SLAA898 September 2022 TAS3251 , TPA3255
Figure 2-1 shows a typical constant-voltage system. A step-up transformer is connected to the output of the amplifier and converts the output from voltage UL to a higher voltage U (usually is 70 VRMS, 100 VRMS, or 140 VRMS). RL and R are equivalent loads of the primary and secondary side of the step-up transformer, respectively. Equation 1 shows the relationship of the voltage and resistances on each side of the transformer.
On the speaker side, the high voltage is converted to a lower voltage to match the impedance of the speaker. R' and R are equivalent loads of the primary and secondary side of the step-down transformer, respectively. So,
So for the whole system, if N number of the same speaker are used is:
UL/U and U'/U0 are used to calculate the turns ratio of the step-up and step-down transformer, respectively.
To make sure the amplifier works at certain loads, resistance must match well with the transformer. For instance, for an amplifier that can deliver 100 W with 4-Ω load, and the power goes to 10 4-Ω speakers. Then RL = 4 Ω, R0 = 4 Ω, N = 10, a turns ratio of step-up and step-down transformers should be chosen precisely according to Equation 6.