If your DI box has a third output, you can use it to feed a monitor mixer, a guitar tuner or other device separately from the FOH mixer and guitar amp.
I was given an LA Audio DI2 DI box, and as well as the link output (which I usually use to send the signal on to a guitar amp, for example) and regular XLR balanced output, it has an unbalanced TS jack output. I’ve not seen this kind of feature before. The manual says it’s “an unbalanced output suitable for connecting to unbalanced equipment inputs”. So, I guess the input and link are connected in parallel, but that third unbalanced jack seems a bit odd to me. What would it be used for?
Also, something else in the manual that really intrigued me was a section headed Speaker Feed. It said: “Plug the output of the instrument amplifier into the input. Now the instrument feed from the balanced or unbalanced outputs of the DI2 includes all EQ, effects and amplifier distortion. The pad switch should be in the pad position for this application.” I assume one has to use speaker gauge cable, not a regular instrument lead for amp out/speaker jacks — is that right?
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: The separate unbalanced output is not that unusual on upmarket active DI boxes, and it can be very useful. It’s a second buffered output, usually with its own active buffer circuitry but sometimes derived from a second output winding on an output transformer. Applications include feeding a local monitoring mixer, a guitar tuner pedal, or anything else you like, all while being electronically isolated from the source. That means that whatever you connect won’t load the guitar’s pickups and change its tone in any way, and neither will it affect the balanced output.
Many DI boxes can also be used to derive a signal from a backline power amplifier, such as in a guitar amp. Sometimes this is a preferred technique because it captures any effects and tones created within the amp itself. But as the manual implies, this approach requires care because there are good and bad ways of doing it! Obviously, the signal level from a power amp is going to be very high, so at the very least you’ll need to use the pad option on the DI box’s input. Most DI boxes employ a 40dB pad for this purpose, and some have a dedicated speaker input socket, separate to the normal instrument input socket, specifically for working in this way.
When connecting a guitar (or other instrument) to a DI box, the usual approach is to plug the guitar into the input socket, and then use the link output to pass the signal directly on to the backline amp. Since the input and link sockets are wired together in parallel, the guitar is effectively connected directly to the amp, and the DI box is just ‘monitoring’ the signal on that connection as it passes through. Its very high input impedance means it doesn’t load the guitar at all, so it doesn’t affect the tone. It’s a convenient and practical way of working.
It’s really not a good idea to connect a DI box between amp and speaker using the input and link sockets, unless the manufacturer specifically recommends it!
In theory, you could work the same way to hook up the signal passing between the power amp and speaker. But this is a high‑voltage signal with hefty currents and lots of power involved! To handle a power amp’s output safely you need chunky cables and thick circuit board tracks and, in reality, very few DI boxes are designed to tolerate such power — it’s really not a good idea to connect a DI box between amp and speaker using the input and link sockets, unless the manufacturer specifically recommends it!
A much better, safer option is to connect the amp directly to the speaker as usual, and then access the speaker signal via either a second output socket on the amp or a second input/link socket on the speaker. For this connection, a standard instrument cable can be used to run that signal over to the appropriate DI box input (with pad selected!).
When connected in this way, there is no power flowing through the DI box, so there’s no need for chunky speaker cables. Instead, the DI box is just monitoring the (high) voltage signal at the amp or speaker. Again, the very high input impedance of the DI box ensures that there is no significant current flow, and no loading effect on the amp or speaker. Just remember that you will need to engage the input pad switch as the signal will be very much bigger than a typical guitar signal!