Koetsu Coralstone Cartridge Lab Report

Lab Report

This longstanding member of Koetsu's 'Platinum' series employs the platinum/cobalt-doped magnet first rolled out in the Urushi MC [HFN Apr '90 and Oct '20] and, in terms of Koetsu MCs reviewed in HFN over the past decade, is most closely allied to the Onyx Platinum [HFN Nov '17]. Both cartridges share the same magnet, silver-plated copper wiring, boron cantilever and line contact stylus, and both have a specified 0.3mV output although, in practice, there are very real differences between the two models. The Onyx, for example, offers a higher 530µV versus 387µV from the Coralstone (re. 1kHz/5cm/sec into 100ohm) and while both models have the same (specified) 1.8-2.0g downforce and (measured) 12-13cu dynamic compliance and high-ish 25-26° VTA, it's the 'older' Coralstone that offers the better tracking performance. Indeed, the Coralstone also bests the Urushi Sky Blue [HFN Jul '13], Black [HFN Sep '06], Red K [HFN Oct '09] and Urushi Vermillion [HFN Nov '19], which all struggle to mount the +15dB test tracks (re. 0dB at 315Hz/5cm/sec), by holding to <0.4% on both channels at +15dB and only then letting go at +18dB with a 2.0g downforce.

The Coralstone also has a 'brighter' presence and mid treble than most other Koetsus despite offering a similar 4-5dB roll-off at 20kHz. Here its ~0.5dB lift at 10kHz [see Graph 1] is traded for a very minor loss in symmetry that delivers more HF output with lateral cuts [black trace, Graph 1] than vertical cuts [red trace]. The pattern of distortion vs. frequency is typical of the breed, however, reaching 7-8%/10kHz (lateral and vertical) – almost entirely 2nd harmonic – while settling out at 0.5-10% with stereo cuts (–8dB re. 5cm/sec, 20Hz-20kHz). PM


Freq. resp. curves (–8dB re. 5cm/sec) lateral (L+R, black) vs. vertical (L–R, red) vs. stereo (dashed)


Lateral (L+R, black), vertical (L–R, red), stereo (dashed) tracing and generator distortion (2nd-4th harms) vs. freq. from 20Hz-20kHz (–8dB re. 5cm/sec)

Generator type/weight Moving-Coil / 12.5g
Recommended tracking force 18-20mN (19mN)
Sensitivity/balance (re. 5cm/sec) 387μV / 0.4dB
Compliance (vertical/lateral) 12cu / 13cu
Vertical tracking angle 26 degrees
L/R Tracking ability 75μm / 75μm
L/R Distortion (–8dB, 20Hz-20kHz) 0.9-12% / 0.3-11%
L/R Frequency resp. (20Hz-20kHz) +0.0 to –3.5dB / +0.0 to –4.8dB
Stereo separation (1kHz / 20kHz) 34dB / 18dB
Price £8498

Tokyo, Japan
Supplied by: Absolute Sounds Ltd
0208 971 3909