Active Loudspeaker Systems with Transconductance Amplification.

Available from UMI in association with The British Library. Requires signed TDF. This thesis aims to show that for the moving coil loudspeaker, current drive offers a superior method of operation to the conventionally used voltage drive, bringing advantages in terms of increased linearity and a freedom from thermally induced errors. The argument is supported by analysis, comparative measurements and non-linear computer modelling of drive unit distortion mechanisms. After considering existing approaches to power amplifier design suitable for current drive and discussing their limitations, a novel technique using a grounded base output stage is presented. This isolates the load from the amplifier feedback loop and maintains a naturally high output impedance. To control the drive unit fundamental resonance, which is no longer damped by the voice coil resistance under current drive, techniques of open loop equalisation and motional feedback are discussed, with the latter being regarded as optimal for low frequency operation. To implement this, a mechanically straightforward arrangement of an overwound sensing coil is proposed, with transformer coupling error from the main driving coil being mulled by electronic compensation. The circuitry of a two-way current driven active loudspeaker system is described in detail, in order to verify the methodology presented. Further, the problem of dynamic current demand under voltage drive is considered and comparisons made with the current driven case, in order to establish operating bounds for the power amplifier. Drive unit protection under current drive is also discussed.