Microelectronics | Audio Sources Selector V.2

Synopsis

After experimenting with the initial version of the 8 audio sources selector, I went for a second iteration. As before, here is the technical data.

audio sources selection is carried by a CD4097 differential 8-channel (dual 8-input) analog multiplexer/demultiplexer
tape recording output selection is implemented around a CD4016 quad bilateral switch
the analog signal path employs a digital volume control circuit, implemented with Dallas DS1669 digital rheostats
a TL072 operational amplifier buffers the output signal to the main line out connectors
the entire selector is controlled by a PIC16F648 microcontroller
state information and menus are displayed on a 1 x 40 character vacuum fluorescent display tube

Schematic Diagram

Here is the complete electrical schematic diagram.

Fig. 1: Electrical Principial Schematic

Fig. 2: Top Layer Printed Circuit Board

Fig. 3: Bottom Layer Printed Circuit Board

Construction and Pictures

The entire project is constructed with DIY techniques. The PCB was made via toner transfer method, and was etched with ferric chloride (FeCl₃). A thin layer of flux dissolved in isopropyl alcohol was further applied to the bare copper for protection purposes.

I'm impressed by the quality of the PCB, given the construction method.

Only a few small sections had to be cut deeper with an X-Acto knife, to remove short circuits.

Testing the display and the software routines to define the name of the individual audio input sources.

The completed PCB assembly and the message displayed upon system initialization phase.

Eventually this iteration implements a better hardware and software design, compared to the initial version. It is not production-ready, but it's still usable. And you could draw inspiration from this schematic diagram, to derive a better implementation of an analog audio stage, or even replace completely the digital CMOS switching with electromechanical relays.

Later edit: As of 2026, I have fond memories of this project. I remember using it for a couple of years, maybe less, before switching to vintage audio gear. Now I have the required experience to build a way better audio sources selector. But the cost of the implementation, easily (out)reaches vintage Accuphase-class audio selectors. Which is not really economically rational.

Thank you for reading this page, and I hope you found it useful.