真空管是高压线性放大，电流低，原理上没有上面的问题。真空管放大器需要很大很重的变压器。
Single-end Class A Valve Amp
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- Class A design is the least efficient but has the highest sound fidelity.
- Class B design is a little more efficient, but has a lot of distortion.
- Class AB design packs a punch with power efficiency and superb sound.
- Class D design offers the highest efficiency but isn't quite as high-fidelity.?
