Recording - June 2015 - 63

and basses don't like being plugged into
the lower impedances typical of consoles.
And if the instrument is simultaneously
plugged into a mixing console and an
amplifier, this means the console's ground
connection is connected to the amplifier's.
That is another book-length discussion, but
the short version is that when the amplifier
and console are powered by different circuits in the building's wiring, a ground
loop can develop. This makes the whole
system hum at 60 Hz, which can be infuriatingly hard to stop or fix.
So plugging electric instruments directly into consoles was a non-starter.
Modern mic preamps often include highimpedance instrument inputs, but that
only works if the player is in the same
room as the preamp (usually the control
room). Engineers realized that many of
the problems came from the high impedance of the instruments, so they decided
that finding a way to lower that impedance would fix it.
To do that, they used a transformer,
which can be thought of as two coils of
wire wound onto a steel core; one winding
generates a magnetic field, which the
other winding picks up, and so the signal
gets passed from one to the other.
Transformers can step the levels of audio
signals up and down by virtue of the number of turns wound onto the core. If one

winding (known as the primary) has, say,
800 turns and the other (known as the secondary) has 80 turns, we say that the
transformer has a turns ratio of 800:80, or
10:1. An audio signal fed into a 10:1
transformer will have its level stepped
down by a factor of 10; more importantly,
the impedance will be stepped down by
the square of that number, or 100. So the
signal from a bass guitar with an impedance of 50,000 ohms will come out of a
10:1 transformer with an impedance of
500 ohms, which is low enough to run
down a long cable without losing too
many highs. That was vital for our session,
since Shlomo's custom bass has a lovely
sound that includes significant high frequencies.
The output of the transformer is also
capable of being connected to a balanced input on the console (we'll talk
about balanced inputs when we talk
about microphone preamps). Because the
10:1 transformer steps the level of the signal down, it needs to be plugged into a
microphone input, which makes up for
the loss in signal level.
Finally, when a transformer-type DI box is
used to send signal to a console, and the
box's "Ground Lift" switch is actuated, there
is no actual wire connecting the instrument
amplifier to the console. That's really important; the instrument and amplifier connect to

the transformer's primary winding, while
the console connects to its secondary winding. The two windings are not wired to one
another; the only connection between them
is the magnetic field which couples the signal from one to the other. No wire? No
ground loops, and no added hum.
Keeping two pieces of equipment electrically separated in this way is called galvanic isolation.
Using a transformer in this way is called
Direct Injection, and a box with a transformer in it, used for this purpose, is called
a DI box; you can buy them (Radial, Little
Labs, and others make really good ones),
or make your own. These days, DIing electric bass is a standard studio practice. DI
boxes are also used, most often in live
sound situations, to send signals from electronic keyboards and synthesizers to a
mixing console out in the hall.
I brought a homemade DI box to the
Ottoman Underground session, but
wound up not having to use it; Shlomo's
amp has a built-in DI that sends a miclevel feed of the amp's output (including
any nice tone-shaping or distortion it
might be adding) to an XLR mic connector
on the back of the box. Using a DI while
simultaneously miking the amp has its
own set of pitfalls, but I'll talk about those
in the article on mic preamps. Stay
tuned.-PJS

http://www.realtraps.com

Recording - June 2015

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