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Audio Mixing Cables Simple Electronic Projects with Julian Edgar Add an extra input to an audio amplifier or mix the sounds from two sources with these easy do-ityourself mixing cables. I have installed two large subwoofers in my roof space, powered by a dedicated two-channel amplifier, that need to work with two different audio systems. One is my home hifi system, while the second is a home theatre system. I could use a line-level switch to connect the subwoofer amplifier to either the home theatre or hifi system, one at a time. However, since the subwoofer amplifier is located in the roof space, that would have made things a bit difficult without adding very long leads. But why not permanently connect both inputs to the amplifier via twointo-one Y cables? Well, I tried that and found it won’t work! The hifi and home theatre system outputs end up ‘fighting’ each other. John Clarke suggested a very simple solution: put together a couple of Y mixing cables. Here’s how I made them; it only takes a short time and costs little. To make a two-channel system for stereo, you will need (see Photo 1): • Some good-quality RCA leads (don’t use cheap ones – the conductors aren’t thick enough to work with). • Four 1kW ½W resistors. • A small piece of plain punched board (laminate). • Two small plastic boxes. Depending on how you mount the boards and cables, you may also need some PCB stakes, standoffs and grommets. Buy sufficient RCA cables to give you the correct number of plugs and sockets for your application. In my case, I needed two mixing cables, each with a male output and one male and one female input. We will now look at making one cable – each assembly is identical. Cut the leads so that you have three connectors and their associated cables. Once you have done this, carefully strip the outer insulation sheath from the end of each cable and then twist the braid (the outer copper sheath) strands together. With some cables, you will need to use a thin pointer to separate the strands of the braid first. When you are twisting the braid into a single wire, be very careful that every tiny strand of copper is twisted together, with no loose strands remaining that could cause short circuits. This twisted conductor is the ground Photo 1: these are all the parts required to build the Audio Mixing Cables for a two-channel amplifier system. Photo 2: the RCA leads should be cut as shown with the wires stripped. 46 Silicon Chip Australia's electronics magazine siliconchip.com.au connection. Strip a short length from the other wire – the signal connection – and then tin each conductor with solder (Photo 2). Next, solder the signal leads of each input cable to the resistors that then feed the single output. To give attachment points on the perforated board, push short lengths of stiff copper wire through the holes to form pins. You could use PCB stakes instead. Let’s now look at the board more closely (Photos 3 & 5). All the connections are visible – there is no wiring under the board. Solder the braided ground connection of each cable to a stake to physically secure it. The two signal inputs each connect to one end of a resistor, with the other ends of the resistors joined to the output signal. The ground connection between the joined input grounds and the output ground is made by the insulated black wire visible in the photos. Once the soldering is complete, the board can be mounted in a box. I already had these salvaged boxes; all I needed to add were some cable grommets through the existing U-shaped holes (Photo 4). You could use a lowcost Jiffy box or similar. Before you close the box up, do some testing with your multimeter. Every plug’s outer (ground) connection should have continuity (near zero resistance) to every other plug’s ground connection. The two input plugs should have 2kW resistance between their inner (signal) connections, and there should be 1kW resistance between each of the input signal connections and the signal connection of the output. Finally, there should not be continuity between any signal and ground connection. In addition to allowing two different inputs to operate a two-channel amplifier, as the name suggests, the cables also allow the two signals to be mixed (both input signals being heard simultaneously) if that is desired. For example, you could play music while watching TV and hear both if you used such a cable to merge the outputs of a CD/DVD/Blu-ray player and television. The signal level is reduced by half in the mixing cable. The resulting disadvantage is that the signal-to-noise ratio of the signal is a little poorer, but that is not so important for my subwoofer use case. And now, I don’t have to climb into the roof space to swap the inputs of the subwoofer amplifier! SC siliconchip.com.au Photo 3: the layout of the Audio Mixing Cables is very simple, so you can either wire it up as shown or choose your own way. Note that we don’t have any connections on the underside of the laminate. Photos 4 & 5: the finished project mounts neatly in a small plastic box. These boxes are around 5 × 7.5cm. Australia's electronics magazine March 2025  47