Voodoo Lab Pedal Power Digital - Manual (Page 9)

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About Isolation
Pedal Power Digital has four isolated output sections. This is sometimes referred
to as “galvanic isolation” and means that there is no direct electrical connection
between each output. Similarly, when you power your effects with batteries, they
are also isolated because there is no electrical connection between the batteries.
Most power supplies are not isolated. Their output jacks are simply bussed
together or use a daisy chain cable. This allows unwanted electrical current to
flow between the outputs, commonly called a ground loop. The result is added
noise. The isolated outputs of Pedal Power Digital eliminate this noise.
Another benefit of isolation is that you can power both negative ground and
positive ground effects with Pedal Power Digital. Negative ground effects are by
far the most common, however the Fuzz Face and Octavia are some well known
examples of positive ground circuits. Attempting to power positive ground pedals
together with negative ground pedals from a non-isolated power supply will not
work and may even damage the supply.
There are a couple of other tricks you can get from an isolated supply like Pedal
Power Digital. You can use a special Y-cable (voltage doubler cable) which
connects two 9V outputs in series to provide an isolated 18V, or use the 12V
outputs to get 24V. Occasionally, a pedal requires a bipolar supply, using two
batteries for +9V and -9V. Isolated outputs work fine for this, non-isolated
supplies do not.