Counterweight decoupling and structural modification for Rega RB tonearms.
The Rega RB series tonearms are great performers for the price. Nevertheless, few improvements make any Rega RB tonearm even better.
The part of the tonearm that tends to amplify unwanted resonances is the counterweight and its stub.
The counterweight is a necessary evil in tonearms design. Still, the nasty side effects of having a considerable mass connected to the arm tube can be significantly diminished if not completely neutralised.
Our solution required a more complex design and an accurate choice of materials. (see third picture)
Most of the parts are made from brass; for centuries, brass has been the metal of choice in many musical instruments where mid and high frequencies damping is a must. 
The stub is made from four parts of different materials. It is a brass grommet that screws into the arm tube and offers first damping of high frequencies; screwed into it, we find an internal stub made from stainless steel 316 that offers adequate mechanical resistance to accidental knocks. 
Over the inner stub, we have a thick silicone tube that efficiently dumps almost all frequencies; over the silicon tube, an outer brass stub completes the stub design.
The counterweight is made from solid brass and the mounting hole off-centre to lower the centre of gravity of the tonearm.
Another part of the design tends to reduce the counterweight mass that tends to oscillate at the opposite end of the cartridge. The original Rega counterweight is a hefty 135grams; we managed to reduce the mass to 85 grams without limiting the ability of the tonearm to install heavy cartridges up to 11 grams; this reduction of weight was made possible by increasing the mass of the stub itself and increasing the stub length of 5mm over the original one.
If your deck doesn’t allow additional 5mm clearance behind the tonearm than this kit is not for you.
Tests show a general improvement of the sound stage with low frequencies reproduced in a tighter and mode defined manner; the improvement varies depending on the cartridge-tonearm-deck configuration.
The installation is straightforward; unscrew the old stub and screw in the new, done.
Of course, the tonearm and the deck are not included.
What is included is: the improved stub, counterweight and 1.5mm Allen key.
The brass parts are not painted and a pleasant patina is expected to form with the pass of time.

Counterweight decoupling and structural modification for Rega RB tonearms