Ohms FAQ

Discussion in 'Amps [BG]' started by IvanMike, Sep 19, 2004.


  1. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    The purpose of this post is to hopefully answer the FAQ about Ohms (as they regard to speakers and the power ratings of amplifiers) , hooking up cabinets, how many cabinets can a given head power, etc. - Hopefully even become a sticky - I spend a lot of time answering posts about the info contained here

    As always - your comments, and corrections are welcome. Be brutal, I’d rather have this as a good post than for you to be nice.

    At some point in your bass career you have found yourself confused by the term “Ohms”, and by this term when it applies to speaker cabinets, (what the heck is the difference between 8 ohms and 4 ohms anyway?), and when it applies to amplifiers (what does 4 ohm min load mean? - I thought this was a 300 watt amp, why do they say it’s only running at 190 watts?).

    Let’s start with a quick glossary of terms used in this post.
    Ohms are a measure of resistance to an electrical signal.
    Impedance Is very similar to resistance and will be treated as the same for our purposes. Impedance is expressed in Ohms. (e.g., “this cabinet has an impedance of 8 ohms“). Impedance actually incorporates resistance, capacitance, and inductance in an AC circuit. But it still holds the same basic principles as resistance in a DC circuit.
    Load: This is a term used to describe the total impedance a power amp has plugged into it in terms of speakers or cabinets.
    Speaker or Driver: These are the round things in cabinets that actually produce the sound. Each speaker will have its own impedance in ohms, (hopefully printed on it).
    Cabinet: A box containing one or more speakers or drivers. A cabinet will have a net impedance in ohms, (hopefully printed on it). Often referred to as a speaker.
    Amp: For our purposes here, when I say “amps” I mean power amps, or the power amp section of a head or combo.
    Power Amplifier: A power amplifier is designed to take line level signals (from a preamp) and convert it into speaker level signals capable of powering speakers.
    Amp Head: An amp head is a preamp and power amp in one box. You’ll know if you have an amp head if all you need is a bass, an instrument cord, a speaker cord, and a cabinet.
    Combo Amp: These are “combinations” of a preamp, power amp and cabinet all in one. All you need is a bass and an instrument cord to rock!
    Solid State Amps: A power amp section that creates power by using solid state devices. Be careful, a lot of heads called “tube amps” really only have tubes in the preamp section, but use a solid state power amp. These are more correctly called “hybrid amps”.
    Tube Amps: A power amp section that creates power by using “vacuum tubes” or “valves”, (same thing).

    Speakers and Cabinets: Speaker cabinets for bass are typically rated at 8 ohms or 4 ohms. An 8 Ohm cabinet will have twice as much resistance to the electrical power coming out of an amp than a 4 Ohm cabinet. This means that the 4 Ohm cabinet will end up getting about twice the wattage from the amp than the 8 Ohm cabinet will. Some players use PA cabinets and guitar cabinets rated at 16 Ohms. I’ve never seen a 2 ohm cabinet for bass or anything else, but they must exist. Some bass cabinets are rated at 5.3 ohms (usually these are the various 3x10 cabinets). For practical purposes, these 5.3 ohm cabinets should be treated as 4 ohm cabs, but I‘ll explain that later. A cabinet’s Impedance Depends on the impedance of the speakers in it and how they are wired. Speakers can be internally wired in series, parallel, or a combination of both. If a cabinet has one speaker in it, you guessed it, the cabinet’s impedance is equal to the speaker’s impedance. When wired in series the ohms of the speakers get added together for more resistance (so a 2x10 cabinet wired in series with 4 ohm speakers would have a net impedance of 8 ohms.). When wired in parallel, the impedance is decreased according to a formula. The formula is based on adding and reducing fractions. (For you in school this should be no problem, for us older people - this will seem familiar (maybe) if a bit hazy - lol. The formula for parallel impedance is as follows;

    Total impedance = 1/(1/a + 1/b + 1/c ... + 1/n).

