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Connecting Speakers
Howard Sharp

Connecting loudspeakers to a system.  Think about a source delivering power to a load.  Your power amplifier is a source and the speakers are a load.  In electronic terms, delivering power to a load requires current.  Electrical current is what does the work of making a noise at the loudspeaker. 

Every source has output impedance. Impedance is the combined opposition to the flow of current and has three components.  Impedance consists of  D.C. resistance, a characteristic of all components, inductive reactance as a result of any coils or wiring in the circuit and capacitive reactance as a result of distributed capacitance between components.  Current delivered by a source must come out through the output impedance of the source.  As a result, some power is dissipated in the output impedance.  The greater the current, the larger the amount of power lost due to output impedance.  The idea here is to lose a minimum amount of power by making the output impedance as low as possible.

Diagram 1

Every load has input impedance.  This is what draws current from the source.  To get the maximum transfer of power from a source to a load, it is necessary to match the input impedance of the load to the output impedance of the source.  The output impedance of your amplifier is usually indicated by the recommended speaker impedance in the specifications.  Where an eight ohm speaker is indicated, this means that you will get the maximum transfer of power delivered to an eight ohm speaker.  Connecting four ohm speakers to this amplifier will result in a loss of power due to the higher output impedance of the amplifier and possible damage to the amplifier due to excessive current being drawn by the four ohm speakers.  Fortunately some modern power amplifiers have virtually zero output impedance and will deliver power to almost any load. 

diagram2

Make sure you know the recommended speaker impedance before making connections.  Does this mean that if your amplifier recommends a four ohm speaker, you cannot connect more than one four ohm speaker?  It depends on how you connect the speakers.  Most amplifiers will have more than one speaker connection, such as speaker "A" and speaker "B".  This makes it easy, connect one speaker to each output. Can you connect more than one speaker to a single output?  Yes, if you match the impedances.  If you have a four ohm output and you connect two four ohm speakers in parallel, the load would be 2 ohms, bad scene.  See diagram 1.  Connecting two 8 ohm speakers in parallel would give you a 4 ohm load.  See diagram 2.  Connecting two 4 ohm speakers in series will produce an 8 ohm load.  See diagram 3.  Connecting four 4 ohms speakers in series parallel will give you a 4 ohm load.  See diagram 4.

diagram 3

When connecting more than one speaker to a single output, make sure the speakers are identical or at least have the same efficiency rating.  Connecting a speaker with 8% efficiency in parallel with a speaker having 11% efficiency will result in most of the volume coming from the 11% speaker and very little being heard at the 8% speaker.

Diagram 4

If your speaker has a 4 ohm impedance, don't expect to measure this with an ohmmeter.  The ohmmeter only measures the D.C. resistance of the voice coil, which is much less than the rated impedance.  because the impedance includes the capacitance and inductive effects, it is frequency selective.  The rated impedance of a 4 ohm speaker or an 8 ohm speaker is only at the reference frequency, usually 1000 Hertz.  At any other frequency the impedance is quite different.

At some low frequency, your speakers will become resonant.  This means that at that frequency the speaker cone will vibrate quite violently.  When this happens you get that boomy sound that occurs usually somewhere between 16 hertz and about 50 hertz.  Usually the lower the free air resonance the better.  A bass reflex baffle system will reduce the boomy resonance as the box resonates at the same frequency as the speaker and acts in opposition to the speaker resonance.  The bass reflex baffle provides a smoother, more realistic bass response.

Biography:

Howard Sharp is the creator of the interactive, multimedia tutorials in basic electronics available for download at www.twysted-pair.com.

 


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