During the long career of the author as a writer of technical articles and books, he have on several occasions addressed the theme “collection of circuits”, also including information. Thus, previously, addressing this theme, he published the series “Circuits and Information” (7 volumes) and “Circuits and Solutions” (5 volumes) containing hundreds of useful circuits and technical information of all kinds. The series has run out, time has passed, but readers still charge us something similar updated and that can still be used in projects of all kinds. In fact, basic circuits using ordinary discrete components, from transistors to integrated circuits, are still widely used as simple solutions for immediate problems, part of more advanced projects and even for didactic purposes, at the request of a teacher who needs an application for a theory. So, he is back with this series, but with a differentiated structure, new projects and a new approach. The difference in approach will be to divide the various volumes of the series by themes, we will have audio circuits,such as amplifiers, preamplifiier, mixers, etc. In his stock of circuits, collected from all possible sources, he already have more than 12,000 of them, many of which can be accessed in a dispersed way in the site of the author. The advantage of having these circuits organized in volumes, besides access anywhere, is in the easy location of a circuit. The information, on the other hand, will be added to the circuits, with internal links, which is only possible in a digital publication. Most of these circuits, collected in publications that, in some cases, may not be very current, receive a special treatment with comments, suggestions and updates that make possible its execution even in our days.
