Optical Cartridge Technology Explained
Benefits of Optical cartridge design;
Many audiophiles will remember Optical cartridges from the 1970’s . The performance of these was hailed as a radical step forwards at the time, with reviewer comments, ‘Pure, like mountain dew, yet strong like a waterfall’. Unfortunately optic technology in the 1970s had many flaws, issues with heat and signal-to-noise ratio meant that with the dawn of Compact Disc and the digital era, these products disappeared from the market – although still revered in audiophile circles. Now, 40 years later, the massive improvements in optic technology have allowed DS Audio to completely redesigned the Optical cartridge for the 21st century.
Whereas MM & MC cartridges rely on magnets and coils to generate the electrical signal, the Optical cartridge uses a beam of light to detect cantilever movement, making the cartridge much lighter, and more agile. This also removes the magnetic field inside the cartridge caused by the coils & magnets in conventional cartridge designs, and it’s subsequent influence on cantilever movement.
Faraday’s law of induction governs the design and operation of both MM and MC cartridges. As a result, both designs are subject to Lenz’s law. In audio terms, Lenz’s law states that the movement of the coil and/or magnet will produce a frictional force that affects the movement of the cantilever and hence the stylus, such that exact stylus vibration can never be reproduced by an MM or MC cartridge system. DS Audio’s optical system, on the other hand, has absolutely no effect on the vibration of the stylus/cantilever system.
Another benefit of the Optical cartridge design is the output from the cartridge itself. Whereas conventional MM & MC designs rely on ‘velocity-proportional method’, in which, the output signal depends on how fast a tiny magnet (or a tiny coil in MC) moves in the magnetic field. At lower frequencies, the magnet moves slowly in the magnetic field, so the resulted output signal level is very small. Whereas at high frequencies, the magnet moves faster, so that the output signal becomes un-proportionally large. This means MM or MC cartridges require equalisation between high and low frequencies.
In contrast the Optical cartridge system uses the ‘amplitude-proportional method’ in which, the output signal depends on how much distance the stylus moves, without the huge disparity between high & low output frequencies seen in MM and MC cartridge design. It results in no equalisation being necessary in the output signal except for RIAA curve correction. This is especially important when it comes to low frequency reproduction, as the Optical cartridge design can theoretically detect signals as low as 1Hz, as it does not rely on the speed of cartridge movement which dictates MM & MC cartridge output declines 6db/octave.
※DS Audio CEO Tetsuaki Aoyagi explained the technology of optical cartridge at Munich High End show2017 Tech Stage.
Please see below video.