Matching

Author: N.H. Crowhurst

How do you match these?

When the word "matching" is used about paint, fabric, or in a repair job, its meaning is obvious; however, when we speak about matching a loudspeaker to an amplifier, the meaning can at first be mystifying! The word is used to refer to electrical properties. An amplifier performs properly when the right kind of electrical impedance is connected to it. If, by some lucky chance, the speaker voice coil impedance is just what the amplifier needs, it is said to match it. This does not usually happen so conveniently, and some circuit finagling, known as matching, is required to achieve the best performance.

Some variation in the electrical impedance, or resistance, of the loudspeaker can be made by winding the voice coil differently. A single layer of turns may have a resistance of 1 ohm. By halving the diameter of the wire, twice as many turns will go in the same layer length, and we can get two layers in the same thickness. The wire has one quarter the cross section, will be four times as long, giving (4 X 4) or 16 times the resistance: 16 ohms. Using wire of one-third the diameter originally used, we can use nine times as many turns at 9 times the resistance per turn, which yields (9X9X1) or 81 ohms.

Voice coils can be wound with different numbers of turns

In this way the voice-coil resistance can be changed to some extent by choice of winding. Common commercial values for voice-coil resistance are 1 ohm, 2 ohms, 4 ohms, 8 ohms, and 16 ohms. Some, for special purposes, have a resistance of 45 ohms, and coils have been wound to resistances as high as 500 or 600 ohms. (These coils have a great many turns of extremely fine wire - too fine for a robust coil, and they tend to give trouble in service.)

Even 500 or 600 ohms is rather too low a resistance to match most amplifiers, hence a matching arrangement is still required. Once this is the case, it is as easy to match from 1 ohm as it is from 600 ohms (or any value between, of course), and there is no longer any point in making such fragile coils.