Radial J+4 Active Stereo Line Driver, -10dB to +4dB Balanced Converter

Radial J+4: Send a Consumer Signal Down a Professional Balanced Line

The Radial J+4 is an active stereo line driver, a -10dB-to-+4dB converter that lets a consumer-level source drive a professional balanced input without losing signal along the way. It accepts the low, unbalanced output of a laptop, smartphone, DJ mixer, or keyboard, amplifies it through a 100% discrete circuit, and delivers a balanced +4dB line that can travel hundreds of feet to a console or interface. When a show or session depends on a clean handoff between consumer gear and a pro rig, the J+4 makes that connection sound like it belongs there.

What a Line Driver Actually Does

A line driver solves a mismatch that quietly ruins signal chains: consumer gear puts out a weak, unbalanced -10dBV signal, while professional consoles expect a strong, balanced +4dBu line. Plug one straight into the other and you get a thin, noisy feed that hums the moment the cable gets long. The J+4 bridges the gap by raising the level, balancing the signal, and buffering the output, so the source arrives at the mixer at the level it was designed to receive.

Why -10dB and +4dB Refuse to Get Along

The difference between -10dBV and +4dBu is nearly 12 dB of level, and it is not just a volume problem. An unbalanced -10dB cable has no common-mode noise rejection, so every foot of run acts like an antenna for hum and interference. Balancing the line to +4dB flips that: the two conductors carry the signal in opposite polarity, and the receiving input cancels any noise picked up along the way. The J+4 performs both jobs at once, which is why a long run off its XLR outputs stays quiet where a raw consumer feed would not.

100% Discrete, and Better Than 95dB Quiet

Radial built the J+4 around a fully discrete audio path rather than a single op-amp chip, and that choice shows up in the noise floor. The unit delivers better than 95dB of signal-to-noise, so the boost it adds does not drag hiss or grit along with it. Careful internal grounding and shielding also keep spurious clocking noise and digital hash out of the signal, which matters when the source is a laptop or a media player sitting inside an electrically busy rack.

Three Inputs for Any Source You Plug In

The J+4 is built to meet whatever cable your source hands it. A stacked pair of 1/4-inch jacks welcomes the main outputs of a DJ mixer or a stereo keyboard, RCA jacks take a playback deck or media player, and a 3.5mm mini jack lets a phone or tablet feed the box directly. A single stereo level control sets the gain for the pair, so you dial the box in once and leave it, whatever is plugged into the front.

Ground Lift and High-Pass Keep the Run Clean

Two switches handle the problems that show up on long cable runs between systems. The ground lift breaks the loop between the source and the destination, silencing the 60-cycle hum that appears the instant two grounded devices share a signal cable. The high-pass filter rolls off subsonic rumble and low-end mud before it reaches the console, tightening the bottom end and protecting downstream gear from energy it never needed to pass.

Where the J+4 Earns Its Place

This box lives wherever consumer audio has to meet a professional stage or studio. A DJ feeds a club's front-of-house console from mixer outputs that would otherwise be too weak and too noisy. A presenter routes a laptop's playback into a house PA over a long, hum-free run. A keyboardist sends a stereo synth into two console channels at proper line level, and a studio drops a phone or media player into the patchbay at +4dB. Anywhere a -10dB source needs to act like pro gear, the J+4 makes it happen.

Who the J+4 Is For

The J+4 is for the DJ who needs a bulletproof, quiet handoff from a mixer to any house system; for the AV tech and presenter who feed laptops and media players into pro consoles all day; and for the keyboard player or studio engineer who wants a consumer source to sit at professional line level without hum, hiss, or level loss. If you plug something small into something serious and the signal has to stay clean, the J+4 is the stereo line driver to standardize on.