Adaptive Optics (AO) systems of the Extremely Large Telescopes (ELT) will incorporate deformable mirrors with an order of magnitude larger number of piezoelectric actuators than the AO systems currently deployed. Simply scaling up the drive electronics offered commercially would substantially drive up the AO cost, pose unacceptably high demands for the supply power, and occupy large volume. We are progressing on the design of a compact high voltage amplifier with the goal to have a Deformable Mirror (DM) drive density of 1200 channels per 6U VME crate. Amplifiers will be driven by multichannel D/A converters, consume no more than 0.5 W power each, be slew rate limited in hardware, and be short-circuit protected. In addition, the cost per amplifier is expected to be drastically lower compared to the integrated circuit amplifier currently used in smaller scale AO systems. Several suitable circuits using inexpensive components have been conceived and investigated by computer simulation and built. In this paper we present experimental results of prototype circuits exposed to both normal operating conditions and foreseeable fault conditions. The performance is evaluated against the AO requirements for the output range and bandwidth as well as the DM actuator operational safety requirements.