Turntables, Arms & Cartridges

Described as ‘Positive Scanning’, Decca’s innovative pick-up mechanism is closest in (magnetic) principle to a moving-permalloy cartridge [HFN Aug ’25] where the stylus moves a ferromagnetic cantilever through a fixed magnetic field, inducing a voltage in the coils. Decca’s implementation is distinct in two key areas – its thin metal ‘cantilever’ is critically bent and retained under tension by a tie wire [the white terylene fibre, inset picture] while the sensing coils are arranged to separately detect horizontal (L+R) and vertical (L-R) motion. The original 1950s mono Decca pick-up sensed lateral motion only via the ‘lateral coil and magnetic pole’ seen in the line diagram [below].

Pictured in the new London [below], the gold-plated radio metal poles are connected to AlNiCo magnets. Also visible is the diamond-tipped 38µm soft iron cantilever emerging from a black ABS bobbin wound with 54swg (0.016mm) enamel-coated copper wire.

The advent of stereo LPs forced an evolution in the Decca mechanism that saw two vertical-sensing coils and one magnet arranged in a ‘V’ pattern above the knee in the cantilever. One of these poles/coils is illustrated in the upper portion of the line diagram while, inside the new London, there are two 52swg (0.019mm) copper-wound coils paired with an AlNiCo magnet. PM