Audio Jack Connector: 100+ Years of History Interfaces with Modern PCBs

The cylindrical phone connector (AKA audio connector, audio jack, stereo plug, etc.) was conceived of in the late 1800s for use in telephone switchboards, allowing operators to literally connect one line with another. It was then adapted for use in audio production and reproduction, along with any number of other quick-change electrical connection applications.

Impressively, these connectors are still very much in use today, and function as a de facto standard in many applications. In this article, I'll outline this connector's history, the terminology, and how you can implement this ancient-but-awesome technology with modern PCBs.

Phone connector development

The original phone connector was invented well before the turn of the twentieth century, making it over 100 years old as of this writing in late 2026. It originally used a two-conductor, 1/4-inch diameter plug, with a non-conductive split separating the two sections. The tip carries a signal, while the sleeve, or base section, connects to ground.

Eventually, the original tip/sleeve (signal/ground) arrangement, known as TS, morphed into tip-ring-sleeve (TRS) plugs that could carry two signals (tip and ring), while still connecting to ground with the sleeve. Further evolutions on the idea include TRRS and TRRRS plugs, which enable three and four signals respectively (though the signal/ground order varies). Two-signal TRS plugs are, of course, perfect for stereo, and are extremely common for wired headphone usage.

Along with 1/4-inch plugs, miniature 1/8-inch and 1/10-inch versions were eventually produced, AKA 6.35, 3.5, and 2.5mm in metric. Audio production tends to favor 1/4-inch mono plugs, while standard headphones are typically 3.5mm stereo. 2.5mm plug usage is a bit of a mixed bag, so do your research if you're going to swap these around.

TRS plug/signal terminology

The physical TRS arrangement is easy to remember with "T" for tip being on the end, and the "R" for ring being a literal ring around the plug. The sleeve is at the base in the same way that a shirt sleeve covers the base of one's arm. For audio equipment, T is the left channel, R is the right channel, and S is ground. Remember R = Ring = Right channel.

Here we'll define plug as the male connector, and jack as the female receptacle. However, other terms are also in use, like phone jack/phone plug and jack plug/jack socket, so you'll need to pay attention to context and industry when discussing this.

Why use ancient phone connectors on YOUR PCB?

With any number of modern connection/communication methods available, why would you want to reach back to the origins of telephony to augment your device? No, phono connectors aren't appropriate for everything, but they form an excellent method for making reliable connections over and over.

Consider ye olde telephone operator, single-handedly jockeying connections with impressive speed and expertise. You soon realize just how durable this technology had to be. After 100 years of use and electromechanical evolution, this connection concept is fully tested and rock-solid, ready to be adapted to your PCB.

Phone jack connector PCB options

When it's time to actually design a board, you have a few options for adding a phone jack/audio jack to your PCB:

Surface-mount technology (SMT) phoneconnector

SMT devices have dramatically increased the number of components that can be arranged on a PCB, while helping to enable modern machine placement of components. Phone connectors are commonly available in SMT configurations, and footprints are available in ECAD for PCB design usage:

Through-hole technology (THT) phoneconnector

THT phone connectors that solder into the proper through-hole footprint are also very much in use. Below is an adapter PCB that I came up with years ago for a foot pedal interface that I never quite finished. The design is available on GitHub if you would like to examine further.

Anecdotally, it seems that 1/4 inch jacks are more likely to be THT, perhaps with the idea that they will be more often used in rugged professional settings. I typically find 3.5mm jacks in SMD format, though I'm generally dealing with low-volume applications in either case.

Panel-mounted jack:

This type of connector doesn't actually solder onto a PCB, but instead uses a nut/bolt type arrangement to keep it secure. Electrical connections are via solder terminals on the end of the jack opposite where the plug is inserted.

This type of jack can also be securely mounted to enclosures or other mechanical structures, such as an electric guitar (as seen below). Or the PCB itself could act as the enclosure, which appears to be how the Pluto electronic instrument is constructed.

What about using monos plug with stereo jacks (& vice versa)?

If a mono plug is inserted into a stereo jack, R and S are shorted together, meaning that any signal to R gets routed straight to S/ground and never reaches your headphones (or other device). You simply lose one channel.

Of course, that's just one mismatch situation. It's typically best to use the correct jack with the correct socket if you don't know what's going on beyond the connector. In the case of my very experimental stereo jack guitar setup outlined below, R acts as an auxiliary switch. This does nothing if a traditional mono plug is used, and the guitar operates normally otherwise.

Experimental Usage

So how did I get on this ancient phone connector kick? It started out with a new amplifier for my guitar, which could be controlled externally via a simple foot pedal switch. Instead of a foot pedal, I decided to run this signal to a button on my guitar using a stereo jack.

Does this idea work? Find out in the video below... or just take my word for it — yes it does. And it's a lot of fun to use!

When you need one or two rock-solid electrical connections that can endure a significant number of mating cycles, the venerable phone/stereo plug connector is an excellent option. Be sure to keep it in mind for future projects, audio or otherwise! -JC

Thanks for reading!

Thanks for reading my biweekly PCB Friday column! I'm looking forward to sharing more PCB-related knowledge and insights here, and I hope you'll follow along for the journey. You can see more of my hacking/DIY exploits on the Cooked Audio YouTube page, read my semi-technical musings at www.TechAdjacent.io, or email me: hi@jeremyscook.com!

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