Inside the sub-mini Jewelery box preamp

I have a weakness for "cute" enclosures, particularly timber ones. The mirror is just a bonus. The circuit board is part of a kit from Oatley Electronics Australia. I have replaced the carbon resistors in the kit with metal film, the 0.1uF MKT caps with 0.47uF polypropylene types, the 220uF electrolytic caps (output) with 470uF low ESR types. Also the PT2308 headphone driver chip was replaced with a Burr-Brown OPA2134 OP amp chip. 2 X 9V lithium batteries provide 9V to a L4949 regulator which controls filament current to the Raytheon JAN6418 sub-mini valves. Attenuation is trim set and volume controlled by external devices.

Three grommets are used as valve dampeners. These tiny valves can be extremely microphonic. Three grommets stops the microphonic problems dead! The amp is extremely quiet. See the JAN6418 Valve (Tube) Preamplifier Kit page for full details.