Most of the DI boxes we've looked at in SOS have been active models capable of matching high‑impedance sources, such as electric guitars, to the inputs of mixing consoles, but there are other applications where active, high impedance circuitry is unnecessary, or even undesirable. In these instances, the active circuitry is replaced by an impedance‑matching transformer, which avoids the question of powering and offers a high degree of electrical insulation between input and output.
Under review is a passive DI box built by Rapco. The circuitry is simple but provides two modes of operation, for use with both line‑level signals and speaker‑level signals from amplifiers, such as guitar amps, making it very versatile. The balanced output may be fed directly into the mic input of any mixer, and because the circuitry is passive, no phantom power is required and no circuit noise is generated.
Unlike speaker simulators, the DB‑101SL can't simply be connected instead of a loudspeaker, as its impedance is higher than that of a loudspeaker and its circuitry isn't designed to dissipate the power generated by an amplifier. Most amplifiers work best when connected to a load of the proper impedance, while valve amplifiers in particular may sustain damage if run without a load. To get around this, the box has both an input jack and an output jack, enabling it to be placed in‑line with either a speaker or line‑level feed without breaking the signal path. In other words, the DI box goes between the amp and speaker (or instrument and mixer) and taps off a tiny percentage of the energy, while the bulk of the signal goes through to the output socket. This is ideal for some types of live recording, as a balanced feed can be obtained without affecting the way the amplifier normally works, but in the case of rock guitar, some further treatment may be necessary, because the DB‑101SL doesn't colour the sound as a typical guitar speaker does.
A rocker switch selects between Instrument and Speaker level inputs, while on the output, a Ground Lift switch is fitted; this is a vital facility as, without it, ground loops and the hum that they cause can be a major problem.
In practice, this DI box goes about its business largely unnoticed — which is the way it should be. Little coloration is evident in the DI signal, and because there's no active circuitry, no noise is added by the unit. Of course, the old adage about lunches and the lack of free ones also applies to passive DI boxes; transformers can either provide current gain or voltage gain but never both at once, as that would contravene the laws of physics (Jim) — and physics wouldn't be best pleased about it. In this case, the impedance‑matching transformer provides current gain, leaving the mic amp on your mixer with the job of handling the voltage gain side of things. This means that you need to have a mixer with spare mic inputs, and if performance isn't to be compromised, they must be reasonably quiet into the bargain. Paul White