Figure 4-18.-Various speaker arrangements.

Figure 4-17.-Output transformer used as an Impedance

matching device.

The primary impedance (Z) of a matching

transformer is the ratio of rated primary voltage to rated

primary current. Similarly, the secondary impedance

is the ratio of rated secondary volts to rated

secondary current. Substituting

for

you get the impedance ratio

Thus, the ratio of the two impedances that a

transformer can match is equal to the turns ratio squared.

Also, the turns ratio is equal to the square root of the

impedance ratio. Example: Find the turns ratio for the

Figure 4-18.-Various speaker arrangements.

transformer shown in figure 4-17. The plate resistance

is 1,250 ohms and the primary impedance is twice as

much to permit maximum undistorted power output.

Such a combination might consist of an 8-ohm speaker

Solution:

and a 16-ohm speaker (fig. 4-18, view B). These two

speakers in parallel result in an impedance of 5 1/3

ohms. The voltage drop across the two speakers is the

same for both and the power division would be 2:1 in

favor of the 8-ohm speaker.

MATCHING SPEAKER LOADS

The alternate arrangement of series connection of

the unequal-impedance speakers produces a branch

Four equal impedances may be connected in a

impedance of 24 ohms with a power distribution that is

series-parallel arrangement to present the same

2:1 in favor of the 16-ohm speaker.

impedance as one speaker to the amplifier (fig. 4-18,

view A). In this case, the power delivered by the

The ratio of power conversion efficiencies of the

amplifier is divided equally among the four speakers.

8-ohm speaker to the 16-ohm speaker is at least 5:1.

For better reliability, a series connection of two speakers

Combining this output efficiency ratio with the actual

in parallel is preferred. Should one speaker open up, the

power taken by the two units gives

other three would continue to operate, but with a slight

power change. However, in a parallel arrangement of

two speakers in series (fig. 4-18, view A), if one speaker

opens up, its other series member becomes inoperative

and the system loses two speakers instead of one.

for the parallel connection (fig. 4-18, view C). This

shows that the 8-ohm speaker, taking twice as much

There may be times when speakers of unequal

electrical power as the 16-ohm speaker, is putting out 10

impedances are coordinated into an impedance-matched

system because they may be the only kind available.

times as much acoustic power.

4-17