Transformers and Hum

Tube amplifiers usually have at least two transformers - power and output. Transformers generate noisy magnetic fields around themselves where the magnetic flux leaks from the core. This happens most profoundly where the flux has to go around corners or across the grain of the core material. Toroidal transformers have no corners or grain of the core material and they leak less than conventional EI transformers, and are preferred for audio signal amplification equipment. However, they do leak around the lead-out wires, so this area should be oriented away from sensitive circuitry.

It is difficult to shield against magnetic fields, so audio circuitry should always be placed as far away from any transformers as possible, especially the power transformer. The output transformer is not so much of a problem because it should have little or no hum current flowing in it. It does handle high primary voltages that generate strong electric fields, but it is much easier to shield against these. Nevertheless, it is usual to mount power and output transformers at the opposite end of the chassis from the input tubes.

The magnetic field around the power transformer will couple with output transformer to produce hum in the speaker, and the closer their proximity to each other the worse it will be. This works against us since ideally we want to mount them both at one end of the chassis away from the input tubes, but their interaction could be minimized by orienting them carefully. In theory, the axis of one transformer core should be kept orthogonal to the other to minimize coupling but, in practice, an apparently random orientation may provide the best performance. The perfect arrangement can be found at an early stage of construction by connecting a speaker or headphones to the secondary of the output transformer, and then connecting the mains supply to the power transformer. The output transformer can then be moved around the chassis while listening to the changes in hum. Obviously, great care must be taken when doing this, since the power transformer is live. It should then be possible to find some position where the hum reaches a minimum or becomes inaudible. If smoothing choke is used, it may be placed between the power and output transformers to provide a measure of shielding between the two. Its orientation is less important, but it it should be orthogonal to the output transformer where possible.

Transformers are usually mounted to the chassis with non-ferrous bolts and an insulating gasket or fibre washes to prevent the core flux from also flowing in the chassis, which might otherwise lead to hum getting into the ground circuit. However, this is unlikely to be a problem in a headphone amplifier, provided a sensible ground scheme has been employed.

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