The impedance of the driver cannot be changed without modifying the driver itself. But if you need to increase the impedance seen by the amplifier, you can insert a resistor between the amp and the driver. But this must be a series connection, not parallel to the driver.
Also, the impedance of a driver depends a lot on frequency. An "8 Ohm driver" therfore does not have an impedance of 8 Ohms a all frequencies. The "8 Ohm" specification just means that the driver impedance is never a whole lot less than 8 Ohms at all frequencies. A driver with a minimum impedance of 6 Ohms might still qualify as an "8 Ohm driver".
8 ohms would be more of a rating than a requirement. The lower impedance speaker might draw more current than the amp was designed for at high volume settings. It should sound OK at moderate levels.
I could say that the amp fuses would protect it, but that isn't sensible advice. If you use the resistor it will probably change the sound and the resistor may get hot.
You could use two same speakers in series.
Speakers tend to have two very different regions of the frequency spectrum where they become very reactive (lots of impedance, most of it not resistive).
The first of these regions occurs at the low frequency end of the speakers range, near the fundamental mechanical resonance of the speaker. There is usually a large impedance peak there. The nature of the this sort of peak is that it behaves like a large inductance just below resonance, and like a capacitance just above it. But there is no actual inductance or capacitance of this size in the speaker - the impedance peak is actually a byproduct of the interaction between moving mass (cone, coil, spider) and the voice coil in the magnetic field.
This impedance peak is an accidental byproduct of the way the speaker works, and in general, life would be better without it. (For example, the design of a crossover network for a tweeter is made much more complicated by the impedance peak at the tweeters fundamental resonance frequency.)
The second highly reactive region occurs at high frequencies, where the voice coil inductance starts to dominate. Adding an external series inductor would increase this effect.
But why would one do this? An ideal speaker would behave like a simple resistor - the reactance of the voice coil is an unwanted side-effect, and speaker manufacturers try to minimize it. (For example, by using a copper shorting ring as part of the magnetic structure.)
So, IMO, the short answer to your question is, yes, one could certainly add external inductance or capacitance to make a speakers impedance curve more reactive. But I can't think of any good reason for actually doing this!
-Gnobuddy
Inductors have low impedance at low frequencies and high impedance at high frequencies. Capacitors are the reverse. Either one would act as a crossover. The inductor would increase the impedance at high freqs and roll off the highs, the capacitor would roll off the lows - but it would increase the impedance down low. Probably not what you want to do
Best solution (apart from buying another amp) would be to use an autoformer.
If you put a resistor in series with a speaker, it will change the speaker's response based on the speaker's impedance curve. This will make the speaker sound different. The resistor forms a voltage divider with the speaker - part of the voltage from the amp will be dropped across the resistor and part across the speaker. If the speaker's impedance curve is not flat (most are not), then the voltage dropped across the speaker will be different at different frequencies. In simple terms this is the same way an equalizer works - changes the voltage to the speaker at different frequencies.
I'd suggest not using the resistor and instead not turning up the amp as much. 6dB should do it.
I'd suggest not using the resistor and instead not turning up the amp as much. 6dB should do it.
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