International projectionist (Jan-Dec 1951)

AMPL.< OUTPUT* Us n*i FIG. 1. When three 8-ohm loudspeakers are connected in series, the total impedance will match that of a 24-ohm tap. The Matching of Loudspeakers An excerpt from the book "Installation and Servicing of Low Power Public Address Systems," by John F. Rider, publisher, 480 Canal St., N. Y. City 13. THE matching of one component to another in a public address system is very important. A bad match between a good amplifier and a loudspeaker will give poor results in terms of power output and fidelity. Given an output tube having a stated plate circuit impedance, it is necessary that the loudspeaker voice-coil impedance match the impedance of the plate circuit. For the best possible fidelity, the source and load impedances should match within about 10%. The impedance of a loudspeaker is the impedance of the voice coil and is always included in loudspeaker specifications. This impedance, which is equal to the voltage across the moving coil divided by the current through it, is given at a particular frequency, usually 400 cycles. Voice-coil impedances generally range from 2 to 15 ohms, with most between 6 and 8 ohms; however, in special loudspeakers it may be as much as 50 ohms. Range of Impedances When loudspeakers are directly connected in various types of series, parallel, or series-parallel combinations, the impedance offered by the total load may be anywhere from 0.1 ohm to 500 ohms in commercial practice. Generally when the distance between the amplifier output transformer and the loudspeaker is about 200 feet or less, the line can be run at the impedance of the voice coil. The term "line impedance" as used here does not refer to any characteristic which the line itself has but means that the conductors are connected to a load of that type impedance. Thus, a low-impedance line means that the wires are connected to a low-impedance load. Any combination of loudspeakers can be connected by a low-impedance line. Matching on Low-Impedance Lines The total load impedance offered by two or more loudspeakers connected in series is the sum of their individual impedances. This total load can match the amplifier output by connecting it across the same value of tap impedances. Thus, if three 8-ohm loudspeakers are seriesconnected, matching is secured by connecting the entire load across a 24-ohm tap on the output transformer as shown in Fig. 1. The total load impedance offered by two or more loudspeakers connected in parallel, when all have the same voicecoil impedance, is equal to the impedance of any one loudspeaker divided by the number of loudspeakers. Thus, if four 8-ohm loudspeakers are connected in parallel, the total load impedance Zr = 8/4 — 2 ohms. For proper matching, the loudspeakers should be parallel-connected to a 2-ohm tap on the output transformer as shown in Fig. 2. The total load offered by four or more loudspeakers connected in series-parallel, when all have the same voice-coil impedance, is equal to the impedance of any series branch line, divided by the number of such series lines that are in parallel. If four 8-ohm loudspeakers are seriesparallel connected so that there are two loudspeakers connected in series in each branch and two branches in parallel, then the effective load is 16/2 = 8 ohms. This load should be connected to an 8-ohm tap for proper matching, as shown in Fig. 3. Matching on High-Impedance Lines Where several loudspeakers are situated at some distance from the amplifier and from each other, then each loudspeaker (or group of loudspeakers) can be matched to a 500-ohm line (or other high impedance) by means of an individual transformer having a primary impedance such that in combination with the other individual transformer primary impedances the total load is 500 ohms (or equal to the amplifier tap impedance used). Four loudspeakers in series-parallel using two matching transformers, or nine loudspeakers in three series-parallel groups using three matching transformers, can each be connected so as to (Foot of Col. 1, Next Page) AMPL. OUTPUT' rj°n°r<] 8n|rr°rrr<J sri[rjrrj<] t^-F^^T1^ FIG. 2. Four 8-ohm loudspeakers connected in parallel will match the impedance of a 2-ohm tap. AMPL. OUTPUT. 16/1 jpoe k8A» <! 1*8*1-1 FIG. 3. When four 8-ohm loudspeakers are connected in two parallel branches of two seriesconnected loudspeakers each, the total load impedance will be 8 ohms. pospd i_r 8n ^ (A) an Oil ^^ rTTrrCI T an _8a ^x Oil ^0+ FIG. 4. Low-impedance loudspeakers can be connected to a high-impedance amplifier in many ways using line matching transformers. Parts (A) and (6) illustrate two such possible configurations. 16 INTERNATIONAL PROJECTIONIST • August 1951