After using my amplifier for a while, I noticed the power would occasionally refuse to turn off. The amplifier receives 36V from a power supply underneath which goes through a large automotive relay to disconnect power when switched off. As the power supply has reasonably large output capacitance and my amplifier has a large input capacitance, the inrush current was in excess of 100A.
The mechanical relay contacts were spot welding together with the inrush! A microscope shot can be seen below showing the severe pitting on the relay contacts from these inrush events.
A solid state replacement was designed in LTSpice to slowly turn on the power using a FET over the course of a second to reduce the inrush dramatically. This then lets large current transients through without impacting the performance of the amplifier.
A small PCB was designed with exactly the same footprint as the original mechanical relay so the replacement could drop in easily with no mods required to the main PCB.
Fred Temperton 