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The output transformer (OPT) is a major component in all mainstream valve power amplifiers, accounting for significant cost, size, and weight. It is a compromise, balancing the needs for low stray capacitance, low losses in iron and copper, operation without saturation at the required direct current, good linearity, etc.
Output transformers dramatically increase the cost of a valve amplifier circuit compared to a direct-coupled transistor alternative. However, in both tube and solid state amps, matching output transformers are required for public address applications where low-loss high impedance/high voltage lines are used to connect multiple distant loudspeakers.
Williamson was confident that the output transformer is the most critical component in any valve amplifier. [24] Even before applying global feedback, the transformer is liable for at least four types of distortion. [24] Their causes cannot be addressed simultaneously, and the designer must make a compromise between conflicting requirements. [24]
Rear view of a valve combo guitar amplifier. Visible are two glass 6L6 output tubes, six smaller 12AX7 preamp tubes in their metal tube retainers and both the power transformer and the output transformer. Guitar amplifiers are often designed so they can, when desired by the guitarist, distort and create a tone rich in harmonics and overtones.
Circlotron valve amplifier is a type of power amplifier utilizing symmetrical cathode-coupled bridge layout of the output stage. Original circlotrons of 1950s used output transformers to couple relatively high output impedance of vacuum tubes to low-impedance loudspeakers.
The Williamson amplifier was published in 1947 as an article in Wireless World, and was a milestone which defined the mainstream topology for the majority of amplifiers thereafter. The design gave particular attention to a very high specification for the output transformer, in addition to being generally a consistently well worked-through design.
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Thus, if the signal source has an impedance of 25 kΩ or more, the noise of the tube is actually smaller than the noise of the source. For a source of 25 kΩ, the noise generated by tube and source are the same, so the total noise power at the output of the amplifier is twice the noise power at the output of the perfect amplifier.