Most Hi-Fi amps require a minimum load impedance of 4 ohms or 6 ohms.

Two 6 ohm speakers in parallel gives a total load impedance of 3 ohms. This is below the minimum of 4 or 6 ohms, therefore it is not advisable to connect two 6 ohm speakers in parallel directly to a Hi-Fi amp.

You can use a 2 zone speaker selector switch which uses a series resistor. This will add a series resistor of 3-5 ohms (depending on the manufacturer.) Therefore the 3 ohms (of the two speakers in parallel) and the series resistor (of the selector switch) will bring the total load impedance above 6 ohms, so all should be good for the amp.  However the resistor will reduce the power available to each speaker (by up to 60%).  See my speaker selector simulator for more details.

You can also simply connect both speakers in series. This will bring the total load impedance to 12 ohms, which will be fine for the amp as it above its minimum load impedance. However, the maximum power of the amplifier will be slightly reduced. For more practical details see my article on How to connect 2 speakers in series.

For more examples of connecting multiple speakers to a HiFi amp see my article on Understanding Speaker Impedance. The video in this article also describes how the different types of speaker selector switches help with impedance.

Generally no, but with some speakers you can.

Many Hi-Fi stereo amplifiers have connections for Speaker A and Speaker B. These connections are **normally in parallel, so it is the same as connecting two speakers in parallel – but the amp provides switches to turn each set on or off.

These are designed to allow easy connections of 2 sets of speakers. Normally these amp have a warning notice. This notice is saying that when connecting to either A or B, then the speaker should have an impedance of 4-16 ohms. But when connecting to both A+B, then each individual speaker should be 8-16 ohms.

So if you have 2 sets of your speakers which are 8 ohms or more, then you can safely use these connections.  This is because two 8 ohm speakers in parallel gives a total impedance of 4 ohms – the minimum impedance the amp is designed for.

However if your are connecting speakers with an impedance less than 8 ohms, or you have more than 2 sets of speakers, then you should not use these connections as the total impedance will be below the minimum 4 ohms.

For these speakers you could connect them in series to Speaker A only. Or you can use a speaker selector switch connected to Speaker A only.

** Most amplifiers with an A/B switch connect both outputs in parallel, but there are the occasional exception where they connect both outputs in series.

When more one than one speaker is connected to an individual amplifier, they can be wired in series, parallel or (rarely) in series/parallel.

Speakers in Series

Two or more speakers can be wired in series. That is, one wire is used to to go from each speaker to the next. This is not normally used for more than 2 speakers. For a practical discussion on wiring 2 speakers in series, see the article on Connecting 2 speakers to an amplifier. 

To calculate the total impedance of speakers in series simply add the impedance of all the speakers together. For example, in the diagram above, if each speaker was 4 ohms, then the total load impedance seen by the amp would be 4 + 4 + 4 + 4 = 16 ohms.

Adding speakers in series is a safe way of adding speakers to an amplifier, but not always practical. The higher total impedance also doesn’t allow the amplifier to produce its maximum power. For more in how impedance changes amplifier power, see the article on How impedance changes amplifier power.

Speakers in Parallel

Two or more speakers can be wired in parallel. That is, two wires are used to connected from one speaker to the next.

This is the easiest and most common way of connecting 2 or more speakers. Wiring each speaker back to the amplifier with separate wires is also wiring them in parallel.

Both of the above diagrams show 4 speakers wired in parallel. If you follow each wire with your finger on the lower diagram you will see it is effectively wired the same as the first diagram.

Due to the total impedance, it is rare you can wire more than 2 speakers in parallel.

If each speaker is the same impedance, it is easy to calculate the total load impedance of speakers in parallel. Simply divide the individual speaker impedance by the number of speakers. For example, in the above diagrams, if each speaker is 8 ohms, then the total load impedance would be 8 divided by 4, which equals 2 ohms.

If each speaker has a different impedance, then it is is easier to use my Speaker in Parallel calculator.

Wiring 2 speakers in parallel is very common in domestic and commercial installs. You just need to ensure the total load impedance of the speakers in parallel is above the minimum impedance required by the amplifier.

If you need to wire more than 2 speakers in a domestic install, you can use a speaker selector switch if the total load impedance is below the amplifier’s requirements.  For more details on wiring 4 or more speakers to an amplifier see the articles Connecting 4 speakers to an Amplifier, Speaker Selector Switch summary, and my unique Speaker Selector Switch Simulator.

If you need to wire more than 2 speakers in a commercial install, there are three alternatives. Firstly you can get commercial amplifiers designed to work with a load impedance as low as 2 ohms. Secondly, you can use multiple amps, That is, one dual channel amp for every pair or for every 2 pairs of speakers. Thirdly, you can use a distributed speaker system.

Impedance is a characteristic of a speaker you need to take note of, especially if connecting more than one speaker to an individual amp. See the article on Understanding Speaker Impedance for details.

The first important point to note is you should not connect a speaker combination with a total impedance lower than the minimum the amp is designed for.

