A jack is a port on a module that you plug a cable into, in order to pass a signal into the module, or to receive a signal out of it. The standard type of jack in eurorack systems is the 3.5mm phono jack, shown to the right. This is the same connector as a “headphone jack” on a phone.
The particular jacks that get the most usage in modules from byte-sized beats are the “Thonkiconn Jacks” made by Thonk. These are panel-mount jacks, meaning they can be affixed to the front panel, in this case with a nut. Often panel-mount jacks and controls are all that is needed to secure a panel to a module, without the need for extra support screws. Thonkiconn jacks are also switched jacks, explained below.
Eurorack jacks are generally mono, although switched jacks will still have 3 pins. For a stereo jack, there are three conductors on the plug, referred to as “tip”, “ring” and “sleeve” (this is why they are sometimes called “TRS jacks”). Mono jacks are the same, but with a single connector spanning the ring and sleeve regions.
A “switched” jack is one that has a special pin used as the default signal for a jack when no plug is inserted. With no plug inserted, the default signal is used. When a jack is inserted, the signal from the jack is used instead. Jacks are said to be normaled to another when they use that jack’s signal as their default signal. An example would be a module with stereo input jacks, one for the left channel and one for the right, where the right input is “normaled” to the left input, so you can plug a mono signal into the left input and it will be forwarded to the right channel as well, without the need for a mult.
Other jack types
While the 3.5mm jack is ubiquitous in the eurorack format, there are other types of jacks as well, used by other modular systems. The “Moog Unit” (or MU) and “MOTM” formats are both larger than eurorack, and use larger 1/4” phono jacks instead. “Serge” and “Modcan” formats use “banana jacks”, which come in a variety of colors and form robust electrical connections. However, they use just a single conductor for signals, so the ground line isn’t carried alongside the signal (this can cause electrical problems if the module grounds aren’t connected together some other way). Occasionally a module may feature headers instead of jacks, with jumper wires to connect them. These are very small and tightly packed together, making them very space-efficient, although they don’t form strong connections and wires may fall out.