab? cheers

    • nope, if you put a 600watt amp through a 300 watt cab that would be silly but other way round is fine.

    • I’m currently(pun) running 720 watts @ 2 ohms into (1) 2×10″ 175 watt, 4 ohm bass cab stacked upon (1) 4×10″ 350 watt, 4 ohm bass cab with “IMpressively EXcellent” sound-quality results.

      Just because an automobile has 500 horsepower doesn’t mean it can idle in park or run slowly down Any Lane, U.S.A. at the posted speed limit.

      So, if you have (1) 600 watt power amp rated at the correct ohms for your bass cab, just don’t press the pedal-to-the-metal and things should run just fine.

      -Ernie Leblanc