For example, if your amp says the speakers should be 4-16 ohms, then connecting a 4 ohm, or 6 ohm or a 8 ohm speaker is fine. But connecting two 4 ohm speakers in parallel (which results in 2 ohms total load impedance) is too low. To  calculate the total impedance of speakers in parallel see the popular Speaker in Parallel Calculator.

If you do connect a total load impedance which is lower than the amp’s minimum, you run the risk of overloading the amp; causing it to turn off, blow a fuse or blow the electronics. You can get away with a lower impedance at low volume levels, but as the amp gets close to its full output, it will get hot and stop working.

The second important point to note is you normally can have a total load impedance above the recommended maximum (Eg above 16 ohms). However the higher the load impedance, the less power the amp will be able to produce. See the article on Speaker Impedance Changes Amplifier Power.

Some amplifiers will detect that the impedance is too high and turn off thinking there is no speaker connected, but most amps will have no problem.

The exception is a tube (valve) amp. Most tube amps require some load and don’t like having no load.

In a perfect world, every cable should be labelled, identifying which speaker they are connected to. However, that is often not the case. Perhaps you are helping a friend, or you have moved into a new house which has speakers in many rooms, or perhaps the kids have pulled the wires out for fun. Whatever the reason, you’ll be pleased to know there is a cheap and easy way to find what speaker each cable is connected to.

You will need two things: someone to help you, and a 1.5 volt battery.

Each cable going to a speaker has 2 wires. Make sure the wires are stripped (that is, the outer plastic is stripped off, allowing around 12mm (1/2″) of bare copper wire to be seen at the ends.

Hold one of the wires (doesn’t matter which one) on the bottom of the battery. With the other wire, tap it on and off the top of the battery (the +ve terminal). The connected speaker will make a scratching noise. Note: the speaker only makes a noise when the wire is tapped on or off the battery. So keep tapping the wire on and off the battery until your assistant finds the speaker which is making the noise.

Once identified, label the speaker cable.

Other Methods

Use an impedance meter: an impedance meter sends a tone (normally 1kHz) down the cable to measure the speaker impedance. Most meters also allow the tone to be held on continuously so you can test and/or identify what speakers are connected. This is a great way as the meter will also tell you the total load impedance.

Connect to amplifier: You can connect the cable under test to an amplifier playing some music at a low volume level. Just be careful not to short the wires when connecting the amplifier. Probably best to turn amp off when making each connection.

You can certainly design your speaker boxes, but I wouldn’t.

I’m an installer, not a speaker designer. I’ve installed many speakers, but I wouldn’t like to try to design them. The internals of a speaker box (cabinet) is complex.

The simple calculation of speakers in parallel is only useful if your don’t use a cross-over circuit (a circuit which splits the signal into different frequencies to best match each speaker in the box).

The complexity of the total impedance of a speaker (box) is not the only issue. Other things I’m aware of (among other issues) that need consideration are:

• The internal volume of the cabinet (less the volume of the other speakers) needs to match the characteristics of the biggest speaker
• The effect of the air movement caused by one speaker might have on the other speakers
• The matching of the crossover circuit to each of the speakers
• The phasing of each speaker relative to the other speakers
• The overall flatness of the resultant frequency response.
• The power handling of each speaker

So, I leave all that to the people who get paid the big bucks to design speakers. We need to simply install and enjoy the product of their effort.

It is not possible to make two 8 ohm speakers into a 8 ohm load.

There are only two ways to connect two speakers together, either in series or parallel.

In parallel, two 8 ohms speakers give a total load impedance of 4 ohms – too low for any amp designed for a minimum load of 8 ohms.

In series, two 8 ohm speakers gives a total load impedance of 16 ohms. This will work with amps designed for a minimum load of 8 ohms. However the amp will not operate at full power. In a domestic install, a simple series/parallel speaker selector switch can be used to do this, as well as enable any one pair of speakers to be connected on their own. However, with a 16 ohm load, the amplifier’s output power will be reduced by 3dB compared to using just one 8 ohm speaker.  For more info on this see the article on how Speaker impedance changes amplifier power. 

For the best power output, you would be better off to use just one 8 ohm speaker, provided it will cope with the full amplifier power.

So, it comes down to what you are trying to do. If you want maximum power, then use just one speaker. If you want a wider area covered than what one speaker will cover, then use the two speakers in series.

Also if maximum power is not an issue, then you could also use a speaker selector switch with a series resistor. This will add a 3-5 ohm* resistor in series with the speaker. This will allow both speakers to be wired in parallel (4 ohms total load impedance) in series with the resistor (3-5 ohms) to give a total load impedance close to 8 ohms. However the resistor will take close to half the power of the amplifier.  This is not normally a problem when using multiple speakers in a house, but not so good in a commercial installation requiring maximum power.

*the value of the resistor depends on the manufacturers design.

To see how much power a speaker selector switch resistor takes, see my simulator here.

For a more a detailed understating of speaker impedance, see the article on Understanding speaker impedance.

For more on how to wire speakers in series or parallel, see the article on Connecting 2 speakers to 1 amplifier.