Bryston uses a somewhat unusual trigger connector as implemented on my 14BSST. It looks something like
this. I had to order a replacement directly from Bryston because the original had been lost over the years.
The specification says 4-12V is required to trigger the input. Experimentation shows that on my unit at least, 3.3V is sufficient to turn on the amplifier with a current draw of 0.84 mA.
This lower value is useful because a SqueezeBox can generate a
3.3V trigger signal.
The 14BSST manual is not clear on the subject of how to enable the trigger mode. I had to set the trigger mode to "External Trigger", the power mode to "Local"
and turn the unit on via the front panel.