Handbook for Sound Engineers

Audio Transformers 299

former with taps for turns ratios of 1:1.4 or 1:2 which
could be used to drive a 70 V line from amplifiers rated
for either 300 or 150 W respectively at 8ȍ. Several
power amplifier manufacturers offer such transformers
as options or accessories.

A line to voice-coil transformer is usually necessary
to step-down the line voltage and produce the desired
loudspeaker power, Table 11-3.
These step-down transformers can be designed
several ways. Fig. 11-44 shows a design where the line
voltage is selected at the primary side and the power
level is selected at the secondary while Fig. 11-45 shows
a design where power level is selected on the primary
side and loudspeaker impedance is selected at the
secondary.
As may be seen from the repeating patterns in the
table above, there are many combinations of line voltage,
loudspeaker impedance, and power level that result in
the same required turns ratio in the matching transformer.
Since the constant-voltage line has a very low source
impedance, and the transformer is loaded by a

low-impedance loudspeaker, transformer high-

frequency response is usually not a design issue. As in

any transformer, low-frequency response is determined

by primary inductance and total source impedance,

which is dominated by the primary winding resistance

since the driving source impedance is very low.

Winding resistances of both primary and secondary

contribute to insertion loss. In efforts to reduce size and

cost, the fewest turns of the smallest wire possible are

often used, which raises insertion loss and degrades

low-frequency response. Generally, an insertion loss of

1 dB or less is considered good and 2 dB is marginally

acceptable for these applications.

Figure 11-43. Step-up auto-transformer

70 V

Line

300 W

150 W

Amp

Com Com

Figure 11-44. Transformer with secondary taps for power

selection.

Figure 11-45. Transformer with primary taps for power

selection.

Speaker

10 W

5 W

2.5 W

1.25 W

100 V

70 V

Line

Com

Com

Speaker

Com

8 7

4 7

1 W

Line 2 W

4 W

8 W

Com

Table 11-3. Transformer Step-Down Turns Ratio Required to Produce the Desired Loudspeaker Power

Speaker Power in Watts Loudspeaker Transformer Step-Down Turns Ratio Required

16 : 8 : 4 : Volts 100 V 70 V 35 V 25 V

32 64 128 22.63 4.42 3.12 1.56 1.10

16 32 64 16.00 6.25 4.42 2.21 1.56

8 16 32 11.31 8.84 6.25 3.12 2.21

4 8 16 8.00 12.50 8.84 4.42 3.12

2 4 8 5.66 17.70 12.50 6.25 4.42

1 2 4 4.00 25.00 17.70 8.84 6.25

0.5 1 2 2.83 35.30 25.00 12.50 8.84

0.25 0.5 1 2.00 50.00 35.30 17.70 12.50

0.125 0.25 0.5 1.41 71.00 50.00 25.00 17.70