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Radial Engineering SB-4 Piezo

Compact active DI for piezo pickups, low-cut filter, 48V phantom

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The Radial StageBug SB-4 is a compact active direct box that has been optimized for use with piezo transducers. This enables violin, dobro, mandolin, cello, upright bass and other instruments that are best amplified using a piezo to be connected without compromising the tone or frequency response.

The design begins with a ¼" input for the instrument pickup. Unlike typical direct boxes that have a 1meg-ohm input impedance, the SB-4 features a very high 5meg-ohm impedance that smoothes out troublesome peaks that give piezos a bad name. The higher impedance also broadens the frequency response for a more natural and pleasing sound. The active circuit performs the high-to-low impedance conversion and balances the signal to enable long runs upwards to 100 meters without loss, delivering spectacular linearity and low distortion throughout the audio range.

To help eliminate feedback on stage, the SB-4 is equipped with a high-pass filter and 180° polarity reverse switch. The high pass filter eliminates excessive bottom end which in turn reduces resonance that can cause the top of the instrument to vibrate. The 180° polarity reverse helps reduce acoustic hot spots on stage by changing the phase relationship between the PA system and room acoustics which can combine at certain frequencies to create peaks. Reversing the polarity eliminates the acoustic peak without resorting to severe equalization which only serves to compromise the natural tone of the instrument. Connections include an AUX out for a stage amplifier plus a standard XLR output to feed the PA. Power is derived from the console's 48V phantom, thus eliminating the need for batteries or external supply. One simply connects, turns on the phantom power and plays!

These features combined with the compact size make the StageBug SB-4 the perfect companion for orchestral artists, blue grass players and pop string quartets that need to turn it up when in concert.


Ultra compact to fit in your guitar case

High pass filter eliminates resonance

Smoothes out peaks, eliminates squawk

Active DI optimized for piezo transducers


Circuit TypePiezo optimized active buffer
Frequency Response20 Hz to 20 kHz
Total Harmonic Distortion0.006%
Dynamic Range100 dB
Input Impedance5 MΩ
Maximum Input+4 dBu
Gain2 dB
Output Impedance330 Ω
Equivalent Input Noise-101 dBu
Noise Floor-97 dBu
Intermodulation Distortion
DeviationPhase: 10° @ 50 Hz
SwitchesHigh Pass Filter, Phase, Ground Lift
Low-Cut Filter150 Hz
FilterSlope: 6 dB/octave
Instrument Input1/4" buffered with piezo optimized load
Auxiliary Output1/4" buffered to drive guitar effects, amplifier, or tuner
Balanced OutputPin-1 ground, pin-2 (+), pin-3 (-)
OthersPolarity Reverse: 180° Ground Lift: Soft lift with 100 Ω to ground
Construction16 gauge steel chassis & outer shell
FinishBaked enamel
Power48 V phantom power
Dimensions (L x W x D)4.50 x 1.85 x 1.35" (114.00 x 47.00 x 34.00 mm)
Weight8.80 oz (249.48 g)
Package Weight0.6 lb
Box Dimensions (LxWxH)8.0 x 4.7 x 1.6"