gaussmarkov: diy fx

Custom gaussmarkov mojo cables kick off this post about 1/4″ phone jacks commonly used in stompboxes. This tutorial supplements a recent post on wiring up a stompbox with off-board components.

Plugs

Here are two artifically coloured 1/4″ phone plugs, mono above and stereo below. The green part is called the sleeve and the red part is called the tip. The stereo plug has an additional part (silver in this figure) called the ring. The tip, ring, and sleeve are all insulated from each other and can be used for different purposes. Stereo plugs are sometimes labeled TRS, the initials of Tip, Ring, and Sleeve.

Guitar cables generally have mono plugs where the tip carries the audio signal and the sleeve connects to ground. Stereo plugs appear on headphone cables. If you put a mono plug into a stereo headphone jack, the silver part is replaced by the grounded sleeve and one channel is grounded and, hence, silent. Because the Ring usually carries the Right channel, the right channel is typically silenced.

Jacks

These colourful images represent three panel mount Switchcraft jacks:

• mono (Switchcraft no. 11 on the left),
• stereo (Switchcraft no. 12B in the middle), and
• mono with NC (or normally closed) switch (Switchcraft 12A on the right).

Following the colour scheme above, I have made the sleeve connection green and the tip connection red.

The role of the silver part in this figure depends on the jack. For the (middle) stereo jack, the ring connection is silver. For the (right-hand) mono with NC switch, the shunt for the switch is silver.

Each part of the jack has a solder lug where wires are usually connected. The tip lug is not always located in the same place. Other manufacturers than Switchcraft may use a completely different configuration of the solder lugs for each of these jacks. You can use the contintuity test of your DMM to figure out which lug goes with each plug connector. Alternatively, you can figure this out visually.

These jacks are constructed in layers, with a single piece of metal comprising the lugs and the plug connectors. I have never taken one apart, but there must be some sort of insulation that separates these metal layers from the barrel that contacts the sleeve. So you can just look from the side and see which lugs and connectors are paired.

Schematic symbols for these three jacks often look like these. Layout symbols are similar.

Connections

Often mono plugs are mated to mono jacks and stereo plugs are mated to stereo jacks as shown above. Notice the position of the tip connection. The “click” that you feel when you plug into your guitar, stompbox, or amp is the (red) metal tab that contacts the tip snapping into the groove around the tip. This holds the plug firmly in place until you pull it out. After repeated use, jacks must be replaced when metal fatigue occurs and the jack no longer grips the tip tightly.

Also, compare the mono and stereo arrangements and you will see how a separate connection is made by the ring of the stereo plug touching only the silver metal tab of the stereo jack. Below we will discuss a case where such a separate connection is not desired.

When inserting a 1/4″ phone plug into these jacks, the tip first contacts the grounded socket before reaching the tip connection at the end of its travel. The sleeve connection comes later. This initial contact between tip and ground without a grounded sleeve causes the pops and hum one hears as a cable connected to an on-line amp is plugged into a guitar.

The figure above shows how the mono jack with NC switch has a tip connection that is closed without a plug and open when a plug is inserted. This is useful in stompboxes like Sean MacLennan’s B Blender. His circuit provides an “effects loop” that is blended with the input signal. By using mono jacks with NC switches for the effects loop, a default connection is possible through the shunt lugs when nothing is plugged into the loop jacks.

This figure illustrates how a stereo jack is used as the input jack for a stompbox to switch the ground connection of a battery. The plug is, of course, a mono plug. The ring part of the jack touches the sleeve of the mono plug. If the sleeve part of the jack is grounded as usual, then the mono plug grounds the ring connection. Without a plug, the ring connection is floating.

So the battery’s negative terminal is wired to the ring solder lug of the stereo jack. When there is a mono plug in the jack, the negative battery terminal is grounded and electrons flow. Without the plug, the ground connection is broken. In this way, the battery is disconnected whenever the stompbox is not in use.

Other Jacks

There are more elaborate jacks than these three. There are jacks with more rings, with more NC switches, and also with NO (normally open) switches. For examples, see the Switchcraft link listed below. Although these exotic jacks do not usually find their way into stompboxes, you will certainly encounter them in guitar amplifiers.

The illustrations above picture “open frame” metal jacks.  There are also panel mount jacks that enclose the plug connections inside a plastic box. For example, see these Switchcraft jacks.  Such jacks are often used to insulate the sleeve connection from the enclosure. This can also be accomplished with open frame metal jacks using nylon washers.

PCB mount jacks offer another possibility. These are also enclosed and quite compact. For an example of their application, see the design described in A Nice Design for 1590B Enclosures.

References and Additional Resources

1. Neutrik Jacks
2. Switchcraft Phone Jacks
3. Wikipedia: TRS Connector