Speaker Impedance
All speakers have resistance to electrical current. Resistance to
Alternating Current (AC) is called impedance. Impedance cannot be easily
measured, it is usually calculated. This is because there are three
components found here: direct current resistance, capacitive reactance, and
inductive reactance. Speakers usually have two components which make up
the impedance: voice coil resistance, and voice coil inductance. Thus, the
resistance of the voice coil is the direct current resistance, which can be
measured with an Ohm meter. The inductance of the coil can be measured
with special tools or calculated. This factor (inductance) accounts for
the inductive reactance. These two components together form to make the
impedance of the woofer.
Typically the impedance of most speakers is between 4 and 8 ohms.
However, the impedance of the speaker is not the same at all frequencies.
Usually, the resonant frequency of the speaker yields the highest impedance.
When multiple speakers are connected to an amplifier, certain decisions need
to be made. How will the speakers be wired to amplifier? Will this
wiring present a proper impedance to amplifier? Will this wiring reduce
the fidelity of the speaker system?
If you have an amplifier with an 8 ohm output jack, and you have one speaker
that is rated at 8 ohm impedance, you simply connect the two devices together.
But if you have two speakers, you now have a decision to make. Should I wire
these in parallel or series? What will the resulting impedance be?
What if the speakers are wired out of phase? These are the questions that
need to be answered.
First, most transistor amplifiers can tolerate a range of impedance loads.
Typical is 8 or 4 ohm loads. Some amplifiers are rated for 8, 4, 2 ohm
loads. However, tube amps are very particular about loading issues. If the
amplifier says 4 ohms, then you better have a 4 ohm load. Transistor
amplifiers operate on completely different basis than tube systems. A tube
amplifier is really an current source device, this is why it requires a load to
operate. A transistor amplifier is really a voltage source device, and
does not require a load. No load is view by it as infinite resistance.
For tube a amp, no load (no speaker) is a disaster, and appears as a short to
the system.
If you wire two 8 ohm speakers in parallel, the resulting impedance is 4
ohms. If you wire two 8 ohm speakers in series, the resulting impedance is
16 ohms. The 4 ohm load will draw more power, because the impedance is
lower, but the amplifier will not have as good of control over the speaker as an
8 or 16 ohm load. The series load of 16 will provide the best control for
the amplifier, but the power will be reduced, and the high frequencies will also
be reduced. This is because the voice coil for each speaker is acting as a
low pass filter to the other speaker.
If you have 4 speakers, they are typically wired together in seriesparallel.
See the above wiring diagram.
This is where two of the speakers are wired in series with each other. The
other two speakers are wired in series too. Then the resulting systems are
wired in parallel. If all four speakers are 8 ohms, then the result from
this wiring practice is 8 ohms.
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