There is a lot of false and unclear information about the output impedance of your guitar amp and what the specific consequences are for not following the golden rule of thumb (see later). In this article we would like to give you the practical basics so you can be confident to hook up your guitar amp without damaging it.
Electrical impedance (Z) is the total opposition to alternating current by an electric circuit*. Don’t worry if you don’t understand this, as it has little value to the practical guitar player. We just wanted to include a definition for the sake of completeness. :)
Every device that passes an electrical signal has an input- and output impedance. If we connect the output of a device (e.g. guitar amp) with the input of another device (e.g. speaker cabinet), we need to follow a specific rule depending on what property of the signal we want to transfer. Between guitar amps and speaker cabinets we want to transfer power. For transferring power the golden rule of thumb is:
The input- and output impedance needs to be the same (or matched as we commonly say)
You can however get away with an impedance mismatch without damaging your guitar amp if you know what you are doing. Below we summarized the do’s and don’ts of hooking up a tube-amp or transistor/solid-state amp. Note that there is a big difference between a solid-state amp and a tube-amp.
If speaker impedance is higher than amp impedance, the amp can’t give away all its power. Part of it gets reflected back into the amps output transformer, heating it up. If speaker impedance is much higher than amp impedance (infinite for nothing connected), all power gets reflected back into the amp causing serious damage to the output transformer.
If speaker impedance is lower than amp impedance, the amp needs to work harder to get the same output level as with an impedance match. Since a tube-amp is limited to how much power it can give, it can never damage itself. The tubes get worn out faster (since they have to work harder) and the amp won’t seem to be as loud as with an impedance match.
As a precaution, most tube-amps automatically short circuit (impedance of load is 0) there output when no jack is connected. This is to make sure that if the amp were to accidently turned on with nothing connected, it wouldn’t cause damage to the output transformer. Just make sure you don’t have a cable without a speaker connected to the output of the amp, as this would bypass the safety precaution!
You can get away with a ½ mismatch for tube-amps. For example, you can connect an 8 ohm output of a tube-amp with a speaker cabinet of 4 or 16 ohm without causing too much harm. But you will lose power and bandwidth causing the guitar to sound thinner and less defined. Never connect a 4 ohm output to a 16 ohm speaker cabinet.
If you need to make a mismatch it’s better to have the speaker impedance lower than the amps impedance (as described above).
If speaker impedance is higher than amp impedance, the amp won’t produce more power than it can give away. It’s therefore safe to turn on a solid-state amp with noting connected to it (infinite impedance). PA technicians sometimes confuse tube-amps with solid-state amps and tell guitar players that it’s okay to turn on their guitar amp with no speaker connected to it, but this can be very dangerous as described above.
If speaker impedance is lower than amp impedance, the amp will increase its output power to drive the bigger load**. Since there is no theoretical limit to how much power a solid-state amp can give, it will simply blow-up when you short circuit (impedance is 0) the output of the amp. Luckily most amps have fuses and short-circuit protection build in.
As you can see a solid-state amp is like the opposite of a tube-amp. So it’s good to know what the output stage of your guitar amp is (some modern amps combine a tube pre-amp with a solid-state output stage). We just scratched the surface with this article, but hopefully you will now have a much better understanding of impedance and what to watch out for.
* This is a very brief definition. Explaining it fully would go well beyond the scope of this article.
** In electrical terms we call the device that receives the signal the “load”. If the input impedance of the load is very low it requires more power to be driven and is therefore called a bigger load.