I sat in on a session last night with some guys who use a Soundcraft UI24R mixer. They used a TRS‑TRS cable from my personal mixer’s line output to route my vocals into the Soundcraft. All worked well, but it seemed like there were far more XLR inputs than TRS ones, and I wondered if they’d have been happier if I’d given them a TRS‑XLR cable instead? This got me thinking: are mixers smart enough to know when a signal coming into the XLR is at line level and to adjust accordingly?
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: The Soundcraft UI24R has 10 combi XLR inputs with TRS sockets at their centre, and 10 more straight XLRs. They can all accept line‑level signals (up to +19.5dBu), with the appropriate gain being set manually with the corresponding channel controls. I don’t know of any mixers that can switch automatically between mic and line input levels, but plenty can accommodate a vast range using the input gain control and/or pad or line switches.
A lot of line‑level equipment is not designed to tolerate 48V DC across its outputs, and applying phantom power can cause catastrophic damage.
But there are some important differences between the TRS and XLR inputs on the Soundcraft, and most other desks. For a start, the TRS inputs in the combi XLRs present a moderately high 12kΩ input impedance, whereas the XLR inputs are 6kΩ — but both values are perfectly acceptable in the context of your personal mixer outputs. A more concerning difference is that phantom power can be enabled on any of the XLR inputs, whereas the TRS inputs are isolated from phantom power. It’s significant because a lot of line‑level equipment is not designed to tolerate 48V DC across its outputs, and applying phantom power can cause catastrophic damage to the source equipment in some situations.
The safest way to connect your line‑level devices to a front‑of‑house mixer is to feed them through the mic preamps via a good DI box. For stereo devices such as the mixer discussed here, a stereo DI box such as the Radial Pro D2 is convenient.On that Soundcraft desk, phantom power has to be manually enabled for each channel individually so, technically, you could connect your mixer’s line‑level output to an XLR input if the phantom is switched off. But can you guarantee that, in the heat of the action, someone won’t accidentally turn it on? Are you prepared to risk your mixer going ‘phut’? Connecting via the TRS inputs avoids that potential problem completely, and would be the simplest solution.
If you do a lot of sessions with different bands, or perform on stage with different PA systems and operators, I would strongly urge you to invest in one or more passive DI boxes and feed your mixer’s outputs to that, and the DI output to the other mixer. This will protect your system from phantom power and other electrical faults with the other people’s gear. It also provides easy ground‑lift switching, should that prove necessary to stop ground‑loop noises. What’s more, the output of a DI box is a standard, balanced mic‑level signal that can be plugged into any stage box and everyone knows exactly how to deal with it at the mixer — there’s no confusion over line level or mic level, high or low impedance, phantom or no phantom. Just plug it into a channel mic preamp alongside all the other mics and it’s job done.
If you’re working in stereo or need to pass two channels, there are two‑channel DI boxes that are very convenient. For electronic outputs, such as from your mixer, a passive DI box is absolutely fine and less expensive than an active box. Active boxes are fine too, though they need battery or phantom power, which a passive box doesn’t (but it won’t care if phantom is present).
In my own rig, the outputs always go via Radial Pro D2 passive two‑channel DI boxes. I wouldn’t risk direct connections to any system I wasn’t 100 percent confident about — which means on any live stage or rehearsal room!