Bob's Bench

Simple and Cheap Audio Mixer

Let's say you have 2 Dream Players and want to connect them to a single set of speakers. You could go to the store and buy a low-cost audio mixer. A really cheap one from Radio Shack might only set you back $59 - $79, but I haven't checked lately. If you choose that route, be careful, they usually are not very good sounding. To get a good sounding mixer will set you back a couple hundred. That mixer would probably have lots of knobs, sliders, and maybe even some cool blinking LED's. Don't get me wrong, a mixer is a wonderful tool, not that I actually own one. Ok, I've wired many studios and churches in my life, but I don't own my own mixer. As they always say, a mixer is a terrible thing to waste. Not sure if I have ever said that, or if anyone else ever has, but I did just type it. I tend to use a software package like Audacity, Cubase, ProTools, Cakewalk, inside a computer to mix files together, but that is another topic.

But what is a mixer anyway? What do we actually need from that snazzy mixer? For some applications, all you really need is to combine 2 signals and never use most of those knobs on a 'real' mixer. A mixer can actually be constructed of 2 simple resistors. No knobs, no LED's, no control, but sometimes that is a good thing. If you look at it that way, then I guess I do actually own a mixer. In fact, I own a bunch of them.

A stereo mixer, built from simple parts could actually be quite useful for a layout with 2 Dream Players and one set of speakers. The applications are endless but we do need to have some reason to bother writing this...

Take the following situation: you have a "background" scene player with great background sounds, including other sounds that are not "triggered". Now add a second "scene" player that responds to "trigger" events. These might be motion sensors, buttons, etc. You could put 2 sets of speakers and let the sounds mix "in the air", but what if you don't have the room for 2 sets of speakers? Combining the output from those 2 players into one set of speakers would be useful wouldn't it?

Click to enlarge imageLooking at the connection diagram to the left, we take two Dream Players, two CD players, or any combination of the two. The point is that you want to take the audio signal from two separate sources. Each of these sources must share a common ground connection with the speaker outputs. In a typical stereo headphone or CD player output, this is the 'Sleeve' on the cable. Just hook all of these together to keep everybody on a common signal reference plane.

Now take the Left and Right signals from each of your audio sources, and pass each through a resistor (10K ohms as a suggested value, available pretty much anywhere). These resistors prevent the output of one player from pushing the other player too hard. This is called a 'summing network'. You may be tempted to use a "Y" cord and just combine the sources, but this is usually not a good idea and can cause damage to the sound sources.

Now just tie the resistors together to feed your speakers (or Dream Switcher). That is all you need to do!

A couple limitations: You can not adjust the level of the sources. You also will not want to run long cables after the resistors feeding the speakers. This is because you have inserted a pretty sizable resistance (10K ohms) into the cable driving capability of your signal sources. If you run a long cable, you will likely pick up extra noise and could potentially loose some of your treble or high frequency response. You won't hurt anything though, so as usual, just do some experimenting and see what works for your application!

One improvement to this circuit would be to use a potentiometer instead of the simple fixed-value resistors. Still need to a drawing of that one. More accurately, need to ask Jim Wells to draw it for me! We might even make a product called a "Dream Mixer" who knows... If you want a Dream Mixer, just shoot me an email and I'll know that somebody is actually reading this!

Bob Scheffler