I am looking for a USB audio interface that I can connect all of my instruments to, and I have been considering a Focusrite Scarlett 18i20. I have an electric guitar, an electric mandolin, a Hammond SK1 organ and a Roland Gaia synth, and I would like to connect them all to the interface simultaneously, which shouldn't be a problem as there are enough inputs for this. However, in the interface manual it says that instruments can only be plugged into inputs 1 and 2 directly. If I wanted to connect all of the above instruments, can I do that directly or will I need some DI boxes? If so, how many would I need? The Hammond requires two inputs (L+R stereo) and the Roland requires another two (L+R stereo again). Would each line require its own DI box?
Via SOS web site
SOS Technical Editor Hugh Robjohns replies: The answer to this question isn't quite as straight forward as it appears, but you really won't need the DI boxes you're worried about. Here's why...
First, the 18i20 has two combi sockets on the front,
marked 'mic-line-inst'. The centre jack sockets in these two connections
are presented to high-impedance instrument inputs which are intended
specifically for receiving the signals from electric guitars and similar
instruments. Depending on the actual impedance, which doesn't appear to
be stated on the web site or in the manual, it may not be ideal (high
enough) for the mandolin if that uses piezo pickups, but you should just
try it and see. It's probably going to work just fine!
The rear-panel has more combi jacks, which are labelled
'mic-line', and you can connect your keyboards here. The keyboards both
output something close to line-level outputs, so you can connect them
directly as line-level inputs to the Scarlett using ordinary jack
cables.
A DI box takes the unbalanced line-level
The
output of most synths and keyboards like this Roland Gaia, for example,
will work fine with most audio interface line inputs, without the need
for a DI box. output and knocks it down to a balanced mic
level which you can then connect to a mic preamp, while also separating
the source and destination grounds. It's a very convenient solution for
stage use, but unnecessary for most studio applications, and potentially
degrades the keyboard signal when there's no need!
The
output of most synths and keyboards like this Roland Gaia, for example,
will work fine with most audio interface line inputs, without the need
for a DI box. output and knocks it down to a balanced mic
level which you can then connect to a mic preamp, while also separating
the source and destination grounds. It's a very convenient solution for
stage use, but unnecessary for most studio applications, and potentially
degrades the keyboard signal when there's no need!
In a perfect world, plugging the keyboards straight
into the Scarlett's line inputs would work fine. But the world ain't
perfect, and while this may well work fine — and is certainly worth
trying — you may find that you end up with an annoying low-level hum or
buzz caused by ground loops, because we're dealing with unbalanced
connections.
If either of the keyboards has a 'double insulated'
(class 2) power supply, there won't be a problem with ground loops
anyway because it doesn't have a mains safety earth to create the ground
loop. If they have normal earthed (class 1) power supplies, then you
can minimise the likelihood of ground loops by making sure that the
keyboards are powered from mains sockets adjacent to the Scarlett's
power supply.
If you still have an audible ground-loop problem then the most cost-effective solution is to head over to the SOS shop (/shop)
and purchase a pair of pseudo-balanced cables for each keyboard — one
cable for each of the left and right channels. This might seem like
shameless promotion of our wares, but these are unique to SOS and break
the ground loop in quite a clever way by using the balanced line input
of your interface. They're available with either TRS or XLR plugs at the
balanced interface end, and have a normal unbalanced jack plug at the
instrument end — and they cost a heck of a lot less than a decent DI box
or line isolation box, which would be the other solution. In your
situation, I'd suggest that the TRS version is the most appropriate.
But first, try plugging the keyboards in to the line inputs with ordinary unbalanced TS jack cables; there's no point spending money if you don't need to!
Published in SOS January 201
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