Since you are native French speaking I would suggest Google using the words Baxandall EQ as there is a ton of information out there (perhaps in French) and this would allow you to learn everything you would wish to learn and more if you like.
A Baxandall tone stack is a high/low shelf EQ. Versions can be found in Ampeg, Orange, Tweed Fenders, Sunn amps and more. With the controls set to 12:00, the frequency response is essentially flat, as opposed to a std TMB tonestack, which has a mid-scoop and bass roll-off with the controls set the same. A Bax stack is capable of essentially a mid-hump, which a std stack is unable to accomplish at any setting.
This circuit has been used in every stereo system since the 70’s for good reason and as a result there is plenty of information readily available. Also here is a link to a great Tone Stack Calculator.
Thanks for the Doc.