[Jackaudio] Noise when using JACK in Linux Mint

Hello to all!<https://sound.stackexchange.com/posts/51899/timeline> I am experimenting with audio production under Linux Mint 20.3 (based on Ubuntu 20.04). I managed to install a low latency kernel, then I have installed JACK, and Qsynth, following the steps described in this tutorial: <http://www.tedfelix.com/linux/linux-midi.html> http://www.tedfelix.com/linux/linux-midi.html <http://www.tedfelix.com/linux/linux-midi.html>Everything seems to work fine if I use Qsynth and pulseaudio: I use the vmpk simple virtual keyboard, and I have good output sound. I have also tested JACK, as suggested in the tutorial using $ jack-play test.wav and I hear the right clean output. However, when I tell Qsynth to use JACK, I have a lot of noise. When I hit a key I have "white noise" for few milliseconds instead of the key sound, and only when it fades I have a little bit of the note that I was supposed to play. I have the same "white noise" problem is I use Rose Garden. I have tried to change the sample rate and the frames/period settings, and also the Timeout, in JACK, but the problem stays the same. Thank you in advance for any help you can give. Best, Fabio