The basic amplifier block is a half-bridge UcD (Self-oscillating, post-filter feedback) module that operates from +-45V providing 100W rms@8 ohm/200W rms@4 ohm. The modules are designed with overcurrent protection, undervoltage protection and overvoltage protection. Any faults trip a timer circuit that causes the amplifier to cease oscillation for a few seconds to allow for the fault condition to be raised, and to allow the high side gate drive capacitor to recharge. The reason for the overvoltage protection is one problem common to all half-bridge class-d amplifiers, rail pumping. The higher the output current, and the lower the frequency, the more severe this problem becomes. This unit is designed to be operated with a power supply that actively balances its supply voltages, effectively eliminating this problem, still the overvoltage protection is provided to avoid any catastrophies during development.

Both PCBs have been designed using one side as ground plane for the sensitive signals, providing lowered inductance for high dI/dt signals, also providing large copper areas dissipating stray RF-energy through eddy currents.

The modulator stage has a direct DC-path, and a potentiometer for offset-nulling, the zener followers that provide voltage for the input OP-amp are designed for low noise at the cost of DC regulation, which is of little importance here.

The modulator (shown above) is provided on a PCB on its own, the motivation is twofold, ease of routing, and to allow the placement of this EMI-sensitive PCB perpendicular to the magnetic disturbance that leaks from the output filter RM-core. The oscillation takes place at 500kHz (no signal) given the component values in the schematic, and leaves a carrier residue of 400mVp-p after the output filter.

The power board (shown above) holds all high power circuitry in addition to all protection circuitry, and interfaces the modulator board through a connector of its own. When and if the protection circuitry is satisfied it tells the modulator board to start the oscillation. The most innovative feature is the use of mosfet on voltage-drop for overcurrent sensing, this is done by careful blanking of both rising and falling gate drive signals to allow only the saturated state of the mosfet to be sampled. This scheme gives one added advantage - the overcurrent trip-point becomes temperature dependent staring at 20A for room temperature and falling to 12A when the heat sink becomes too hot for my hand to touch.