Conn Imp. Precision 3 vs Kelly 3C
The Kelly 3C has a wider cup diameter, the Kelly 3C features a deeper cup.
Cup Diameter
The Kelly 3C has a cup diameter of 17.02 mm / 0.6701 in compared to 16.41 mm / 0.6461 in on the Conn Imp. Precision 3 — a difference of 0.61 mm / 0.0240 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 3C has a very deep cup while the Conn Imp. Precision 3 has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Material
The Conn Imp. Precision 3 is described as Silver-plated brass, and the Kelly 3C as Lexan polycarbonate. Material affects mass, warmth, and how quickly the piece responds.
Which is better?
Both mouthpieces are categorized as "General" — the physical dimensions above will determine which suits your embouchure and playing style better.