Radial SB-4: Natural Piezo Tone in a Case-Sized Active DI

The Radial SB-4 is a compact active, piezo-optimized direct box (DI), a phantom-powered active DI built around a very high 5-megohm input impedance that finally lets a piezo pickup sound the way the instrument really sounds. Ordinary direct boxes load a piezo transducer down, exaggerating the harsh peaks and midrange "squawk" that give piezo pickups their bad name; the SB-4's ultra-high impedance smooths those peaks and broadens the frequency response for a rounder, more natural voice. Small enough to tuck inside an instrument case, it delivers the deep bass and sweet top end that a classical guitarist, violinist, or upright player has been chasing through the PA for years.

Why Piezo Pickups Need a Special Kind of DI

A piezo transducer is a high-impedance source, and when you plug it into a standard direct box or a low-impedance input, the mismatch robs it of low end and pushes a brittle, quacky peak into the upper mids. The fix is input impedance: the higher the load the pickup sees, the more of its true frequency range survives. The SB-4 presents a 5-megohm input, dozens of times higher than a typical DI, so the pickup is barely loaded at all. That single design choice is what turns a thin, honky piezo signal into the full-bodied, natural tone that makes it worth plugging in direct instead of miking up.

Active by Design: Phantom Powered, No Batteries

The SB-4 is an active direct box, meaning it uses a powered buffer circuit rather than a transformer to do its work, and that active buffer is what makes the enormous 5-megohm input impedance possible. It draws its power from standard 48V phantom off the mixing console or preamp, pulling just 5 milliamps, so there is never a battery to change or a wall wart to lose. Plug the XLR output into any phantom-capable channel and the SB-4 wakes up and stays on all night, delivering less than 0.006% distortion and a noise floor near -100 dB from a box that fits in your palm.

High-Pass Filter and Polarity Reverse: Winning the Feedback Fight

Amplified acoustic instruments live or die by feedback control, and the SB-4 gives you two switches aimed squarely at it. The high-pass filter gently rolls off excessive bottom end, taming the low-frequency resonance that makes a hollow body ring and howl at volume. The 180-degree polarity reverse flips the phase relationship between your instrument and the PA, moving the acoustic hot spots that build up in a room so a note that fed back a second ago now sits quietly in the mix. Together they let you turn up on a loud stage without fighting the squeal.

Buffered Aux Output for Stage Amp and Pedals

The SB-4 carries two outputs, and the difference between them matters. The XLR output sends a balanced, low-impedance feed to the PA that stays clean over runs up to 330 feet (100 meters). The separate 1/4-inch aux output is buffered, which means it can drive a stage amplifier or a chain of effect pedals without its load reaching back and changing the high input impedance that makes the box sound so good. You get the pickup's true tone to front of house and a personal amp or pedalboard feed at the same time, with no interaction between the two.

On Classical Guitar, Violin, Upright, and Dobro

This is the box built for the acoustic and orchestral player who plugs in direct. On a nylon-string classical or a concert guitar it smooths the piezo peaks into a warm, natural tone. On violin, mandolin, or electric violin it tames the harsh top end that forces engineers to pull you down in the mix. On an upright bass or a dobro it captures the body's depth and controls the midrange squawk, all without a separate preamp in the signal chain. And because it travels inside the instrument case in a rugged 16-gauge steel chassis, the smallest piece of gear in your bag is the one that fixes your sound.

Who the SB-4 Is For

The SB-4 is for the classical or fingerstyle guitarist who wants the true voice of a piezo pickup through the PA; for the orchestral string player who needs to plug in direct without a rack of preamps; and for the touring upright or resonator player who wants to shrink the gear without shrinking the tone. If you play a piezo-equipped acoustic instrument and have never loved how it sounds direct, the SB-4 is the compact active DI that changes your mind.