2-Channel Passive Audio Mixer

I recently decided to install a HAM radio in the car and needed a way to mix my radio’s audio signal with my stereo’s audio signal. I wanted to avoid any active (powered) mixing  because that would add complexity and signal degradation to the circuit. I also didn’t want to spend too much money on parts and have to wait for them to be shipped. I looked into several pre-made passive mixers to get some ideas and possibly to pick one up if it wasn’t terribly expensive, but couldn’t bring myself to spend $50 on such a simple device.

I went out to my local electronics store (Rat-Shack), picked up a couple parts, and put a 2-channel passive mixer together in 15 minutes, saving myself $40 along the way. It may not look as good as the store bought ones, but it works!

Parts:
6 x Panel Mount RCA Jacks
1 x Project Box
4 x 4.7 kΩ Resistors (1/4 watt or larger)
Wire

Tools:
Soldering Iron
Wire Cutters
Solder
Drill with 1/4″ Bit

The build is pretty straightfoward. The first thing I did was chose where I wanted to have my connectors and drilled the holes in the project box. I recommend putting your inputs on one side of the box and your output on the opposite side to avoid confusion.

Next, I inserted the RCA connectors and tightened them in place. I also chose which side would be “Right” and “Left” and marked the “Right” side with a red sharpie.

After that, I soldered the ground wire to each connector. I used some bare wire I had and just added solder to the connector while covering the wire. I didn’t cut separate pieces of wire, I just used the same strand and soldered it in multiple places.

The next step was to solder the resistors from the inputs to the output. Since this is a 2 channel mixer I used 4 resistors, one for each “Right” and “Left”. These resistors are used in order to avoid crosstalk between sources and to keep the signal from being cut too much when one source is off. 4.7 k Ω resistors were chosen because it’s what I had on hand, but anything from 1 kΩ to 10kΩ can be used. Keep in mind that the greater the resistance, the more your signal will be attenuated.

Resistors!

Solder one end of each resistor to every input, connect the two coming from the “Left” inputs to the “Left” output, and the other two to the “Right” output!! Once your resistors are soldered, just close up your project box and test it out! You should be able to hear both sources at the same time!

Simple, eh? Here’s what mine looks like.

You'd almost think I bought this!

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