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Magnetostriction distortion in audio transformers

I recently came across a recent article that may be of interest to asylum members (see the link). It is about magnetostriction in transformer cores. The authors are mainly concerned with noise produced by large power transformers, but the conclusions are applicable to transformer distortion.

Magnetostriction is magnetization-induced expansion and contraction in ferromagnetic materials. It is not large, less than 1/10,000 of a percent in silicon steel. However, under certain circumstances it may pack a lot of energy. One example is magnetostrictive ultrasound transducers with conversion efficiency of ~80%. From distortion point of view, an audio transformer can be regarded as a big moving magnet cartridge: magnetostrictive core vibrations are picked up by coils and added to output signal distortion.

The main conclusions of the article:

1. In grain-oriented steel, magnetostriction is lowest along the grain and highest across the grain, so different parts of the core expand and contract at different rates.
2. DC bias increases magnetostriction.
3. Mechanical strain, e.g. clamping the core, increases magnetostriction.
4. Magnetostriction produces harmonic content (~15% second and ~10% third).

There is one more aspect of magnetostriction that tube amp designers should be concerned with: tube microphonics. If an audio transformer or choke is rigidly attached to chassis, magnetostrictive vibrations are mechanically transferred to tubes. The corresponding mechano-acoustic transmission lines may have multiple resonance peaks. The consequences are not difficult to imagine. Output transformer is the worst offender under this scenario. Ideally, it should be acoustically uncoupled from the chassis. Some old radio designs did exactly that by attaching the OPT to speaker frame rather than to the tube chassis. Alternatively, OPT can be cushioned on the chassis using rubber mounts.

One big reason why permalloy is superior to iron is much lower magnetostriction. 80% permalloy has practically zero magnetostriction.


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Topic - Magnetostriction distortion in audio transformers - sser2 15:25:38 03/06/17 (1)

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