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For those brave souls who crave the quad experience, I have compiled a few factoids for 4-channel vinyl reproduction which may provide useful pointers.

It's not strictly necessary to have a pick-up cartridge that was specifically designed for  quadraphonic reproduction, but bear the following in mind before lowering your diamond tip on that precious Quadraphonic or Quadradisc:

CD-4 cartridges are so called  because they have a cantilever and stylus that can track  ultrasonic frequencies. They also usually have coils with lower inductances than usual, to reduce ultrasonic frequency attenuation. Low capacitance (100pF or less) cables between the deck and demodulator can help here too. Frequencies up to 45 Khz for CD-4 reproduction are involved, 10KHz lower for SQ. Within the cartridge, excellent channel separation at these frequencies must be maintained to enable the decoding process to work correctly.

“Contact-line” or “Shibata” tips provide for optimum contact with the record groove. A conventional eliptical or spherical stylus may seem  to work initially, but may quickly damage  the high frequencies within the record grooves.

Suitable cartridges are available from manufacturers such as Audio Technica, Grado, Dynavector but there are other compatible cartridges on the second-hand market from manufacturers like Empire, Pickering and one from Shure specifically for four-channel, the M24H.

Ed Saunders, best known for manufacturing Shure stylus replacements has a cartridge specifically designed for CD-4 reproduction. Although I have no personal experience of this, by all accounts it's very good.

Other cartridge contenders which have yielded success and are worth investigating:

           Ortofon 2m Black, Grado Prestige Blue, Grado Gold.

Audio Technica AT440MLa , AT12Sa, AT20ss and AT15ss, AT15sa and AT20sla

If using a MC cartridge, make sure the phono amp is capable of at least a frequency response up to 50Khz for CD-4, a little less for SQ (35KHz).  

Cartridge tracking adjustments must be undertaken carefully, especially anti-skating.

Early CD-4 records were possibly inferior, wearing out more quickly than their successors.

The term "demodulator" refers to CD-4 devices which turn 2 physical channel information into 4 channels of information. A matrix device (for SQ, QS, EV, etc.) is not referred to as a demodulator, but rather a "decoder". Technically, a CD-4 device could also be called a decoder because it does "decode" the 2 physical channel information but, during the quad heydays, the two terms were differentiated so one knew to what was being referred.

A generally useful site manned by Quadraphonic enthusiasts:

Other articles 
Grace 707 Quad-Master tone arm box

A 4-channel compatible cartridge will probably have a high compliance cantilever. This will benefit from a low mass pick-up arm, so consequently some  arms may not be suitable.

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