A Practical Commercial Output Transformer-less Amplifier,
- J. Futterman
- J. Audio Eng. Soc.,
www.audioXpress .com I f you have spent $500 or more on 5m or so of exotic loudspeaker cable, have you ever wondered about the 500m of standard copper wire in the output transformers of your tube amplif ier? Audio output transformers are large, expensive components that require complicated winding arrangements in order to work properly at high frequencies. They are the prime culprits for the soft bass sound associated with tube amplifiers. The main causes of this are iron core saturation distortion and the winding inductance which bypasses the loudspeaker at low frequencies. Also, the winding resistance typically wastes 10% of the output power. Hence, a lot of iron and copper are required in order to minimize these problems. An alternative is the output transformer-less (OTL) tube amplifier. However, this concept is not easy to realize in practice, otherwise there would be more of these around.