Ground Problems

Author: N.H. Crowhurst

A special ground connection is needed to make transformerless equipment safe. The supply-circuit "ground" is connected to one side of the supply, which may be the "live" side, if the power plug is not put in the right way around. This is a hazard because metal parts of the amplifier become capable of giving shocks. For this reason, two ground points must be provided. The circuit ground is wired so that it is not accessible at any point. A very small capacitor is connected between this high-voltage negative line and the metal parts that are accessible and thus liable to be touched. (This capacitor has to have a very high breakdown rating if the metal parts are to remain isolated from the line voltage.) The use of this capacitor insures the user that touching a metal part of the amplifier can give him only a slight tingle, (due to the microscopic currents that pass through the capacitor). To avoid introducing hum, the chassis has to be carefully arranged so that any accessible metal parts are not close to the low-level audio wiring.

This means the chassis construction must be double. All of the amplifier circuit is arranged to be within a chassis that is connected to supply negative, while a second chassis, insulated from the first, encloses it, eliminating the risk of shock.

Using a safety ground