MattKrass.com

Summary of Project

Using 3.5mm breadboard compatible breakout boards, and jacks to solder to those boards, I’m intending to prototype a design I sketched up for a 2 channel audio mixer on a breadboard. If it works I’ll try to expand it to a 4 channel mixer and if that works, I’ll have a PCB made and build an actual nice little enclosure for it. This project is on-again/off-again as things come and go in my life!

Type of Project

Personal

Current Status

In Progress/Prototyping

Goal

Design a simple analog audio mixer that can mix up to four audio sources. I’d like to have a volume control for each individual input as well as the output. I’d also like to do this fairly inexpensively but without compromising audio quality. Mainly this a good exercise for practice designing practical analog and operational amplifier circuits.

Details

The original design

Originally I went through several revisions and concepts of the design, and most of those have been eaten over the months by forgotten backups and system reinstalls. However the ‘original’ design, as far as this instance goes anyway, was a simple 2 channel deal with a dual op-amp used with summing circuits. That has since evolved in to a design that would take two audio inputs and have volume controls on each, as well as a volume control on the output. Images are below for the final iteration of the original design I started with:

2 Channel Mixer - Original Schematic

The prototype

Before purchasing the (somewhat) expensive logarithmic slide potentiometers and the nice audio op-amps and a PCB and all that,  I wanted to know that the circuit worked. On hand I have some breadboard compatible 10k rotary linear potentiometers, and some nice general purpose LM358 single suppy op-amps. I also have some 3.5mm stereo jack breakout boards that fit in the breadboard. I put together a prototype and after some experimenting, I settled on some changes in capacitor values and proved I could successfully mix two channels this way. My first attempts at volume control didn’t go so well, but I’m about going to redesign that part of the system next.

Here is a photo of the final prototype build before I started the volume control experiments:

Final try on the breadboard

4 channels vs 2 channels

Initially I only planned for two channels, but I realized if the design worked as advertised it would easily allow for multiple channels for minimal extra cost, simply requiring another volume pot and another mixing resistor. Because of this, once I get all the bugs worked out in the prototype I’m going to re-lay the original PCB for 4 channels, and incorporating any other changes I need. I also am investigating the complexity of being able to switch off individual channels.

Notes

During the course of the design and (mostly) prototyping, I made several little notes, I’m not endorsing the technical competency of any of these notes, because some were very late at night and some still don’t make sense to me.

• Without the DC-blocking caps, battery powered audio sources (e.g. my BlackBerry) really screw up the signal. I think this is a differing ground problem. I connected all the input and output grounds together, but it seems that didn’t fix it. I think this might be because the characteristic impedance of the audio cable might be enough to successfully drop the voltage difference across it.
• Again, without those caps, my powered headphones and my 2.1 speaker set are perfectly fine with the mixed signal, despite the ~4.5V DC bias in the output signal. Perhaps they’re designed properly with good DC-blocking caps on their inputs?
• With the caps, there is no problem mixing battery powered sources with wall-powered sources, but there was initially a substantial drop in volume and an annoying background hiss. It seems that making the output capacitor substantially larger (see photo above for my cluster of caps!) deals with this effectively. I think this is attenuation from capacitive reactance, because the formula for reactive impedance includes the capacitor value in the denominator. So bigger cap, lower impedance, less attenuation? Makes sense. With the latest cluster of capacitors the mixed signal sounds fine on my speakers, there is still a slight hiss on my headphones. I think these values are getting a little ridiculously large though so I’m looking for a better explanation and a proper fix.

Relevant Blog Posts

Mixer Prototype Rev 1 – 2/26/2010

Project updates – 2/11/2010

LED Madness, Audio Mixer and more! -2/3/2010

2 new projects! – 1/15/2010