The Oakley Power Bus
In an ideal world I wanted the Oakley power bus to be based on a five way 0.156” MTA or
Molex connector. This would c15V, -15V and three grounds. One ground would be
the safety ground; this would be connected to the front panels and then directly onto the
main’s supply earth. The second would be a clean ground for all the analogue modules to take
their supply reference, the zero volt line, ie. 0V. The third would be a dirty ground. This
would be the ground reference for things like the noisy digital circuitry and LED switching.
However, this system would be incompatible with the MOTM modular which has a four way
connector for its analogue modules. So the question now remained, how could I make my
system work with MOTM, yet still retain some of the features I needed.
The chosen Oakley power bus comprises of +15V and -15V lines with two grounds. These
grounds are not connected together as in the MOTM. They are joined only at the power
supply in a wholly Oakley Modular.
In an Oakley modular one of the grounds, pin 2, goes straight to the power supply’s star
point. This is the analogue reference point and is correctly called 0V or module ground. This
is the same as the MOTM ground. The second ground is on pin 3. This is also connected to
the power supply’s ground. But this ground is connected only to the metal lugs of the sockets
on each module and nothing else on the module. It is this connection that sets the potential of
the patch lead’s screening and the metal case of the modular. This way it is impossible for
ground currents to travel down the inserted patch cords, since the panel is isolated from the
system ground except at one point.
However, this only works if ALL your modules follow this rule. If you insert just one MOTM
module into this system, the ‘benefits’ are lost. MOTM modules can be modified to allow full
Oakley compatibility although this should be done as you are building the module. Modifying
a completed MOTM module is possible, but the reverse side of the PCB must be accessed so
that you can cut the required tracks.
But I said earlier that the MOTM and Oakley power systems were compatible. And indeed
they are. Any Oakley module will work in a MOTM system. And vice versa. But you will not
get the inherent advantages of a two ground system if you mix and match systems.
Having said all that I have yet to hear any problems related to ground currents in any MOTM
or MOTM/Oakley systems.
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