    When all the cabinets or speakers are the same impedance, all you have to do is add up the # of speakers and divide the resultant number into the # of ohms of each speaker. For example, three 16 ohm speakers wired in parallel in a cabinet = 3/16, or 5.333333.… ohms. That’s how they get those 5.3 ohm 3x10s btw! (The 2x10 cabinet with 4 ohm speakers mentioned above wired in parallel would have a net impedance of 2 ohms). When the impedance of the different cabinets or speakers is different, you have to use the full formula. With a 4 ohm cabinet and an 8 ohm cabinet run in parallel the formula is 1/(¼+1/8). You can’t add ¼ to 1/8 without having the same denominator (I told you this was gonna sound like school), so you have to reduce ¼ to 2/8 so they match. Then adding 2/8 to 1/8 gives you 3/8, dividing 8 by 3 gives you 2.67. So, hooking up an 8 ohm cabinet and a 4 ohm cabinet in parallel to your amp gives the amp a load of 2.67 ohms to power.

    For the most part, you’ll never have to worry about the impedance of individual speakers, only the net impedance of the cabinets. All that math junk is a pain, but needed to figure out the total impedance of a cabinet combination you plan on using. Most cabinets are designed to be “daisy chained” together. On the back of the cabinet you’ll probably see 2 ¼ inputs labeled “parallel inputs”. This is the most common method of “daisy chaining” cabinets together. Almost always this results in the cabinets being run together in parallel. You may want to check with a particular brand of speaker, but I’ve never run across a cabinet that daisy chains in series.

    Solid State Amps are rated for the amount of watts they will put into various loads. A typical rating might be “200 watts @ 8 ohms, 350 watts @ 4 ohms, 425 watts @ 2 ohms“. They are also rated for the minimum safe load they can power before the amplifier is likely to become damaged, (typically 4 ohms or 2 ohms). You should never run the amplifier below its minimum safe load - doing so will most likely damage the amplifier, and void your warranty!! For example, for an amplifier rated at 4 ohms minimum safe load you could run it with one 8 ohm cabinet, two 8 ohm cabinets (for a net load of 4 ohms), or one 4 ohm cabinet. Running with three 8 ohm cabinets gives you a net impedance of 2.67, as does running with one 8 ohm cabinet and one 4 ohm cabinet. So in the case of the 4 ohm minimum amp, you can’t do it. Even worse would be running with 2 4 ohm cabinets for a net impedance of 2 ohms!. Before I mentioned that for our purposes we should treat 5.3 ohm cabinets as 4 ohm cabinets. That’s because when figuring out net impedance of cabinet combinations the result is almost the same as using a 4 ohm cabinet in the equation. If you use one 5.3 ohm cabinet in parallel with one 8 ohm cabinet the formula is (1/8+1/5.3) = (8/42.4 + 5.3/42.4) = 13.3/42.4 = 3.13 ohms. You still can’t use that combination with a 4 ohm minimum amplifier. A solid state amp, head, or combo, will have speaker outs on the back of it. Combos may not have any speaker outs, and this is generally because they are already running at their minimum safe load. A head or power amp with more than one speaker output, (per side if stereo - see below), will almost always have those outputs run in parallel, (but as always, check if you’re not sure).

    Most solid state power amps and a small amount of amp heads are “stereo” containing 2 separate power amp sections. This allows you to connect more cabinets to the amp, and of course gives you more wattage. Most of these stereo amps can be run in “bridged mode”. This connects the two amplifiers together and requires you to hit a switch when the amp is off, and to hook up your speaker cable differently, (usually by connecting a banana jack to the positive terminal of both 5 way output jacks on the back of the amp. When running in bridged mode each amp “sees” ½ of the total impedance of the cabinets connected to it. Therefore, an amp rated at 4 ohms minimum per side that will put out 300 watts into 4 ohms per side can only be safely bridged into an 8 ohm load, and will put out 600 watts into the 8 ohm cabinet. Most amplifiers only safely power a minimum of 4 ohm loads, however, more and more amplifiers have been produced that handle 2 ohm minimum loads, which is good news for us, as we can run a good number of cabinets in different combinations. As a last point, you may have noticed that the ratings for our “typical amplifier didn’t double each time the impedance of the load was cut in half. Why not? Well, in a perfect world, an amplifier that put out 200 watts into 8 ohms would put out 400 watts into 4 ohms and 800 watts into 2 ohms. In the real world, other consideration, (such as thermal limits), limit the actual number of watts an amplifier will safely produce at a given impedance.

