5.5mm x 2.5mm, according to Q&A
https://www.amazon.com/Acoustic-Revive-RR-777-Frequency-Generator/dp/B01M3YVR49
Use adapter if the power supply comes with 5.5mm x 2.1mm plug.
https://www.amazon.com/OMNIHIL-Adapter-Converter-Female-x2-5mm/dp/B01N812NKL/ref=pd_lpo_3?pd_rd_i=B0...
https://www.amazon.com/Acoustic-Revive-RR-777-Frequency-Generator/dp/B01M3YVR49
Use adapter if the power supply comes with 5.5mm x 2.1mm plug.
https://www.amazon.com/OMNIHIL-Adapter-Converter-Female-x2-5mm/dp/B01N812NKL/ref=pd_lpo_3?pd_rd_i=B0...