Mastering parallel speaker wiring: An essential guide

What is parallel speaker wiring?

Many modern amplifiers have terminals to which two pairs of speakers can be wired, along with ‘Speaker A/B’ switching: it’s a convenient feature should you want to add more speakers, perhaps wiring them into another room, or using installation speakers to fill a large space with background music, but it usually comes with a warning, either in the user manual or printed near to the speaker terminals, to pay close attention to the impedance of the speakers used in such a set-up.

This is what’s meant by parallel speaker wiring – two speakers on one amplifier channel shown here:

You’ll see that the cable is split to feed the two speakers, and the overall effect is to halve the impedance – in very general terms, the resistance of the combined speaker system –which can be a good thing, as a properly-designed amplifier will deliver more power into a lower impedance. So, using two 8ohm speakers, whether you use an amplifier with A/B switching or just run two cables from a single set of terminals to the two speakers, the amplifier will ‘see’ an impedance of 4 ohms.

So far so good, but if you use speakers with a lower nominal impedance, the combined effect of the two speakers may reduce the impedance to a point where the amplifier begins to struggle. Most consumer amplifiers quote a power output into 8ohms, this rising into a 4ohm load – in well-designed amplifiers the power may double into 4ohms – this being principally because most speakers’ impedance isn’t a fixed value but may change with frequency. You’ll find many speakers quote a nominal impedance and a lower minimum impedance figure to take account of this.

Understanding speaker impedance and power handling 

It may seem logical to think that lower impedance is a benefit, allowing the amplifier to deliver more power, and thus increasing it’s a ability to play louder, but it’s not as simple as that: to simplify, the lower the impedance, the faster the energy flows out of the amplifier to the speaker, and thus the faster that energy needs to be replaced by the amplifier’s power supply section. 

And if the energy isn’t replaced as fast as it’s rushing out of the amplifier, the output stages will be starved of energy, and are likely to ‘clip’ – literally lopping off the peaks and troughs of the waveform they’re trying to reproduce. If you’ve ever played a modestly-powered amplifier too loud into your speakers, the hardening of the sound is that clipping effect, which will rapidly cause the amp’s protection circuit to kick in or, at worst, damage your speakers. Severe clipping can overheat the voice-coils in your speaker’s drive-unit, at worst fusing the windings of wire into a solid mass; the result is a defunct driver, needing replacement.

Hence the need to be careful about your impedances when parallel-wiring multiple speakers to and amplifier output: stick to speakers with an 8ohm nominal impedance, and pairing them up will reduce the impedance to 4ohms, with which most amplifiers will cope; use speakers of 6ohms or even 4ohms, and you’re very likely to tip the amplifier into protection and shut-down – unless, that is, you’re using a monster power amp capable of doubling its output repeatedly as impedance halves.

That’s why amplifiers with multiple speaker outputs will usually caution the user to stick to speakers with an impedance of 8ohms or greater if more than one pair is to be used: the designers know just how much abuse their amp can take! 

This kind of parallel speaker set-up, as already mentioned, is very effective for background music duties, whether with dedicated custom-installation speakers hidden in ceilings or walls, or just a second pair of speakers in another room to help fill your home with music. Just don’t expect it to make your main system music louder: ‘stacking’ two pairs of speakers in that way will probably give you a more confused sound, and what you want would be better achieved with a more sensitive single pair of speakers and/or a much more powerful amplifier. 

Connecting speakers to amplifiers in series

Wiring your speakers in series has the opposite effect when it comes to impedance: the amplifier now ‘sees’ the combined impedance of the two speakers, so a pair of 8ohm speakers will present a 16ohm load to the amplifier, which will decrease the output of the amplifier to the speakers.

Wiring your speakers in series has the opposite effect when it comes to impedance: the amplifier now ‘sees’ the combined impedance of the two speakers, so a pair of 8ohm speakers will present a 16ohm load to the amplifier, which will decrease the output of the amplifier to the speakers.

As you can see in the diagram above, the connection method is different: here the positive terminal of the speaker on the right – remember this shows two speakers on one amplifier channel, not a stereo pair – connects to the negative on the second speaker. To simplify, the two speakers now form a loop, rather than effectively being connected side-by-side.

However, the complexity of wiring up a configuration such as this may be daunting, so for most set-ups where multiple speakers on each channel are required, parallel wiring will be the most practical approach.

Techniques for wiring speakers in parallel

• Use speakers of the same type and model throughout: mixing speakers will give a confused sound

• Check for any impedance restrictions on your amplifier, and remember using two speakers on one channel will halve the impedance

• Take the time to run identical speaker cables to all the speakers, both on one channel, and on both channels

• Make all the connections with the amplifier turned off, then check and double-check them before powering up again

• Remember the effect of that halved impedance on the way your amplifier works: if you hear any hardening of the sound as you push the volume up, back off on the level – you’ve probably reached the limits of the system.

• Pour your favourite drink, invite some friends round, put on some lounge music and enjoy your new-found sophisticated party ambience!

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