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<blockquote data-quote="helotaxi" data-source="post: 395498" data-attributes="member: 550915"><p>If you look at the impedance plot for a speaker throughtout its frequency range, it is not flat. There is a peak around resonance (very pronunced on a woofer/mid usually and not as bad on a tweet.), a dip right after resonance and then a gradual rise as the inductance of the voicecoil becomes a factor at higher frequencies. A passive filter network (a crossover is just a frequency filter) depends on the impedance of the load it is connected to to determine its rolloff point. All a crossover does in fact is increase the impedance above the crossover frequency for a lowpass filter and below the crossover frequency for a highpass. If the impedance of the speaker is not relatively flat from the xover point and up for a low pass and from the xover point and down for a high pass (basically through the frequency range you are trying to attenuate with the filter) then the filter will not work correctly. What this means is for a woofer you need to counteract the gentle rise in impedance causes by voice coil inductance with a cap and resistor and tame the peak at resonance for the tweet with an inductor and resistor. This gives the drivers flat impedance curves through the crossover region and allows the crossover to be designed for a fixed load impedance and predictable frequencies and rolloff and response characteristics. A Zobel network is the name for the flter use to normalize the impedances. The values of the caps, coils and resistors used in the network are determined by the characteristics of the speakers so unless you are swapping out a driver for one with the same impedance characteristics, the Zobel won't be correct. If the Zobel is not correct, the crossover cannot be correct. You will either have a shift in frequency or a really ragged response.</p><p></p><p>The other function that a good crossover will perform is level matching. If, for example, the tweeter has a sensitivity of 92 dB/Wm and the woofer has 89dB/Wm the two drivers will not blend together nicely because the tweeter will be louder than the woofer. To fix this, you increase the impedance of the tweeter with an attenuation circuit and burn up a bit of power with a resistor. This presents the crossover with an even different impedance than just the Zobel and must be accounted for in the values of the components in the crossover.</p><p></p><p>As you can see, the design of a crossover is pretty complicated if you want to do it right. Its function is entirely dependant on the drivers being used and you can't take a crossover designed for one driver and expect it to work well with another.</p></blockquote><p></p>
[QUOTE="helotaxi, post: 395498, member: 550915"] If you look at the impedance plot for a speaker throughtout its frequency range, it is not flat. There is a peak around resonance (very pronunced on a woofer/mid usually and not as bad on a tweet.), a dip right after resonance and then a gradual rise as the inductance of the voicecoil becomes a factor at higher frequencies. A passive filter network (a crossover is just a frequency filter) depends on the impedance of the load it is connected to to determine its rolloff point. All a crossover does in fact is increase the impedance above the crossover frequency for a lowpass filter and below the crossover frequency for a highpass. If the impedance of the speaker is not relatively flat from the xover point and up for a low pass and from the xover point and down for a high pass (basically through the frequency range you are trying to attenuate with the filter) then the filter will not work correctly. What this means is for a woofer you need to counteract the gentle rise in impedance causes by voice coil inductance with a cap and resistor and tame the peak at resonance for the tweet with an inductor and resistor. This gives the drivers flat impedance curves through the crossover region and allows the crossover to be designed for a fixed load impedance and predictable frequencies and rolloff and response characteristics. A Zobel network is the name for the flter use to normalize the impedances. The values of the caps, coils and resistors used in the network are determined by the characteristics of the speakers so unless you are swapping out a driver for one with the same impedance characteristics, the Zobel won't be correct. If the Zobel is not correct, the crossover cannot be correct. You will either have a shift in frequency or a really ragged response. The other function that a good crossover will perform is level matching. If, for example, the tweeter has a sensitivity of 92 dB/Wm and the woofer has 89dB/Wm the two drivers will not blend together nicely because the tweeter will be louder than the woofer. To fix this, you increase the impedance of the tweeter with an attenuation circuit and burn up a bit of power with a resistor. This presents the crossover with an even different impedance than just the Zobel and must be accounted for in the values of the components in the crossover. As you can see, the design of a crossover is pretty complicated if you want to do it right. Its function is entirely dependant on the drivers being used and you can't take a crossover designed for one driver and expect it to work well with another. [/QUOTE]
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