Recording - September 2014 - 72

Leave It On, Or Turn It Off?

Guest editorial by Mike Rivers
For nearly as long as electricity has
been around, there have been arguments
over this simple question: When you're
not using it, leave it on or turn it off? Our
Editor recently posed that question for a
specific device, a computer audio interface that's powered from a computer's
FireWire port and has no power switch of
its own. Like so many things in this business, the short answer is "It depends."
Another answer, valid for most modern
electronic equipment, is that an off/on
cycle makes little if any difference to either
the unit's performance or to its expected
service life. However, knowing the facts
behind how power cycling affects electronic devices will help you decide how to
manage your power. Here's a primer.
Think of the incandescent light. When its
time has come, you flip the switch on, the
bulb flashes briefly, and then dies. It's less
common for a light bulb to fail while it's on.
The light, and along with it, heat, comes
from current flowing through a thin wire filament inside the glass bulb. Heat causes
metal molecules to boil off the filament wire,
weakening it until it eventually breaks and
current can no longer flow. But why does it
commonly fail just as you've switched it on?
Incandescent lamps are rated by operating voltage and wattage (power). At
the rated voltage, the filament draws
enough current to reach the rated
wattage and glow brightly. For a common 120 volt, 60 watt household bulb,
Ohm's Law tells us that the filament must
have a resistance of about 240 ohms and
draws a current of 0.5 amps. This is the
filament's resistance when it's hot.
However, the resistance of most metals is
lower when cold and increases with temperature. The cold resistance of a 60 watt
lamp is around 21 ohms. When the voltage is first applied, the filament initially
draws nearly 10 times its normal current. It
takes about 150 milliseconds to reach
operating temperature and where the resistance is 240 ohms. This brief but large initial inrush current can cause a filament
that's already weakened from use to fail
suddenly. This is the failure mode that's at
the root of most "Leave it on" arguments.
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RECORDING September 2014

Vacuum tubes have filaments and can
fail in the same way. However, their filaments run a bit cooler, red-hot rather than
white-hot, making them less subject to
fatigue than an ordinary lamp. But can
we extend the life of tube gear by leaving
it on, or by turning it off?
Back when all studio electronics used
tubes, leaving the gear on all the time
was fairly common, though actual practice was determined mainly by studio
usage, not science. Major studios of yesteryear worked nearly around the clock,
so there was little reason to turn anything
off. Keeping tube mics powered up was
particularly important since certain mics
didn't sound right without an hour or so to
warm up.
Early power supplies rarely regulated
the filament voltage, so turn-on stress was
a consideration. Modern tube devices,
including mics, use more sophisticated
power supplies that often stabilize the filament voltage as well as the high voltage, and offer a soft turn-on to limit inrush
current. Tube instrument amplifiers, however, still typically use simple power supplies (it contributes to the dynamics of
tone). Tubes also generate heat that, if not
properly vented, can shorten the life of
other components in the unit.
Does any of this justify leaving modern
tube gear on all the time? Not really. If it's
something that's always in your signal
chain, leave it on while you're working.
However, if it's something that's used only
occasionally, leave it off until you need it,
though once it's on, you might as well
leave it on for the rest of the session
because you might need it again.
With the introduction of solid-state
gear, we traded the old problem of tube
burnout for some new problems.
Semiconductors don't wear out and have
a failure rate of tens of thousands of
hours, though they're still subject to
instant death from overloads. A solid-state
unit, however, typically has more individual components than its tube counterpart,
creating more possible points of failure.
Vintage solid-state gear used many discrete transistors, while modern designs
typically replace dozens of components
with a single integrated circuit (IC), resulting in fewer parts subject to damage from
current surges.
While nothing gets red-hot (or isn't supposed to!) semiconductors dissipate some
heat, too. Good designs deal with heat
properly, but this still worries some owners. It's OK to turn off a device when it's
not in use, but there may be a gotcha-
many of today's low-power solid-state
devices have no power switch.

This isn't a hint to leave it turned on, it's
to save cost. The intent is that you'll plug
several switchless devices into a power
strip and that will become your master
switch. Arrange your outlet strips for what
makes sense in your workflow. As a counterpoint, you won't preserve a poorlydesigned device by minimizing its on-time
or number of times switched on!
A good number of modern computer
audio interfaces have no dedicated power
supply, but rather are powered through the
same port that carries their data. Here, the
power-on status follows the computer-on
when the computer is on, off when the
computer is off, and, depending on the
computer, may go either way when the
computer is "sleeping."
Should you unplug the USB cable when
you're using the computer but not the
interface? The USB port's external device
power (5 volts at up to 500 mA, or up to
900 mA if it's used only for battery charging) is current limited, eliminating high
inrush current when "hot-plugging" (connecting the device with the computer
turned on) so it's unlikely to cause damage, but is it worth the hassle? I doubt it.
The warning: "Be sure everything is off
when connecting or disconnecting" came
from USB 1.1 days, and for early
FireWire audio interfaces. We're often
warned not to disconnect a USB device
unceremoniously, but that's to prevent
data loss, not electrical damage.
USB 2 and 3, FireWire, Thunderbolt,
and Lightning bus-powered devices all are
designed for hot-plugging, so you can
unplug them if that makes you feel better.
On the other hand, since your studio computer should be primarily for audio work,
you'll probably need the interface whenever you're using the computer anyway.
What about an AC powered, bus-connected audio interface? It's no different from
any other AC powered solid-state device.
One final thought: USB connectors
wear out after a large number of mate/unmate cycles. It's rarely the cable plug that
wears out; it's the socket on the hardware.
Plug and unplug it too often and you might
find yourself with a computer or interface
port that no longer works. Score one for
"Leave it connected."
So: Do or don't? As usual, it depends,
but in general doing what makes practical sense also makes the best electrical
sense. The days of instant death when
switching power on/off are over.
Mike Rivers (rivers@recordingmag.com)
is a recording engineer and audio gear
designer. He thinks hard about this stuff.

Recording - September 2014

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