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