    Tube Amplifiers are a different animal, and many have switches to match the impedance of the cabinets to be connected to them. A given tube amp might be able to send a 400 watt signal to an 8 ohm, 4 ohm, or 2 ohm load, but the switch on the back must be set to the load being used). Generally it’s safe to be up to 100% off on the load hooked up to a tube amplifier from where it’s selected. For a tube amp set for a 4 ohm load that would mean that you could run it into anywhere from an 8 ohm load to a 2 ohm load and still be reasonably safe. The important difference between tube amps and solid state ones is that it’s dangerous to run tube amps above their impedance rating!! Running a tube amp set at 2 ohms into an 8 ohm load can dangerously raise the plate voltage of the amplifier and ruin it. Also, while most solid state amps will do ok with no speakers plugged into them (infinity ohms), tube amps need to have a load connected to them, or they will generally self destruct.

    Volume A general discussion of decibels (dB), and volume is too big to include here, so I’ll keep to the basics as it applies to this post. You would think that doubling your wattage would result in twice the volume, but it’s just not so. Doubling your watts results in a 3 dB increase in volume, which is generally the smallest change in volume a person can hear. That’s why using a 4 ohm cabinet instead of an 8 ohm cabinet with your head doesn’t always blow you away. Cabinets also have sensitivity ratings (measured in dB). The higher the sensitivity a cabinet, the louder it will be. So, using a head with a 4 ohm cabinet with a sensitivity rating of 100 dB will be about as loud as using it with an 8 ohm cabinet with a sensitivity rating of 103 dB! These ratings aren’t generally printed on cabs, you have to do some research to find them out. Using multiple cabinets (say, two 8 ohm cabs on a head instead of one 8 ohm cabinet generally results in a volume increase of from 3 - 6 dB. This is because not only are you increasing the wattage, but you’re also moving more air with the additional drivers. Of course, if you are using different cabinet configurations,
    ( like a 4x10 and a 1x15), your results may vary considerably. Also, if you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one. Depending of the sensitivity and size of the cabinets, you may notice a significant difference in volume between the 2, or none at all. Even using two 8 ohm cabinets of different sensitivities or configuration on a 4 ohm min amp may result in volume differences. You’ll just have to experiment.
  2. oddio

    oddio

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    I haven't finished reading yet, but I found a glitch that you might want to fix. Obviously you meant ohms, but let's not confuse anyone with inaccuracy. Back to reading
  3. Tim__x

    Tim__x

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    I have to admit, I skipped the last third of your post. But anyway, I think it was a great post and a good idea. Let me be the first to say, Sticky?
  4. DubDubs

    DubDubs

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    (say, one 8 ohm cabs on a head instead of one 8 ohm cabinet...)

    Another typo. Obviously it should be 2 not one.
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  6. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    those two typos fixed - thanks for the input
    please let me know if i've been misleading in any way too
    also, sorry the post is so verbose, i tried to pare it down as much as i could :rolleyes:
  7. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    let me know if reading this answered any questions you had - especially if reading this prevented you from posting a question on this forum
    thanks
  8. My bass in your

    My bass in your

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  9. Juddium

    Juddium

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    Didn't read past the glossary, but I did find a minor glitch in the glossary I felt lead to point out (even though it's really nitpicky)
    Impedance is not the same as resistance. It's similar to, but not the same - it incorporates resistance, capacitance, and inductance in an AC circuit. But it still holds the same basic principles as resistance in a DC circuit.

    just thought I'd muddle things up a bit for good measure :)
  10. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    definition of impedance corrected. :p
  11. lucas vigor

    lucas vigor Banned

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    What does the word "bridged" actually mean?

    I had a Carvin RC210 combo with head. In the back, the outputs were " 4ohms hi-freq, 4 ohms lo-freq" (I understand that these two are the bi-amped outputs) and then there were two separate outputs labeled "bridged only".
  12. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    as i understand it the two power amps are connected together or bridged while the polarity of one of them is reversed. The net result is akin to having one larger power amplifier.
  13. sandman

    sandman

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    thanks for the post!

