Considering that speaker/amp connections, impedance, the different combinations, and incorrect wiring are a recurring source of confusion for audio heads and musicians, we decided to try to clear things up a little.
We will try to explain the most common cases and you’ll have the possibility to post your specific questions in the forums.
The two most common notions that we will use here are impedance (stated in Ohms, Ω, and which has a value from 2 to 8 or even 16 in most cases) and output power (stated in Watts, W, with a value ranging from 1 to 1,000 or even more, since some bass amps produce over 1,200 watts of power!). Following, we will briefly refer to the efficiency of a speaker, which is stated in decibels (dB) in most manufacturer spec sheets, and shouldn’t be confused with the output power stated in the same unity; don’t worry, we will explain you why.
So, we will focus on impedance and power, both for speakers and amps, and explain how to combine them for the best results and to avoid any problems. The output power delivered by an amp, regardless of whether it’s a tube or solid-state amp, is given for a certain impedance. If you fail to follow the indications given you could damage your amplifier more or less seriously, which will surely be detrimental to your wallet. Besides this tutorial, you should read the product manual of your amp first to know how to connect the system the best way.
From now on, we will use the term “amp” for “power amplifier.” In most cases a so-called “amplifier head” combines a preamplification stage (gain, EQ, compression, FX loop, one or more channels, etc.) and a power amplification stage (including the volume control and a presence parameter on some tube amps). Thus, when we speak of tube or solid-state amps, the term applies to the power stage. The fact that a preamp stage features tubes has no consequence on the behavior of a solid-state power amplification stage.
On the other hand, a combo amplifier system includes an amplifier head (preamp + power amp) plus a speaker — all within the same cabinet. But from an electronics point of view, each element has to be considered separately. The internal speaker of a combo has its own impedance and has the same electronic behavior as an external speaker cabinet. Everything that we state here applies to an amplifier head with one or two external speaker cabinets, or to a combo — an amplifier head with a speaker plus an eventual additional speaker cabinet.
We address guitar and bass players from the point of view of a musician. P.A. amplifiers work under the same theoretical basis, but they show a specific behavior. To answer your specific questions about P.A. systems, please refer to the P.A. forum.
Faithful to our “from musicians to musicians” philosophy, we will simplify everything, which will result in electronics specialists thinking our descriptions are inexact. Our goal is to explain to you how to connect your gear, and although it is always nice to understand how it works it is not necessary to know every single detail!
Let’s start with some easy theory: what is impedance?
The impedance of a speaker is the “electrical resistance” to the flow of current for a given frequency. The symbol for impedance is “Z.” The higher the impedance, the more the speaker opposes the current being delivered, and the less it develops output power for a given initial power. The resistance of an 8-ohm speaker is two times higher than that of a 4-ohm speaker. In other words, a 4-ohm speaker will let more current pass through the circuitry than an 8-ohm speaker. The lower the impedance, the more the load increases for the amp feeding it, up to the point where it could short circuit.
The impedance of an amp refers to the resistance it can handle (or it expects to experience) when a speaker is connected. In fact, the electronic circuit is conceived to work with a given impedance, which is often very low according to its construction (about tenfold less than the speaker’s), making output stages very vulnerable to excessively high loads. That’s why manufacturers state the impedance the amp can support in the user’s manual. If the resistance is too low or too high the amp could suffer damages.
In electronics there are two ways to wire a circuit: in series and in parallel. A serial connection means that the components are wired successively in the circuit like in a string of pearls (or like stomp boxes connected one after another). A parallel connection means that the circuits is divided in several branches that feed different elements and are later on brought together again.
In the case of an amp with speaker cabinets, most connections between the amplifier head and the speaker cabinets are made in parallel. For example, the great majority of bass amplifier head manufacturers offer amps with two parallel outputs. The signal is common in the beginning, but it is divided among the two connectors, it goes through one or several speakers, and comes back (via the same cable) into the amp where it meets again with the ground/earth. The same applies to the great majority of guitar amps with two parallel outputs.
It might happen that inside the speaker cabinet some speakers have a serial wiring (i.e. they are directly connected to each other) while the others have a parallel wiring… But then again, if you already open or wire the speakers of a cabinet, you don’t need this tutorial!
Except in some rare cases (for example if you have to connect together a lot of speakers or you have weird speaker cabinets), guitar players will usually find only parallel connections, and will have to connect the amp output directly to the speaker.
Some speaker cabinets provide pass-through outputs that allow you to chain several speaker cabinets without coming back to the amp. Although such wiring is electrically correct, it’s better to avoid them in practice. To avoid any potential problems, don’t leave room for doubt (“which connector should I use to link my two speaker cabinets?”). Regarding such pass-through outputs on speaker cabinets, some manufacturers choose parallel wiring while the others use serial wiring. In short, it’s a headache. Use such connectors only if you know exactly what you’re doing!
Instrument cable, speaker cable…
An instrument cable is a small-diameter wire that includes a single isolated conductor surrounded by a shield. A speaker cable includes two isolated conductors and no shield. It can be an actual risk for amps (especially tube amps) if you don’t use the proper cable: to connect a speaker, you must always use a speaker cable. If you’re not sure, don’t do anything: the conductor of an instrument cable has a very small diameter, which can lead to overheating and destruction of the conductor. As a consequence, the full power cannot be transmitted anymore, which can cause damages to the amp’s output transformer.
Now let’s dig in deeper…
After this brief subjective digression, let’s come back to more objective things. When you choose an amp head and matching speaker cabinets or a combo plus an additional speaker, pay attention to the output power of the amp and its output impedance, as well as to the power handling and the impedance of the speakers to be sure it’s a safe system. Afterwards, it is also crucial to pay attention to connections: a wrong wiring can damage your speakers, your amp or both. So, read the specs of the product carefully and connect the right speaker to the right amp with the right cable… and make your sound rock!
To read the full detailed article see: An Ohm Story