When a power supply is “isolated,” it means that each
power jack (known henceforth as “tap”) is separated
from the rest by way of a transformer. Many non-isolated
power supplies provide individual nine-volt wires but
they share a common ground, which creates a ground
loop between each connected pedal. Ground loops are
like gates that allow stray magnetic and RF fields to
send noise into the power line and crap up your tone.
That said, the perversion of ground loops is completely
dependent on environment, so some folks can use a daisy
chain forever and never encounter a tone-killing loop (or
seven), especially if they play primarily at home.
Voltage and current draw are two relatively easy concepts
to understand in the world of pedals. Each pedal is rated
to run at a certain voltage and draw a certain amount
of current with a certain polarity. Each tap on a power
supply is rated for a certain voltage, a certain amount
of “milliamps” (abbreviated “mA”) and a certain polarity
(center-positive or center-negative). The trick is matching
these things to appropriate taps on a power supply.
As a general rule, too much voltage and too little
milliamperage is harmful, while too little voltage isn’t
desirable but is harmless, and too much milliamperage is
completely fine, as pedals will only take what they need.
Think about milliamperage as a buffet; the pedal will eat
as much as it can until it is full, and it will leave the rest. As
long as the following parameters are correct—matching
voltage and polarity, a surplus of milliamperage—all
systems are go.
Thankfully, nine volts and center-negative power are nearly
standardized at this point, and any derivation from the
convention is meticulously labeled to insure against incorrect
power applications. Also thankfully, most power supplies
on the market today offer 12 and 18 volt taps, and some
even offer AC power for connecting a DigiTech Whammy 4,
Hughes & Kettner Rotosphere or some T.Rex units.
And now, the supplies!
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TONE TALK //
Power Up: The Definitive Power Supply Buyer’s Guide