    You mentioned about different cab ratings,
    what happens when you plug say, a 4ohm cab together with an 8 ohm cab, do they draw power evenly?
    Tombowlus?
    :eyebrow:
  14. matthewfoote

    matthewfoote

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    Great post :hyper:

    Sticky sticky sticky :bassist: :bassist: :bassist:
  15. Surgeon

    Surgeon

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    1st: Great post, I actually logged on to TB today because I'm looking at getting a new bass amp and wanted to better understand what all the ohm/watt stuff I was reading meant.

    2nd: I'm assuming that cabs that have multiple impedences listed have the capability of switching the order (parallel vs. series) of speakers, is this correct?

    3rd: So it is bad to have an impedence that is too small for your amplifier, and therefore connecting multiple cabs in parallel can be bad because the impedence decreases as you do so. So can cabs be connected in series, and if so is 8 ohms the maximum impedence that can be overcome?

    4th: I've read about "headroom" and I am not sure what this means. Is it more power in the amp than the speakers will need or the ability of the cabs to take more power than the amp can provide.

    And I always thought physics would be useless.

    V=I*R
  16. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    and i quote..........."Also, if you hook up a 4 ohm cabinet and an 8 ohm cabinet to an amp that can power a 2 ohm load, the 4 ohm cabinet will get twice the wattage as the 8 ohm one. "

    looks like someone didn't read all the way to the end of the post :p
  17. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    the only cabs ive heard of with variable impedances are accugroove - i think they actually have a switch on them. I'm not sure how they are wired internaly, but your guess is as good as mine
    yes, as the post states, it is bad to run your solid state amp with an impedance lower than the minimum safe load printed on the amp. You can do some rewiring of the cabinet's parallel outs to wire them in series, but it kind of defeats the purpose of having multipe cabinets. pretty soon you end up with a 32 ohm load and every cabinet in your huge setup gets a grand total of 7.75 watts.
    i have no clue what you're talking about when you ask if 8 ohms is the maximum impedance that can be overcome?? Are you fighting it? to the death maybe?? :D :p
    I left "headroom" because the post was already too long. To answer the question in a short and incomplete way, "headroom" refers to the amount of dB your amp can cleanly reproduce over the average program volume/level. A doubling of the wattage = a 3 dB gain, ten times the wattage = a 10 dB gain. Most cabinets can handle "peaks" of twice their RMS rating, (my bag ends can handle 4 times the RMS rating :p ). So if you are running a cabinet that has an rms rating of 300 watts, running it with an amp that will put 600 watts into it will give you 3 dB of "headroom" as long as you don't run the amp flat out. Once an amplifier runs out of headroom it starts to "clip" or distort, and solid state amps sound terribly nasty when they do this. With a powerful enough amp, this clipping can even fry your speakers. A lot of players intentionally use amps rated for more than their cabs are rated for in order to have headroom, but they are also very careful not to run the amps so high that they will overdrive the speakers.
  18. Surgeon

    Surgeon

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    Eureka! :eek:

    It suddenly makes some sense.

    Thanks again.

    Sticky! Sticky! Sticky!
  19. MichaelScott

    MichaelScott

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    Great post.

    The only thing I could think to make it better is to add some pics of the possible ways to wire the speakers in a cab for different ohm combinations.

    Another TBer posted a pic about a week ago and when I saw it everything made sense.

    Sticky +1
  20. IvanMike

    IvanMike Player Characters fear me... Supporting Member

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    actually i'm kinda against that. Most of the time you end up with an 8 ohm cabinet someone mods into a 2 ohm cab (and even if it is marked well that causes the risk of someone using it with an amp that can't deliver into a 2 ohm load and frying their amp), or a 4 ohm cabinet modified into a 16 ohm cab (which is just freakin quiet).
  21. Magman

    Magman

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    With decibels being an exponential measurement scale --- 3db can be quite a noticeable diffence at higher pressure levels if I understand the math correctly.

    the difference between 70 and 73 db maybe barely noticeable, but I would image that the difference between 110db and 113db will be quite significant (at least, much more noticeable than the difference between 70-73)

    Wouldn';t it all be easier if amplifiers max output were measured in voltage? :)



    Jolee likes this.

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