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Loudspeaker impedance.


So what is speaker impedance and just what is being impeded?


The “impedance” referred to is a restriction of the flow of power from a source to a destination in an electrical circuit. In this particular case, we are concerned about how the current flow from an amp can be restricted or impeded from powering the speakers in the most efficient way.



Within every speaker manufacturers specification sheet, you'll see a figure for nominal impedance. Typically this will be 4 or 8 ohms but occasionally 15 ohms, this latter value found most often in vintage designs.


In this little piece, I'm only going to deal in detail with the interaction between the power amplifier and speakers. Impedance is a huge subject but briefly, having a good impedance match between all types of electronic equipment will always allow that connection to work as effectively as possible.


Here are some areas where you may encounter impedance considerations in your audio system:


Connecting digital sources CD players, DAC's, Web music streamers.


RF or radio frequency device links such as an FM aerial connection to a tuner.


Phono cartridges to pre-amplifier inputs.


The list is endless, but some cases of impedance matching are more critical than others. In the case of  a loudspeaker  connection to a valve amp, the importance ranking is quite high.


So, what is impedance in the real world. What causes this “restriction” in the flow of current?



Why do valve amps merit special attention?


Primarily, this is because only valve amps have large output transformers that need to be properly “loaded” with the right impedance. The output stage transformer of a valve amp produces a voltage across it's output terminals, and this must be distributed to a correct load – in other words, a speaker of the right impedance. Otherwise we have what is called a “mismatch”. Correct impedance matching prevents wasted or lost power, possible distortion at certain frequencies, and other undesirable attendant problems.


Transistor amps don't have this problem insofar as they don't have an output transformer, and the design is generally more tolerant of speaker impedance mismatch. They’re certainly more forgiving of having an unconnected speaker. In any event with a transistor design, you will encounter audible distortion as a warning, before any permanent damage is caused to the amplifier as a result of an impedance issue.


A summary so far


If your valve amps should see a 15 ohm output load, your speakers should have a quoted nominal impedance close to 15 ohms. However to allow for more flexibility, a valve amp may have additional speaker terminals for 8 ohm and sometimes 4 ohm speakers as well. It's not a good idea to connect an 8 ohm speaker to a valve amp that is expecting a 15 ohm speaker, as current drawn from the power amplifier will be excessive. Likewise it's not a good idea to connect a 15 ohm speaker  to an 8 ohm terminal on a valve amp, otherwise the current cannot be transferred fast enough - remember the car over-revving?


The general rule is, make sure the nominal impedance of your speaker and valve amp output are closely matched and never power a valve amp without the speakers connected.

cont...

A good analogy is what happens if your stiletto got stuck in a car mat, and this keeps your foot hard on the accelerator with the car in neutral. The engine would scream away until something gave out - a piston through the sides of the engine block, bearings wearing out, getting overheated and seizing and so on. This is akin to the effect of a speaker load for a valve amp being non-existent,  i.e no speaker connected. In other words, if there is no load on the output, voltages can rapidly rise uncontrollably to extremely high levels in the output transformer and cause flashover in the windings, with consequential damage to the amplifier.




This is an extreme case, but imagine our car is in entirely the wrong gear for the conditions, and the engine is either revving too much, or else labouring. Well, this is more akin to having a bad impedance match between our valve amp and speakers, rather than a completely unconnected speaker. In the correct gear the car is working safely and efficiently, and it’s the same effect with a good impedance match between our amp and loudspeaker.

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