I have not been able to underclock my T9+ with the supposed settings that were suggested here but playing around with the ideas from here I found a solution that I wanted to post / share.

So the problem was that that my miner also already had autotuning available and changing "bitmain-freq": "some_value" would be ignored after rebooting.

So first I recognized using top that there was a process /usr/bin/bmminer --version-file /usr/bin/compile_time --api-listen --default-config /config/bmminer.conf running.

Thus I checked /usr/bin/bmminer --help to recognize that there is an option --fixed-freq        Set bitmain miner use fixed freq
which probably could also be set in /config/bmminer.conf.

Thus I added the line "fixed-freq":true, to the bmminer.conf[\b] so that in total it looked like that:



In this way I was able to underclock the miner and tune down the fan. actually the PSU is now more noise as it's fan cannot be regulated by the software (what a pitty because it still produces 50db)

running the following firmware: