Question by flight777: How does Yamaha’s Active Servo Technology work?
I have some small Yamaha speakers with Active Servo Technology that have very good bass for their size. I’m wondering if a similar circuit could be assembled from electronic parts for use on other small speakers? Thanks for any insight.

Best answer:

Answer by Paul in San Diego
I think it would be prohibitively expensive and difficult for the garage engineer to manufacture.

Yamaha’s AST works by having a secondary coil on the speaker cone that induces a signal based on the movement of the cone itself. This is then fed back to an amplifier that adjusts the bass response depending on the signal that gets fed back.

When current flows through a voice coil, it gets attenuated by the impedance of the coil. Especially bass signals, which require a lot of energy (therefore current) to reproduce. This causes a roll off in the bass response the higher you turn the volume up. By using the secondary coil to measure the amount of speaker cone movement and sending that back to the amplifier, the amplifier can be automatically adjusted to compensate for the the increased low-end attenuation caused by the higher volume audio signals.

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