Conrad,
I just want to know what to do with the PWM and tacho wires if
I do NOT want to use them.
The tacho wire is an open collector output. You can simply leave it
disconnected, and it shouldn't cause any problem. Cut it off close to
the fan, to minimize noise pickup, just in case. You can also ground it
there (to the negative fan supply wire), but that's unlikely to be
necessary.
The PWM wire is an input, intended to be driven from an open collector.
Cut it as short as possible, and connect a bypass cap from it to the
negative fan supply line, as close to the fan as possible. A capacitor
value like 100nF is fine.
If after this the fan still gets interfered by the amplifier, you will
need to install some shielding, using wire mesh where necessary, and
bypass the fan supply lines to that shielding. Adding ferrite beads
individually over the supply wires, close to the fan, can also help.
Manfred
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