This circuit was originally designed to power a motorcycle intercom from the vehicle supply system. This type of intercom, which is used for communication between driver and passenger, generally requires quite a bit of power. In order to improve intelligibility there is often elaborate filtering and a compander is sometimes used as well. The disadvantage is that a battery doesn’t last very long. You could use rechargeable batteries, of course, but that is often rather laborious. It seems much more obvious to use the motorcycle power supply instead. A 9-V converter for such an application has to meet a few special requirements. For one, it has to prevent interference from, for example, the ignition system reaching the attached circuit. It is also preferable that the entire circuit fits in the 9-V battery compartment. This circuit meets these requirements quite successfully and the design has nonetheless remained fairly simple. In the schematic we can recognize a filter, followed by a vo...