Why are some DI boxes so damned expensive? Aren't they all just pretty much a transformer in a box? And what if any advantage is there when tracking in using a dedicated DI over the instrument input on a decent-ish mic preamp?
Preston Fleming via email
SOS Technical Editor Hugh Robjohns replies: As with everything, the cost of a product is partly down to its construction, partly its complexity and R&D investment, and partly its reputation and marketing. As you've found, there's a very wide range of DI boxes on the market, from the amazingly cheap — but usually not very good — up to the scarily expensive and (hopefully, but not always) sonically transparent devices.
In general terms, the more expensive units tend to be better built, mechanically, and better designed electronically. Consequently they're more robust, are often more flexible (with more features and options), they usually sound better — with lower distortion, more headroom, less noise and wider bandwidth — and more transparent.
In my own experience, the better designed and usually more expensive products will last for decades and will never become the weak link in your recording chain, so the high initial investment is amortised over a very long period, making them good value for money in the long term. Bargain basement products are more likely to fail or become damaged, and sound quality issues may well become apparent as your own abilities, awareness and expectations rise.
As for the 'transformer in a box' comment, passive DI boxes do involve little more than a transformer, a case, a few passive components and some switches and connectors. However, the cost of switches and connectors can vary wildly depending on their quality and reliability. The same is true of the case: some will easily survive being dropped and even driven over, while others will break if you look at them funny. Some designs afford far greater protection for the switches and connectors than others, too.
The most significant cost of a passive DI box is usually the transformer, though. The cheapest far-eastern units can cost as
little as £1$1 in bulk, while the likes of a specialist Jensen or Lundahl tranformer may cost upwards of £50$80. While they all do the same basic job, the more expensive models are better screened, introduce negligible distortion, enjoy a wider, flatter frequency response, can cope with high signal levels without saturation, and provide excellent electrical safety isolation. Cheaper transformers won't perform as well, and the results are often audible.
Active DI boxes obviously involve active amplifier circuitry, and again there are quality differences due to the circuitry involved: discrete FETs in class-A circuits, op-amps, or even valves, for example. Most active DI boxes still use transformers to provide the output balancing and isolation, but certainly not all, and some involve more sophisticated power-supply schemes than others to maximise headroom and minimise noise, as well as to maintain electrical isolation.
The other aspect to consider is that while most DI boxes are designed to be as transparent as possible, some deliberately impart certain desirable sonic characteristics to enhance the sound of a bass or electric guitar, for example. Some are also designed to accept piezo pickup inputs instead of or as well as normal magnetic pickups. Again, these tend to be more expensive than their vanilla cousins.
Everyone has their own personal favourite DI boxes, and many of mine are well over 20 years old now and still going strong. In the active line my favourite remains the Canford Audio Active DI (originally produced by Technical Projects). This has a very useful transformer-isolated balanced link output mode for signal splitting duties. I also have an original BSS AR116 (no longer available, and replaced with the AR113), but more often these days I use the ever-popular Radial J48 (with its handy input-merge facility and clever power-supply technology). Challenging the trend for costly DI Boxes, I have several extraordinarily cost-effective — but very high quality indeed — Orchid Electronics transformerless active DIs. However, there are many other great-sounding active DI boxes around with a wide variety of features, facilities and quality.
Of the passive DIs, I like the EMO and Radial boxes, the latter being better engineered mechanically, but also more expensive. Radial's J range employs high-quality Jensen transformers while the Pro range uses less costly alternatives. The bandwidth and frequency response suffers slightly in comparison, but not to any serious extent and only the most demanding situations would reveal any weaknesses.
One advantage of a DI box is that it can be placed close to the instrument itself, minimising cable losses, which is obviously important for on-stage and some studio live-room applications. However, a lot of microphone preamps and computer interfaces now include instrument inputs, which are essentially transformerless active DI boxes integrated into the preamp itself. The majority employ a fairly simple FET input buffer to provide the high-impedance unbalanced input. They generally work very well, sound good and avoid the additional expense of a separate DI box.