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How do inductors help impedence matching?

Newbee question: Can someone explain to me how a coil used for low pass filtering helps impedence matching with the amp? My speaker designer told me that the purpose of the 1.5mH inductor on my woofer crossover is primarily impedence matching, rather than as a filter. When I remove the coil from the crossover circuit, the sound changes very little. I would like to bypass the inductor, unless this might cause other problems. Thanks much.
scott
skushino

Showing 3 responses by eldartford

eldartford
4,262 posts
 
Presumably there is a capacitor that couples the tweeter to the amplifier for higher frequencies. This lowers impedance at these frequencies. The inductor that couples the woofer to the amplifier increases the impedance of the woofer circuit for higher frequencies. If you remove the inductor the woofer will be paralleled with the tweeter at higher frequencies, so that there will be a lower overall impedance at these frequencies.

The frequency response of the woofer is probably going downhill at the crossover frequency, so you may not hear much of a change when you remove the inductor.

However, manufacturers generally don't use components that are not needed. I would leave it alone, or, if you must tweek, replace it with an air core inductor of the same electrical value.
eldartford
4,262 posts
 
Skushino...About the impedance...correct.

Think of the transformer as being like an automobile transmission. The taps are like different gears. Suppose you want to get 2 volts out of the amp, and you set the volume so as to get this from the 8 ohm tap. The 4 ohm tap will have 1 volt, and the 16 ohm tap will have 4 volts. So you would need to boost the volume if you connect to the 4 ohm tap, and you could reduce it if you used the 16 ohm tap. Use of the 4 ohm tap might mean that the amp circuit couldn't put out enough voltage, and use of the 16 ohm tap might mean that the amplifier circuit couldn't put out enough current. Sort of like trying to go up a steep hill in high gear. The speaker impedance, modified by the transformer, is the load seen by the amp circuitry. Use of the correct tap will optimize power delivery from the amp circuit.
eldartford
4,262 posts
 
An inductor of 1.5mH would correspond to an 850Hz crossover for 8 ohm driver (and 1700 Hz for a 4 ohm driver). It must be in series.

If the speaker is 8 ohms, the 8 ohm tap will maximize power. Try the others if you want...it will not hurt anything. If your ears are happy without the inductor by all means leave it that way. What you have done is to use the 15" driver to a higher frequency, and attenuated the horn tweeter by leaving the woofer in parallel with it. Attenuating a horn tweeter is not a bad idea (IMHO).
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