Fred, thank you for your kind words!
For the moment I have no plans to make the decay variable, I tried to approach it best to the decay of the C-String of a real Cello.
The ghost tone which you found is most probably due to sampling issues, I recorded mic-less, directly from the audio out into the sound card. The "original signal" (at least for voice I ) is very rich in harmonics, the second harmonic being almost at the same level as the fundamental. This may also be a reason for this slightly metallic undertone. There is still some fine tuning needed in the wave shaping circuit. I hope to be able to optimize that and then to make a true acoustic recording soon.