Microelectronics | Audio Sources Selector

Synopsis

I like listening to music and I have a lot of audio reproduction gear, such as cassette decks, reel to reel tape decks, and turntables. So I decided to build an audio sources selector, to be able to connect and easily switch between multiple audio program sources. My solution was to use a differential 8-channel analog multiplexer in CMOS technology. The design consideration are revealed within the following lines.

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 PIC16F876 microcontroller
the selector is controlled by means of tactile buttons, retro-illuminated with LED

I have crafted the printed circuit boards on my own, using a permanent marker to lay out the tracks, and ferric chloride (FeCl₃) to etch the copper.

The selector software handles the following functions.

detection of the actively pressed command buttons by means of analog to digital conversion (ADC2)
detection of analog audio signal and processing it for displaying on a VU-meter (ADC0, ADC1)
generation of individual BCD codes the CD4028 decoder (LED driver) and the CD4097 analog signal multiplexer, based on the actively pressed command button
adjustment of master volume and individual volumes per each audio program source, including preservation in EEPROM
audio signal muting
loopback outputs selection
storing of all operation state variables in EEPROM for the next hardware startup, or reset
command routines for the vacuum fluorescent display VU-meter (PORTC)

Disclaimer: Since this is an entirely experimental project, the schematic diagrams below are to be taken as educational content only. There are errors (presented in an errata, later on) and there are improvements that I've brought while debugging the hardware and software. These are not reflected in the electrical schematic diagrams.

Schematic Diagrams

The idea was to split the selector hardware in two distinct parts. One should be the digital command and control section, and the other one the analog signal processing section. Also, I've designed the power supply as a separate circuit board assembly.

Fig. 1: Electrical Principial Schematic - Analog Signal Processing

Fig. 2: Electrical Principial Schematic - Command and Control

Errata: In the schematic above, ULN2003A is not correctly connected since the inputs are active high, and outputs are active low. I've corrected this with bodge wires and a CD4049 hex inverter.

Fig. 3: Electrical Principial Schematic - Power Supply

The power transformer should be capable of outputting 35 Vac at 300 mA and 8 Vac at 1 A.

Software

The software program is totally experimental, as the audio sources selector itself. Thus, it will not be available for download.

Construction and Pictures

Here are some of the electronic components that will be part of the this project.

This is the bare dual-layer PCB for the analog signal processing circuit, already drilled, waiting for me to mark the traces.

I began laying down the various connections tracks on the top layer.

Followed by the bottom layer.

Here is the bottom layer of the already etched PCB.

And here is the top layer of the same PCB.

I soldered the parts already.

Here's a view of the RCA connector array.

And an overview of the completed printed circuit board assembly.

Here is the bottom layer of the command and control printed circuit board.

The components have been planted, the ICs are on the bottom layer, and the buttons are on the top layer.

Let's see the circuit board from a different angle. The brain (PIC microcontroller) of the circuit is visible in the distant side of the PCB, as photographed.

A view from the top side reveals the retro-illuminated command buttons.

As I said, the command buttons are all lit by a white-bluish-violet LED.

Which lights up sweet during complete darkness.

While this project is somewhat functional, I'd like to state again that the schematic diagrams can only be used as educational content. A follow-up project can be found by following the Audio Sources Selector V.2 link. It addresses some of the issues and design mistakes from this design.

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