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, Id 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 its only running at 190 watts?).
Lets 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. Youll 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. Ive 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 Ill explain that later. A cabinets 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 cabinets impedance is equal to the speakers 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. Thats 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 cant 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, youll 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 youll 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 Ive 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 cant 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. Thats 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 cant 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 youre 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 didnt 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 its safe to be up to 100% off on the load hooked up to a tube amplifier from where its 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 its 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 Ill keep to the basics as it applies to this post. You would think that doubling your wattage would result in twice the volume, but its 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. Thats why using a 4 ohm cabinet instead of an 8 ohm cabinet with your head doesnt 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 arent 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 youre 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. Youll just have to experiment.
