07.
Power Source
www.LeonAudio.com.au
Power for the FET input amplifier needs to be returned to the XLR
connector. This can not be achieved when the Earth Lift switch is open.
There are 4 possible options ...
1:
Use batteries. This option was considered unacceptable in a
professional D.I. box
2:
Do what other active D.I. boxes do and allow the Phantom Power to
return via the instrument’s mains earth connection. A great way to pick
up lots of electrical noise.
3:
Do what other active D.I. boxes do and use a partial Earth Lift. This
concept works well in the showroom. It fails in the real world when
electrically noisy environments are encountered.
4:
Use a complex isolated DC/DCinverter for the FET input amplifier.
Option 4 was chosen as it is the only one that offers genuine earth
isolation between the input and the XLR output of the DI Box.
Block diagram of the Active D.I. Box
The drawing below shows that there is no direct electrical connection
across the Earth Isolation Barrier.
A DC/DC inverter is used to get the FET amplifier’s power across Earth
Isolation Barrier. Phantom Power is first extracted from the centre tap
on the bifilar wound secondary of the audio transformer. This drives a
DC/AC converter running at 100kHz which in turn drives a small power
transformer. The resultant 100kHz AC voltage on the secondary of the
power transformer is rectified and filtered to provide 18 volts DC to run
the FET amplifier. As a result, there is no interruption to the FET’s power
supply when the Earth Lift switch is operated. As the FET amplifier is
operating on an 18 volt supply, it is able to handle much higher signal
levels than if it were operating from a 9 volt supply, such as a battery.
This means that the DI Box can handle input signals up to +15dBu
without the need for an input Pad switch. (0dBu = 0.775Volts)
Output
to mixer
Buffered
Out
Direct
Out
Input
+
3
2
1
Earth Lift
switch
Earth Isolation
Barrier
Audio
Isolation
Transformer
FET
Amplifier
100kHz DC/DC inverter
DC
AC
AC
DC
