It's a sad fact of life that not all studio equipment is compatible when it comes to signal level and impedance, and even on the occasions when it is, ground loops can still arise to cause hum problems. That's where the DI box comes in, but even there the choice is far from simple, as there are both active and passive versions to choose from.

Passive DI boxes, such as the EMO model reviewed here, contain no active electronics and so require no power. The heart of the circuit is an impedance-matching transformer with the addition of a switchable, resistive attenuator to allow it to handle loudspeaker-level signals up to 100V in amplitude. Because professional studios require several DI boxes, EMO have designed a two-channel model which saves on both space and cost. It can accept line-level signals up to 20dBu, instrument-level signals down to -20dBu and also loudspeaker signals up to 100V or 42dBu. All these inputs are unbalanced and on quarter-inch jacks, while the outputs are balanced on XLR connectors. The output level can be up to 0dBu, but as this is a purely passive system, the actual level will depend on the input signal level and on which input socket is used.

The instrument input may be used with electric guitars and basses, though the input impedance is inevitably less than that of an active DI box, so a slight loss of tone may be experienced with some instruments. This input is also suitable for low-level electronic instruments such as home or semi-pro keyboards, which often work at a nominal -10dBv or -20dBv, and a parallel socket is provided, enabling the instrument signal to be fed to an amplifier at the same time if required.

The line-level input is more suitable for professional equipment having an output level between 0 and +4dBu, while the speaker input may be used with most small and medium-sized power amplifiers to obtain a direct feed from the speaker outlet. There is no dummy speaker load in a DI box of this type, so the loudspeaker must be left connected, and for this reason, there are two parallel loudspeaker sockets, one to connect to the amplifier and the other to the loudspeaker.

One big advantage of transformers is that there is absolutely no electrical signal path between the input and output signals so it's easy to avoid ground loops. Because some situations demand that there is a common ground path from input to output, the unit in question has a ground or earth-lift switch which can be used to select either option.

I was interested to see how well a passive DI box would work on electric guitar, and I was pleasantly surprised, both by how quiet the result was and by how little tonal detail was lost using my Strat. Going into a desk with no EQ, the sound is clean and solid but without the brilliance you'd expect from a guitar amp. This isn't down to the DI box though; it's because guitar amps have a deliberate high boost to create the required tone — no guitar amplifiers have a flat frequency response. By adding around 8dB of boost at 4.5kHz using the equaliser on my desk, I recreated the classic Strat tone without increasing the noise to any noticeable degree. In fact I achieved one of the best clean guitar sounds I've ever managed to get, and by patching in a compressor and adding little reverb, a beautifully produced guitar sound can be achieved.

The instrument input is equally clean and transparent, but a few words of caution are in order concerning the speaker input. This is not filtered in any way and so the signal you get will be a direct copy of what is being fed into the speaker. In the case of instruments which don't rely on speaker distortion, this is fine, but if you try to DI a guitar amplifier in this way, you'll find that the overdrive sounds produce unpleasant, raspy harmonics. These harmonics are actually there all the time but a typical guitar speaker filters them out and generally smooths out the tone.

Ultimately, the EMO DI box achieves its objectives admirably. It is very sturdy, well-designed and uses good-sounding transformers. The instrument level input really can handle a guitar without choking the life out of it, while the sound of some keyboard instruments can be improved by virtue of the better signal matching this box provides. I like the idea of having two channels in one box, though single DI boxes are also available. The cost is a little more than you'd pay for a budget DI box, but this is a truly professional piece of kit and will last a lifetime.

Further Information
ES25 Dual DI Box, £90.46; E520 (single), £52.26. Both prices include VAT.

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