Decca Mark V phono cartridge

[Mr Spock]

The Decca Mark V is the latest version of that English firm's unique "tip-sensing" stereo-matrix-ing pickups. The "matrixing" apellation refers to the fact that the Decca pickups do not use 45°–45° sensing coils, but use instead a combination of vertical and lateral-sensing coils. There is a single coil for lateral sensing, with its pole pieces brought down next to the tip. This is the tip-sensing feature, whose major attribute is that the stylus motions don't have to be conveyed along the length of an armature before reaching the transducing pole pieces. Thus there is virtually no possibility of the stylus vibrations being modified through flexing of the armature prior to their transduction into audio signals.
Then there are two vertical-sensing coils with pole pieces located directly above the top of the armature (fig.1). These are not tip-sensing, but since the armature is a straight, vertical rod, the only thing that could modify tip vibrations on the way to the pole piece is longitudinal compression of the rod, which is likely to be of negligible magnitude.

It is important to note though that, while the lateral sensing arrangement is push-pull (the stylus approaches one pole piece as it leaves the other), the vertical sensing is single-ended (becoming more sensitive as the stylus lifts up towards the polepiece). In theory at least, this must cause more waveform distortion from vertical modulations than from lateral ones. More about this later.

One end of the lateral coil is connected internally to the connection between the two vertical coils, with the latter connected in opposite electrical polarity. And here's what happens:

Whenever the stylus traces a 45° stereo modulation, representing one channel of recorded information, it moves both laterally and vertically (fig.2). Sensing takes place in both the lateral and vertical coils, but since the vertical ones are out-of-phase, only one will match the polarity of the signal from the lateral coil and produce output. The other will cancel the lateral-coil signal, producing no output. Result: stereo separation.

As the groove angle changes from 45° for a change in stereo direction, vertical-lateral cancellation will increase in one channel and decrease in the other, and the relative signal-output levels will change accordingly and appropriately.

Lateral groove modulations (mono, or center-stage stereo) will produce sensing in the lateral coil only. The vertical coils won't respond, so no cancellation will take place. Vertical modulations (rear ambience or, when mixed with lateral, stereo difference signal) will produce equal out-of phase signals. The lateral coil won't respond, so no cancellation will take place.

An odd way to play stereo discs? Not really. All London discs, and many other European ones, are made with cutterheads that use the same matrixing principle only in reverse.

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Description: Tip-sensing matrixing phono cartidge. Stylus: 0.5 to 0.6-mil spherical. Recommenced tracking force: 3 grams. Cartridge weight: 5 grams. Nominal output: 7.5mV.
Price: $109.50 ($135 for Export model) (1974); $1600 (Jubilee, 1995); $4495 (Reference, 2005).
Manufacturer: Decca, New Malden, England (1974); Presence Audio, Overdale, Haglands Lane, West Chiltington, Pulborough, West Sussex RH20 2QR, England (2005). J. Wright Audio Services, Unit 12, Stanley Lane Industrial Estate, Bridgnorth, Shropshire, WV16 4SF, England. Tel & Fax: 0044 (0)1746 761739 (2017). US Distributor: Paoli High Fidelity Consultants, Inc., Paoli, PA 19301; The Audiophile, Chicago, IL 60601; Electrostatic Sound Systems, Sacramento, CA 95837 (All 1974). Pro Audio, Ltd., Barrington, IL 60010 (1995). May Audio Marketing, 2150 Liberty Drive, Unit 7, Niagara Falls, NY 14304-4517 (2005). No US distribution (2017). Web: www.london-cartridges.co.uk/.





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