What is loudspeaker 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 an audio system in addition to the speaker consideration:
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.
Some cases of impedance matching are more critical than others. In the case of a loudspeaker connection to a valve poewr amp, the importance ranking is quite high.
So, what is impedance in the real world. What causes this “restriction” in the flow of current?
A good analogy is what happens if your stiletto got stuck in the car mat, keeping 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, seizing up 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 (the engine), voltages can rapidly rise uncontrollably to extremely high levels in the output transformer, causing 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.
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 amp 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.
So why all this talk about valve amplifiers. After all, they're a minority group?
I'm far from being prejudiced or transistor-phobic in any way, despite the sheltered life I've led, so I'll talk about transistor amps and impedance right now, but this will apply equally well to my true love-the tubed babies.
Now, the lower the impedance of the speaker, the more current is drawn from the amplifier, tranny or valve. For the sake of ease, I'm briefly going to scribble about impedance (Z) as if it were resistance (R). There are significant differences between them as impedance changes with frequency, but pure resistance doesn't. I'll come back to that. Both impedance and resistance are measured in ohms.
Now, the output voltage of an amplifier does vary through loud and soft passages of the music, but again for illustrative purposes in this case I'll keep it fixed at say 10 volts AC. Imagine the maximum output of our amp is rated at 50 watts. So to get our 50 watts with an 8 ohms speaker the calculation is:
V(Voltage)/R(Resistance)=I(Current), 10volts/8 ohms =1.25 amps
But to get our 50 watts with a 4 ohm speaker the calculation is:
V(Voltage)/R(Resistance)=I(Current), 10/4 =2.5 amps
This shows a doubling of the current requirements for the same power.
If the speakers' impedance became 2 ohms, the current requirement would now be 5 amps for 50 watts. Not brilliant!
This is an ideal opportunity to point out the folly of connecting more than one pair of speakers to each set of output terminals of an amp. By paralleling them up on from a single connection, you are reducing the load impedance by half, (if both these speakers have the same impedance). Two pairs of 8 ohm speakers connected to a single terminal set will give the amp an impedance of 4 ohms to deal with. Three pairs would provide a nominal impedance of 2 ohms. Notice I've slipped in the word “nominal”. All will be revealed, but meanwhile.…
The conclusions from this bit.
The amplifier will deliver maximum power to the speakers when the speaker impedance matches the output impedance of the amplifier. If the total speaker impedance is too low, it will draw too much current and cause the amp to overheat. The sound quality may become distorted, and in bad cases could cause damage to the amplifier. If the speaker impedance is higher than the output impedance of the amp, the current flow will be diminished and reduction of optimum power output is very likely .
Despite these Armageddon scenarios, in practise with transistor amps, it's fairly unlikely that there will be any lasting major tragic event over impedance issues, unless you are very carefree with the volume control or unwisely pile speakers onto each terminal.
So far I've spoken about impedance as if it were resistance, but it's not. The difference between resistance and impedance is that the value of a resistance remains fixed, regardless of the audio frequency passed through it, but impedance is frequency dependant and it’s resistive properties change markedly at different frequencies. This is where that little word “nominal” comes about, as in “Nominal impedance”. In this context nominal means “around” “should be”or “typically”-just take your pick. Hardly a definitive figure and not that reassuring. Basically our speaker impedance of 8 ohms will vary depending on the frequency of the audio signal, so it's quite possible that it could fall below 4 ohms and/or rise to as much as 20 ohms and everything else in between, over the audible frequency range. In other words, the speaker is presenting a reactive load to our amplifier. This is normal and to be expected, but some speakers are worse culprits than others. If your local dealer takes an intake of breath and tuts like a good'un when you mention that you are using stacked Quad electrostatics, he's probably going to say “they're a difficult to load mate, better buy my latest Superbox Compact 3's”. Don't, of course.
So what does this mean to us? Well, bearing in mind our discussion about the effect of speaker impedance values and amplifiers, this additional variable of fluctuating impedance needs to be taken into account when creating a good combination of speaker and amp. With an underpowered amplifier, even with speakers that have the correct nominal impedance rating for the amp, if they present a difficult load to drive (the impedance fluctuates wildly), they could be making excessive demands on the amp and cause distortion, even at low levels. Incidentally, this distortion can sound very similar to record tracking distortion, so don't blame your beloved deck prematurely. Borrow a more powerful amp and check if the distortion still exists. Now, what makes you think I'm speaking from experience here, 'cos you may just be right!
What causes this variation in speaker impedance then?
A number of factors. The design of the individual drive units and the interaction between them is important. Each driver is a mechanical device and has it's own physical resonance. This causes the impedance of the drivers to rise drastically at their resonant frequency, especially in the case of the bass and midrange units. The resonance of a high frequency unit may be designed to be above the audible range and easier to tame by the manufacturer. The crossover network is very important to the overall design here. A crossover network 's function is not to just filter which frequencies go to which driver, but to mange and control big impedance fluctuations of the complete speaker system. A crossover network is full of reactive components such as capacitors and coils, and these can be used to good effect to tame impedance changes, but having said that, a poorly designed network could introduce peaks and troughs of it's own to the overall speaker impedance curve
It's also interesting to note that a speaker impedance value can change at at the resonance frequency of the port in a reflex type enclosure. After all, the port has it’s own physical resonance and this affects how the drive units behave and consequently the electrical impedance of the system. Everything is interlinked. Of course the loudspeaker unit coils have their own resistance too and this could change as the coils heat up at higher power levels.
Try and match the nominal impedance of your speakers to the output impedance quoted in the amplifier manufacturers specification. If an amp is under-powered for the speakers to begin with, a difficult or incorrect load impedance with the speakers could well tip the amplifier into distortion. Impedance matching is more critical with valve amps than transistor designs and let me reiterate, avoid running a valve amplifier with no load connected. It’s best not to connect more than one pair of speakers to one pair of amplifier output terminals. It may appear to be working okay, but it could cause harm to the amplifier further down the road, especially on party night.