Dear Thierry, can you recommend a simple addition to your circuit that changes the timbre? (but so as not to significantly worsen the thermal stability of the circuit) Something for a small, but timbre control? At least in the direction of softening, reducing high and mid frequencies, sounding flat and dry. I tried different options, but only adding one amplification stage (I took it from the network) with changing the potentiometer of the transistor base mode worked (not significantly).
I still have not learned how to accurately select transistors for your circuit, although I assembled a set-top box for measuring the parameters of the cutoff voltage and current. But the transistors I bought (specially for these parameters) did not meet the parameters required for this circuit (?!!). But (the circuit works with them) at the same time, some of them produce a sharp, unpleasant, dry sound, and some - a pleasant, soft, musical one.
(you have to select transistors by trial and error, finding a soft, musical set.)
But, the secret of the soft, musical mode, with the presence of low frequencies, remains undisclosed)).
So I decided to ask you, as the creator of the circuit, for advice on timbre.