One cause of the low system efficiency is the low equivalent resistance seen by the amplifier. The equivalent load can be increased by reconfiguring the 18737. From Table 3-1, R_L can be about 8 Ω if using tap 1, 3 as primary and tap A, B as secondary (or tap 4 as primary and tap C as secondary). With an 8-Ω load, η_amp increases from 0.8 to about 0.85. The theoretical system efficiency is:

The final test result is about 0.61, which matches with the estimation. Figure 4-3 shows the THD+N test result with new configuration.

Figure 4-3 THD+N vs Frequency Results Based on TPA3255 With One Peavey® Step-up Transformer and 10 Step-Down Transformers (1, 3 as Primary Side and A, B as Secondary Side)