Various
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
Not
all good DI boxes are expensive. This Orchid range is particularly
affordable, but many of the more expensive boxes do justify their price.
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 in bulk, while the likes of a specialist Jensen or Lundahl tranformer may cost upwards of
$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.
