Audio      How-To

Introduction


A reader with Zenith Allegro speakers emailed me about speaker-wire polarity.

He noticed the non-polar crossover capacitor, which all loudspeakers have (at least any that I can think of).

So he asks:  Do we have to be careful of polarity when connecting stereo speakers?

Let's see. 


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In This Article

Speaker Wires

Capacitors

Speaker Cone Direction

Speaker Phase

Conclusion




Speaker Wires


Many stereo amplifiers have red and black speaker connectors.  By convention they are often marked "+" and "-".



Speaker wires, too, are often marked for polarity.  Often there will be a stripe or dashed line down the insulation of one wire.

Most of the time, the wire with the stripe or dashed line is supposed to be the "positive".


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Capacitors


The crossover capacitor in a speaker is non-polarized.  Even if they use electrolytic, it has to be a special type that works in both directions.

That's because the signal is a type of alternating current.  It contains multiple frequencies;  the capacitor acts as a high-pass filter so that only the higher frequencies go to the tweeter.


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Speaker Cone Direction


Will reversing the polarity cause speaker cones to "pull" instead of "push"?  Well, it's true when you test the speaker with a DC source such as a AA battery.  And the reason it moves the speaker coil only once is that speaker movement depends on a changing electric current.  So, beyond the initial current impulse (dI/dt), the DC current will not move the speaker element.  (It will heat the coil, though...)

A real stereo output signal is not a DC signal, so it doesn't have a set "polarity".  In fact, there should not be any DC voltage on the speaker outputs of your stereo, because DC is only going to heat the speakers up without doing anything useful.

It took moderate volume before I could read 100 millivolts AC on here.  At room-filling but tolerable volume, it would peak around 2.5 volts AC. 



By the way, this is a great example of analog vs. digital multimeters.  The Simpson 260 works almost like a VU meter here.  In the 2.5-volt range, accuracy doesn't fall off until at least 30 kHz.  Perfect for audio signals like this one.


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Speaker Phase


A single AC wave does not have an "absolute phase" reference.

But since stereo speakers have two AC voltage signals (because they're stereo), each one has the other for reference.  Theoretically, they should be in phase. 

If you hook both speakers up "backwards", the audio would still be in phase.  But if you hook one up correctly and the other backwards, then you would have one speaker out of phase with the other.

Does this always matter?  Sometimes, no.  Room acoustics can already cause apparent phase shifts.  So it gets complicated.

If you already have speakers placed to your liking, you may notice a difference if you then connect one speaker with reversed polarity.

You'd probably notice this more in the bass frequencies.  That's because the time between each wave peak is longer at these frequencies.  So you're more likely to perceive it.  At 30 to 60 Hz, you might notice a phase shift as muddy-sounding bass. 


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Conclusion


When hooking up speakers, make sure they both use the same convention.  The striped wire, or dashed wire, or whatever distinguishing feature they've used, should go to the same connector on each speaker. 

If you work on speakers, do the same thing when soldering the wires in place.  It's not unheard of for one speaker to be wired backwards internally.  It's an AC voltage signal going to an electromagnet, so the speakers will still work, but we've seen why they should both have the same polarity.

As long as both speakers have consistent wiring polarity, and you hook them up with consistent polarity, then it will be alright.  Just make sure not to reverse the polarity on one but not the other.


    


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