This tutorial will overview creating a cable which can take a balanced stereo signal from two XLR terminated outputs and turns it into an unbalanced stereo signal terminated by a TRS connector. It includes information about the basics of unbalanced versus balanced audio signals as well as instructions for creating the pad which will deal with impedance and ground loop issues.
This instructable assumes you're familiar with soldering and some basic electrical theory.
Step 1: Gather Your Materials
There are a few materials and tools we'll need before we get started.
- Three-conductor audio cable
- Two 3-pin XLR female connectors
- One TRS connector (I used a 1/4", but the instructions also apply to 3.5mm)
- Wire cutter and stripper
- Soldering iron and solder
- Any tools for affixing connectors to wire
A note about resistors: For this tutorial, I am using 330Ω, 3.3kΩ, and 10kΩ resistors. This is mostly just for illustration purposes. If you use this tutorial for your own needs, you will need to perform research on your equipment to adjust the resistances. The details of selecting resistances for impedance matching (a necessary step in order to minimize signal reflection) is too detailed to get into here. The equipment I was building for was two balanced outputs with impedance at 600Ω and an unbalanced input with impedance at 10kΩ. If you have similar equipment, these instructions should work, but tinkering with resistances will likely be necessary to get a good sound (remember - what you're building for here is for it to sound good).
Step 2: Prepare Your Wires
You'll need a total of three lengths of wire - one for each connector. We'll need to use three-conductor audio cable here to carry all of our signals. Cut off the lengths you want, strip the outer insulating layer on one end, and expose the three wires inside. There should be a red wire, black wire, and an uninsulated wire. The red and black wires carry the actual signals used in the audio equipment. The uninsulated wire is known as the "shield" wire and acts as a grounding wire. Now you'll want to strip the inner wires.
Generally, for a balanced connection, the red wire is known as the "hot" wire and carries the main audio signal, while the black one is "cold" and carries the flipped signal (more on this later). In an unbalanced stereo connection, the red is right-side audio and black is left-side audio. We will keep this in mind while building our cable.
Step 3: Solder on the XLR Connectors
Now it's time to break out the soldering iron. If you haven't already, turn it on and let it heat up.
XLR connectors follow a standard wiring procedure, and should have the numbers printed on them as a guide. The pins are numbered:
Note that the physical layout for a female connector (what we're using) and a male connector are flipped about the vertical axis. So, while the 1 pin is on the left for these connectors, the 1 pin is on the right for a male connector. The 3 pin is always on the bottom. Be sure to keep this in mind if you'll be doing any work on a male connector!
Solder the wires onto the XLR connectors using the provided pictures as a guide. In the pictures, you'll see a view of the XLR connector from the rear along with a picture of how the wires should be connected. For this soldering, it's helpful to have clamps or someone to hold the wires while you solder them. The final picture is me affixing the connector's body to the wire itself. That process will be different for each connector model out there.
Step 4: Solder on the TRS Connector
Next, you'll need to solder your third wire onto the TRS connector. This procedure isn't as standard as XLR connectors.
TRS is an acronym for "Tip-Ring-Sleeve", and is the more technical term for what's commonly called a "phone connector" or "headphone plug". It's the industry standard plug for any pair of headphones and earbuds. The TRS refers to the different bands of metal you'll see if you look at the connector. The tip is, of course, the tip of the connector. This carries the red/right/"hot" signal. Below that is the ring, which carries the black/left/"cold". The shield is the tallest band at the base of the connector and is the shield's connection.
I have provided a picture showing the specific wiring for this model of TRS connector.
Since the pins on TRS models do vary, it's helpful to have a multimeter to test connections before you solder. Put your multimeter into its continuity mode and hold one end to the tip, and the other end to the various soldering points. The multimeter will make a tone if there's a connection between the two probes. In this way, you can confirm the exact parts of the connector that each soldering point connects to.
Step 5: Prepare to Build the Audio Pad
Now that you have two female XLR terminated cables and a TRS terminated cable, it's time to build the audio pad. Simply strip the ends of cable and the wires inside, similar to Step 2.
Take a look at the provided circuit diagram. This is the wiring guide for the pad. "Pad" is derived from "passive attenuation device", which is a technical way of saying that it lowers a signal's strength. What's shown in the schematic is the middle section of one half of our final cable. On the left is our balanced signal coming from our XLR cables, and on the right is our unbalanced cable.
A balanced audio signal works with three wires. The hot wire carries the actual audio signal, while the cold wire carries the same signal, but reversed. So if the hot wire at any moment is at 5 volts, the cold wire would be at -5 volts. A balanced input will take the cold signal, flip it, and add it to the hot signal to create a doubled original signal. The theory behind it is that both the hot and cold signals will pick up the same noise and distortion as it travels the wire. By flipping the cold signal and adding it to the hot, the original signal will be doubled while any noise will be canceled out.
What this pad does is add the cold signal to the hot without flipping it, but the R2 resistor attenuates (reduces the strength of) the cold signal so it doesn't cancel out the original signal completely after the addition. The R3 resistor attenuates this combined signal further and "slows" the signal down as it travels (this is called impedance matching) to our unbalanced input. From here, the signal goes to either the left or right channel on the TRS connector.
We don't connect the shield wires of the XLR and TRS cables together so that we don't create any ground loops, which are unintentionally completed circuits through ground wires. These introduce noise and can damage equipment if not taken care of.
Since it's a stereo connection, we'll need to do this twice - once for each XLR cable/channel.
Step 6: Build the XLR Side of the Circuit
Following the circuit schematic, wire each XLR cable together. My specific physical wiring was chosen for demonstration purposes, but might not be ideal for a production wire. If you want to build a more permanent solution, you could use a printed circuit board or a physical housing for your circuit. However, the circuit diagram itself will not change between implementations.
Connect one R1 resistor to each of the red and black wires, and then connect the R2 resistor between the two R1 resistors. Go ahead and cut the shield wire down so that it's not exposed, since we don't want it connected to anything. Do this step the same way for both XLR connectors.
Step 7: Wire the TRS Cable to the XLR Cables
Strip the TRS cable's wires and again clip the shield wire so it's not exposed. Connect an R3 resistor to each of the red and black wires. You'll want to connect and solder one of R3 resistors between the R1 resistor on the red/"hot" and the R2 resistor on each XLR cable.
It's helpful here to mark the XLR connectors in some way so you know which one is connected to the right channel on the TRS cable and which one is connected to the left channel. Remember - red is right!
Step 8: Finish Up the Cable
Now that all the connections have been made and everything is soldered together, we can go ahead and add finishing touches. For my cable, all I did was wrap electrical tape around the exposed wires. This was cheap, quick, and easy, but you can consider a more advanced housing for your wire if you want. If you do use the electrical tape method, keep the resistors uncovered so any heat they generate can escape.
That's the end! Managing balanced and unbalanced connections is an essential part of audio engineering, and DIY solutions like this are important for the hobbyist or those with limited budgets.