Koch Guitar Electronics 2020.
Voltage, Current, Impedance of guitar speakers
In this article I will explain the basics of guitar speakers such as the Voltage, Current, Impedance, Series and Parallel connection of speakers.
Voltage is the height of the audio signal that an amp offers to a speaker. If you compare it with a tap water installation, voltage is the height of the water in a tank or the water pressure in the water system.
An amplifier sends this audio signal as a current to the speaker. This current flows through the speaker and pushes it to create soundwaves. In a tap water installation, the current is the water flowing from the tap.
Impedance or resistance is the matter in which the speaker resists the audio signal. In a tap water installation resistance is determined by the tap. The more you open the tap, the lower the resistance gets for the water and the higher the water current becomes, and vice versa.
So remember: higher resistance = less current and lower resistance = more current.
Series & Parallel Speaker Wiring
If you connect two speakers in series (see diagram below), both speakers will resist the audio signal more than one speaker and the total current will get less. In a tap water installation if you place a second tap behind the first tap (in series), the second tap lets only a part of the water through that comes from the first tap. So the total water current gets less. As we saw above less current is the result of a higher resistance.
So if you connect two speakers in series the total resistance or impedance becomes higher, it will increase to double the value.
On the other hand, if you connect two speakers in parallel (see diagram below or, by plugging the second speaker in the extension output of you amp or speaker box), both speakers will draw current from the amp. So the total current will increase to twice the value. In a tap water installation this is the same as using a second tap next to the first tap. Both taps together will supply twice the water current. As we saw above more current is the result of a lower resistance.
So if you connect two speakers in parallel the total resistance or impedance becomes lower, it will decrease to half the value.
Since the introduction of the first “Radiola” moving-coil loudspeakers in 1926, we use the word “impedance” to define how much a speaker loads an amplifier. The value of this impedance is specified in “ohms”. But why in ohms? Isn’t ohms used for resistors? And isn’t a speaker a totally a different thing than a resistor?
This is all true… let me try to explain.
Ohms & Frequency
A resistor has the same ohm value throughout the whole frequency area, this ohm value is called resistance (R). A speaker on the other hand has an ohm value that depends on the frequency, therefore this ohm value is called AC resistance or impedance (Z).
The picture to the left shows how the impedance of an average 4 ohm speaker varies with the frequency.
In the graph you can see that the 4 ohm (3.99) impedance of this speaker is an average (or nominal) value. The lowest value is 3.47 ohm (at 400Hz) and the highest is 12 ohm (at 20 KHz). Also, you can see that there’s a peak value of 6.5 ohms at 100Hz, this is called the resonance frequency of the speaker.
Impedance vs. Output Power
As you might know, the output power of solid state amps varies with the load resistance of the speaker. A solid state amp generates a constant ouput voltage and therfore if the impedance of the load increases the output power decreases, this happens in a lineair way. Normally, full power is given at 4 ohms, half power at 8 ohms and a quarter power at 16 ohms. This means that when a speaker loads the amp, the power which the amp generates depends on the frequency, because the impedance of the speaker varies with the frequency.
However, with a tube amp this works differently. Especially tube amps for guitar do not generate a constant output voltage. The output voltage increases when the load decreases. If a tube amp gives full power at a 4 ohm load, it might generate even 70% or more of it’s power at 8 ohms (instead of 50%). This is one of the reasons that tube amps sound different than solid state amps, they react different to the speaker’s dependence of frequency.
Tube Amps & Speaker Output
As described above solid state amps lose their output power in a lineair way: the higher the impedance, the less output power. If the speaker is disconnected, there’s no load and the amp’s output power is zero.
With tube amps this is totally different. A tube amp tries to maintain its output power and as a result of that it raises its output voltage if the impedance of the load gets higher. So if you play a tube amp and disconnect the speaker the load gets unendlessly high and the amp will raise its output voltage to very high levels (thousands of volts inside the amp).
Parts such as output tubes and output transformers are not able to withstand these high voltages which often results in severe damage to those parts. So keep this in mind if you have a tube amp; always connect a speaker or a dummy load to your tube amp, never play it without speaker or power attenuator/dummybox connected.
Our ATR series of hybrid amps the Jupiter, Startrooper and ATR4502 (see also the blog article about these amps), are designed in such a way that they react the same way to speakers as tube amps. Although they operate with mosfets to drive the speaker, their ouput power does not fall back to 50% if the impedance is doubled. This is one of the reasons why our ATR series of hybrid guitar amplifiers sound like real tube amps.
Although their speaker outputs behave similar to tube amps you can safely disconnect the speaker and play them without a speaker or load connected. If you do that you will not damage anything.