If you’ve seen a black dot on the face of some cellos, it’s the sign of a Krentz internal modulator. Inserted through the f-hole, it latches on to the external magnet which allows it to be easily moved as you adjust for wolf notes and tone. While this works well in many cellos, it does not help all cellos solve the problem. If you’ve tried other gadgets unsuccessfully, this is definitely worth investigating. This will fit through an f-hole eye no more than 14.1 mm in diameter.