The purpose of this article is to explain how amps and cabs work together in a rig. I'm going to explain how the power from the amp is distributed to cabs, and what effect the cab load has on the amp's power output. I'll show the three most common combinations of amp and cabs, and explain how you can calculate loads.
First off I'll explain the terms used in this article.
Watt is a unit of electrical power. 1 watt is the same as 1 Joule / second. Joule is a unit of work, so watt is actually an expression of work done over time. More watts = more power. As an example, a 60W light bulb is brighter than a 20W light bulb when they are fed the same voltage.
Ohm is a unit of electrical resistance. More ohms = more resistance. Since electrical current always chooses the path of least resistance, we can use resistors to guide the current to where we want it in circuits. Zero ohm is a short circuit, and materials used for isolation usually have a resistance of several thousand ohm.
Headroom is a popular term to describe to excess power available above the cabs rated power handling. Imagine a weight lifter with a personal record of 200 kg (about 400 lbs) in the bench press. If he only lifts 120 kg (240 lbs), he still has a lot left to give. In the bass world this "extra" power would be called headroom.
A bass amp has a power rating of a certain amount of watts. This indicates the maximum power this amp can deliver at its minimum load. The speaker output on most amps have a minimum load rating printed next to it. This is usually 4 ohms, but 2 ohms also occur. What this means, is that the minimum total load that this amp can handle, is 4 ohms. At this minimum load, the amp can deliver its maximum power output. In this article, I'll be using a 500W, 4 ohm minimum load amp, and 4 and 8 ohm cabs rated at 300W.
Bass cabinets have a rating of how many watts it can handle, and its total impedance (that would be the ohms). The cabs in this article are rated at 300W and either 4 or 8 ohms. The impedance of the cab is the sum of the impedance of the speakers in the cabinet. Depending on the internal wiring of the cab and the resistance rating of each individual speaker, multiple speakers can give the cab a total impedance of any number of ohms. I'm trying to keep this as simple as possible, so all I'll offer in the area of calculating total ohms are the formulas for serial and parallel connection of two speakers. The power rating of the cab tells us how many watts it can handle continuously. It can handle peaks around two times its rating, but not for any long periods of time.
To better understand the difference between series and parallel, try making a mental picture of this and refer to the figure below:
When it comes to connecting cabs to amps, parallel connection is the way to go. I've never heard of anyone connecting bass cabs in series, and that's probably because it's not a very good solution. On some bass amps you'll find two speaker outputs. These are wired internally in parallel, so even if you connect one cab to each output they'll be connect in parallel. On most cabs you'll also find two inputs. These allow you to chain two or more cabs together, and all the cabs in the chain will have a parallel connection to the amp. It may look like a series connection, but that's not the case. This method of linking cabs together would be used to connect more than one cab to an amp with only one speaker output. To calculate the total resistance the amp "sees", use the formula for parallel connection. On cabs with multiple speakers, the internal wiring can be a combination of series and parallel to give the desired resistance for the cab.
The image above shows the rear panel of the Ashdown ABM 500 Evo II bass amp. You can see the minimum load rating printed over the speaker outputs. In the case of this particular amp, it can deliver 575W when the load is 4 ohm. This is when the amp is at it's full potential. If we decide to connect a single cab to an amp, we have two choices, connecting a 4 ohm cab or an 8 ohm cab. If we connect one 4 ohm cab to the amp, the amp can deliver all it's rated power. The problem is that we are already at the minimum load the amp can handle. We can't connect a second 4 ohm cab since that will result in a total load of only 2 ohms, which is below the amps rated minimum. The result would be a damaged amplifier. If we decide to connect an 8 ohm cab, the power output from the amp will be roughly cut in half. That means that the amp is only running on half speed. However, we do have the option of adding another 8 ohm cab to get the maximum out of the amp.
In the figure above, you can see the two options compared. The setup on the left shows a single 8 ohm cab connect to the amp. The cab is rated at 300W, but the amp can only deliver a maximum of 250W. The setup on the right shows a single 4 ohm cab connected to the amp. In this case the amp can deliver all it's 500W to the cab. If you end up with either of these setups, the one on the left appear to be the best match. There is only a 50W difference in the power delivered to the cab, and the cab's rating. However, for a single cab rig with no need to expand with another cab, the setup on the right would be the best one. Please keep in mind that this is my opinion, and some players might disagree with this. The reasoning behind my choice is that overpowering a cab is better than underpowering it. To get the volume level you need from the setup on the left, you might have to push the amp very hard and risk clipping the power amp. This results in a distorted sound that can damage your speakers. In short, the amp runs out of breath as the headroom available dwindles away when you crank the volume. The setup on the right offers better working conditions for the amp. To get to the volume level you want, you won't have to turn up the amp as much. The amp has more headroom available, and better control of the speakers. However, if you feed a cab too much power you can still risk damaging the speakers. By then you're probably playing at uncomfortable volume levels, so it's not as risky as with underpowering the cab. In my example setup the difference is only 200W, so the risk of damaging the cab through overpowering is minimal. In my experience modern cabs are a lot tougher than their rating suggests.
The figure above shows the third option. Connecting two 8 ohm cabs to the amp gives a total load of 4 ohms (try inserting the values in the formula for parallel connection above. You should come out with 4 as the answer). The amp can now deliver all it's power, but the power is equally split between the cabs since both cabs are rated at 8 ohms. In this case, underpowering isn't normally a problem since you double the total speaker area (if for example you use two 4X10" cabs), and you'll be able to reach a comfortable volume without pushing the amp too hard. Another option that is rarely used, is connecting a 4 ohm and an 8 ohm cab the same amp. This will result in all sorts of problems. First of all, the resulting load is 2,67 ohms so you'll need an amp with 2 ohm minimum load rating. Secondly, the 4 ohm cab will be fed with more power than the 8 ohm, resulting in a volume difference between the cabs. Of course, if you have a 2 channel power amp you can work around these problems, but that's beyond the intended scope of this article.
My suggestion when it comes to amps and cabs, is to get as many watts as you can afford and an 8 ohm cab. This gives you the most room to expand your rig at a later stage. Just be careful with the volume knob.