Bring Audio Into Your Computer: Build An Audio Breakout Box
Recently, I was forced to replace my trusty MacBook Pro, and opted for a newer iMac. It was a great choice in many respects, but raised a problem. One of the core applications for me was audio recording--and one thing the old MacBook had that the iMac does not was an audio input jack. So, how was I to get my audio to the iMac?
One solution is to use an audio USB adapter. Many interface boxes provide just this functionality, but standalone units are also available for a few dollars online. Although I have not used such a product, online reviews seem in general to be positive, and this would likely be a successful choice for most users.
However, there is an alternative. I said above that the iMac does not have an input jack, but that isn’t quite true. Strictly speaking, one should say that it doesn’t have a separate input jack. There is an audio input; it’s just that it’s combined into one jack with the output.
Apple accomplishes this by the use of what’s called a ‘TRRS jack.’ TRSS stands for Tip, Ring, Ring, Sleeve. These terms denote the 4 separate sections of the TRRS plug’s shaft, visible in the photo below. Each of those sections conducts either a single channel of audio, or provides a ground (return path).
TRS vs. TRRS plugs
The sections’ assignments are shown in the table below.
TRRS section assignments
Output audio, left channel
Output audio, right channel
Input audio, microphone
So you can simultaneously use that TRRS jack to input a channel of audio into your iMac (or similar computer) and to output your stereo mix from it. What you need is an interface box, containing the correct wiring to separate the input audio from the output audio. Luckily, this is not hard to build.
(Note that this means you will be limited to recording mono channels, since there is only one input channel available via the jack. If you want to record in stereo, you’ll need to go the USB adapter route. For me that was not a problem, as I almost never have occasion to record in stereo. On the other hand, this direct audio connection guarantees zero latency due at the hardware level.)
Project requirements: Time, Skill, tools, supplies
I was able to build my breakout box in an afternoon--perhaps about 4 hours, being very careful at every step. You need basic wiring skills--the ability to strip and solder wire, and the ability to read a simple wiring diagram--and tools to match: wire cutter and stripper, probably a small Phillips screwdriver for disassembly purposes, and a soldering iron with solder and desoldering braid (or the equivalent). You’ll also need electrical tape and/or shrink tube to insulate connections. A multimeter will also be extremely useful in checking terminals and solder joints, and a TRS extension cable will be a very handy accessory to it.
For salvage items, I used a game controller and a dead Discman salvaged from the local Goodwill store, and a cheap headphone set from Walmart. The three together cost less than $10.
Items used for salvage
Item salvaged from it
“Boggle” game controller
2 RCA plugs with cords
Stereo audio input to breakout box
Defunct Sony Discman
Stereo audio output jack
Stereo audio output from box to headphones
Smartphone earbud set
TRRS plug with cords
I/O plug connecting box to Mac
Item #1 will vary according to the audio outputs you are using or wish to use. I’d been using the RCA “2-channel” output jacks from my mixer and opted to continue using that format. But one could use ¼” instrument jacks or even (balanced) XLR cable.
Similarly, item #2 can also be almost anything with a stereo audio jack, either ¼” or ⅛” (3.5 mm), according to your preference. The Discman turned out to be a slightly unfortunate choice in that the line output jack turned out to be mono, and the headphone jack was physically housed with a jack labelled “remote”. This made for a somewhat clumsy installation, as we shall see.
Item #3 is common; any set of phones or earbuds that includes a microphone will of necessity be in TRRS format.
You’ll also need some sort of case. It could be a commercial ‘project box’. But in the spirit of salvage, I used the plastic cap from a bottle of laundry detergent, glued to a base made from scrap plywood. Supplies for that part of the job were a little black spray enamel and some silicon adhesive.
The steps required to salvage the bits you need will vary according to your ‘finds’. In my case, they can be summarized as shown in the table below.
Cut cords free, allowing ample length for your anticipated needs
Cut cords free below mic (that is, leave the earbuds and mic connected as an ‘offcut’)
Strip insulation and tin leads for later soldering
Carefully desolder and remove jack from circuit board
Carefully strip the outer insulation and separate leads
Burn off insulation from each lead--discussed in detail later--and tin leads for later soldering
Note that the earbud cords, if they are typical, will have a micro-coating of plastic insulating each wire. These can be removed by burning them off with a flame, such as that from a lighter.
The plastic is pretty volatile, so the process is very fast. Just a second or so will do! The wires are very fine, and relatively fragile.
Once all the salvaged pieces are prepared--including any drilling or cutting needed for your chosen project case--you are ready to start wiring. Getting it right the first time requires care and patience--plus your multimeter and that TRS cable mentioned above.
Before diving into the diagram itself, you should know that the stereo headphone jack will be in TRS format. (There's a TRS jack pictured with the TRRS one above.) As you might by now guess, it’s quite similar to the TRRS format, but with one ring fewer, since it is meant to carry only two audio channels, not three. As shown in the wiring diagram below, the tip and first ring coincide with the TRRS scheme, and the sleeve provides the ground.
Wiring the box
Depending on your box, it may be quite important to do as much wiring as you can prior to assembling the components to the box, so that you don’t have to try to manage a hot soldering iron in too small a space. As shown below, the headphone jack in particular may benefit from have leads pre-attached before mounting the jack into the box. Be careful to avoid shorting between terminals; the space is quite tight.
- It’s convenient to start by connecting all your grounds: R2 on the TRRS jack; S on the headphone jack; and the sleeves, S, from the RCA plugs. Check that all connections are solid, using your multimeter.
- You may have difficulty identifying which terminal on the headphone jack should be soldered to which wire. Simply insert a TRS (headphone) extension cable into the jack. Then you can use your multimeter to determine which pin corresponds with the sleeve on the extension plug.
- Speaking of the RCA plugs, I opted to tie the wires RCA tips at the last point possible, which is where these wires are soldered to the conductor leading to the sleeve on the TRRS plug. This is essentially a “Y-connector” combining two audio signals into one. Of course, since I only intend to record mono, these will be identical signals which will simply reinforce one another. Use the multimeter to identify the wire connected to the TRRS sleeve, then check your solder connections.
- Finish wiring by connecting the left channel (headphone jack T to TRRS plug T) and right channel (headphone jack R1 to TRRS plug R1). Once again, the extension cable and multimeter will help you to identify the correct conductors.
- Finish any assembly not done prior to wiring. Provide “strain relief”--that is, make sure the wires are secured against pulling. You can use knots, clamps, silicone or electrical tape to physically protect your wiring. You may also be able to use strain relief built into your salvaged plugs, as I was with the RCA jacks, by slotting them tightly into the case and securing with silicone.
- Paint your project, if desired. My paint job was distinctly 'rough and ready'; yours can be as refined, or wild, as your heart desires!