Last week, a moment of reverie was interrupted by the ringing of a bell I hadn't heard in a while: "DECCA CARTRIDGE." I stopped doing what I was doingwhich, as suggested, was nothingand got on the phone to Brian Tucker, of Pro Audio, Ltd., US distributor for London Cartridges, the trade name under which the present-day licensees of the original Decca cartridge design do business (footnote 1).
All right: I lied about the timers. (I did not lie about JA's red telephone.) What really happened was that I visited my friend George, to help him sort out his record player. George had, for a while, lost the use of his right arm, and was unable to cue records without risking and occasionally causing severe damage to his cartridge. In the space of just over a month, two good cartridges had fallen victim, and now George was down to the last three working cartridges in his stable, all of them Deccas.
As it turned out, only one of the three was functional. I installed it on George's Rega Research RB300 tonearm, itself mated to a nice old Rega Planar 3 turntable. I couldn't recall if I'd ever before put a Decca cartridge in a Rega arm; for his part, George didn't recall which, if any, of his tonearms would best suit the semi-lowcompliance Decca (see below). We didn't know what to expect. But I was knocked out: I knew well the sound of George's system, in which tube electronics drive a late-1980s pair of Quad ESL-63 electrostatic loudspeakers, and I'd never heard it produce anywhere near this level of touch and force and dynamism and sheer pep. Neither had George.
That was when I went home and called Brian Tucker.
Three goes into two
The seminal single-channel Decca phono pickup, which first appeared around 1958, is a moving-iron type: It traces the record groove with a stylus attached to an armatureL-shaped, in this casemade of magnetically permeable metal, the latter held within 1mm of a stationary coil, adjacent to which are two tiny, flat magnets, also stationary; the movement of the iron-like armature modulates the current induced by the magnet's flux lines, creating an electrical analog of the modulations in the groove.
The engineers in the Special Products division of Britain's greatest record company must have been pleased with their creation: That same design endures, virtually unchanged, to this day. But it endures as a stereo pickupand that's a bit of an odd duck.
Throughout the phono-pickup industry of the late 1950s and beyond, almost all stereo cartridges have been designed with two separate, identical generators, of whatever technology, driven by a common stylus and compliantly mounted cantilever. Those elements were, and still are, constructed so that each generator responds to modulations on opposite walls of a V-shaped groove, each wall being at a 45° angle to the record's surface, and thus at a 90° angle to each other (ie, their relationship is orthogonal). That's stereo phonography in a nutshell.
But a Decca/London stereo cartridge is, in fact, a mono cartridge design modified to enable stereo playback. It has three coils: the coil that was part of the design all along, for turning purely lateral groove modulations into a signal, and two more coils added later, to produce a signal from only vertical motions of the stylus.
Getting stereo out of such a thing is tricky: On the one hand, the lateral coil produces a signal that is the sum of the electrical representations of both left and right channels: L plus R. On the other hand, reading a 45°+45° groove with a purely vertical generator produces a signal that represents the difference between the electrical representations of the left- and right-channel groove walls: L minus R.
The engineers at Decca Special Products took note of a simple mathematical relationship: By adding to the L+R signal the LR signal, they wound up with 2L, which they directed to the cartridge's left-channel output pin. By subtracting, by means of antiphase, the LR signal from the L+R signal, they produced 2R, which went to the right-channel output pin. Thus, by means of sum-and-difference wiring, Decca produced a two-channel signal from a three-coil cartridge (footnote 2).
Clever though Decca's solution surely was, one can't help noting a couple of departures from perfection. For one thing, in the lateral generator of the Decca stereo cartridge, the coil is almost at the level of the grooveit nearly touches the record's surfaceand the vertical leg of that L-shaped armature passes through its center: an ideal set of conditions. The vertical-plane generator of the Decca stereo cartridge doesn't match that ideal: It uses pole-pieces that straddle, rather than coils that surround, the lateral portion of the armature. (Thus, the Decca's vertical generator has slightly more in common with a variable-reluctance cartridge, which is a different animal altogether; in its lateral generator, the Decca is a purely moving-iron type.)
The other noteworthy idiosyncrasy of the Decca stereo cartridge derives from the mechanical rather than electrical differences between its two generators: Its stylus-armature assembly can move either laterally, which involves some degree of torsional bending, or straight up and down, but it is considerably less compliant in the vertical plane, thanks in part to Decca's use of a piece of very fine thread looped around the stylus shank and held taut, to restrain the armature's movement. (At first glance, that filament resembles a traditional cantilever, and is sometimes mistaken for same by newcomers to the breed.) One consequence is that the Decca cartridge exhibits different vertical and lateral resonant frequenciesmeaning, among other things, that not every tonearm will mate perfectly with this distinctive little pickup.
The iron age
Never mind all that. The big thing about a Decca cartridgeprobably the best, most revolutionary thingis its lack of a traditional cantilever.
We've been taught that a magnetic cartridge generates electricity in proportion to the mechanical movement of its stylus, as it follows the modulations in a record groove. But the real story isn't quite so simple, owing to the length of its cantilever: a lever whose fulcrum is not at the midpoint of its length, but much nearer the generator than the stylus. The obvious consequence is that very large stylus excursions result in very small generator excursions, thus potentially compressing the recorded signal. Which ain't good.
Footnote 1: London Cartridges, c/o Presence Audio, Overdale, Haglands Lane, West Chiltington, Pulborough, West Sussex RH20 2QR, England, UK. Tel: (44) (0)1798-813133. Web: www.presenceaudio.com. US distributor: Pro Audio Ltd., 111 N. South Drive, Tower Lakes, IL 60010. Tel: (847) 526-1660. E-mail: email@example.com.
Footnote 2: If I understand correctly, the great Alan D. Blumleinwho, in 1933, was awarded the first patent in the field of two-channel phonographydescribed a sum-and-difference disc-cutting system as the basis for his "binaural" format. A few years later, Arthur Keller, of Bell Labs, noted the qualitative differences between vertical and lateral cuts, and opted for a purely 45°+45° approach to both mastering and playback. In 1958, when Westrex came out with the first commercially practical stereo-LP technology, they based it on Keller's system.