Passive Line Isolators

Published August 2025

What does Radial’s latest compact pair of boxes have to offer?

Canadian manufacturers Radial have a surprisingly wide range of products for the studio and stage, from load boxes and audio‑over‑Cat devices to colourful summing boxes and 500‑series modules. But they’re particularly well established as one of the leading brands making useful little problem‑solver boxes such as DI and re‑amp boxes (and even both in the same box!), splitters and switchers. As well as boasting superb technical specifications, their compact gadgets are housed in robust folded‑steel boxes, with very clear labelling and usually with an overhang to protect the connections and controls — in other words, they’re built to last a lifetime on the road.

Two High

The latest additions to the portfolio are two boxes that form the Highline range. Both are line‑level isolators, one being mono and the other stereo, and the intended application is to sit between either a pedalboard chain or hardware amp simulator and a PA system, where their internal transformers provide true galvanic isolation — in other words, it protects your gear’s outputs from DC phantom power voltages being applied from the FOH mixer, and it also serves to break/prevent the ground loops that can result in unwanted hum when using unbalanced outputs, such as are often used on amp sims and pedals.

A grey, latching button changes the Thru output to an input, summing it with the main input signal.

The Highlines’ Thru outputs can be used as a secondary input, which is summed with the main one.But for their size and the fact that one can isolate two channels, the two pedals are almost identical and at their heart lies a 1:1 ratio Jensen transformer (or two in the case of the stereo version). Starting with the mono version, it’s incredibly simple from a user point of view. On one end, there are two TRS jack sockets, one labelled Input and the other Thru, and these can handle balanced or unbalanced sources. Between them is a grey, latching button that changes the Thru output to an input, summing it with the main input signal. So this box can receive stereo outputs from an attached device, and send a mono sum on to the PA. At the other end, the output is presented on a male XLR connector, and there’s a ground‑lift button that lifts the audio ground path at the XLR. Helpfully, the labels appear both on the end panels and on the top of the steel case. And that’s as complicated as it gets!

On the output side, there’s a ground‑lift facility.The stereo version is a couple of centimetres longer and about 1cm wider, and this time there are four TRS sockets and two XLR outs, to cater for stereo devices and setups. Again a switch lets you use the Thru jacks as inputs and sum them with the main Input jack. A second button, though, sums everything to mono.

So the stereo box has more potential uses; engage both buttons and it would be possible, for example, to sum four mono inputs to a single or dual‑mono output. Again, there’s a single ground‑lift button that acts on both channels.

Verdict

As is so often the case with Radial gear, it’s hard to know what to say about these boxes — they just work! They successfully blocked phantom power, and I was able to eliminate the hum caused by a ground loop that I’d contrived to create for test purposes. As well as being ruggedly built, the technical specs are exemplary, and to my ears this translates to them having zero audible impact on the sound.

Though intended for amp sims and pedalboards, there’s no reason you couldn’t use them (with suitable cables) for any other line‑level gear, and if you have gear not at line level... well they do boxes for that too. There are plenty of line isolators to choose from today, but there is much to commend the Highlines, and if you have the budget they deserve to sit at the top of your audition list.