Ask an expert: Arm Effective Mass and Resonant Frequency

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Gentlemen. I have recently upgraded my Goldring 1042 moving magnet (MM) cartridge to a MC and was originally considering Denon DL-103R or Audio-Technica AT-OC9ML/II.

After deeper investigation into Arm Effective Mass and Resonant Frequency due reports that the Denon only being suitable for med to high mass arms I found that my Roksan Tabriz (standard model) at 11g mass would not be suitable. (cartridge/arm matching is very well explained > http://www.theanalogdept.com/cartridge___arm_matching.htm

My system consists of Meridian 502/557 pre/power amp, 507 24bit CD player, Sony ST-SDB900 Tuner, Manticore Mantra with Origin Live standard motor upgrade/Tabriz/ AT33PTG via Cambridge 640P phono stage and Wilson Benesch Orator speakers with Chord Silver Siren/QED Silver Anniversary cables and own made screen power cables and junction box. Part of the reason for the upgrade was due to a recent upgrade to the 502 with a MC board on board which replaced my existing 551 which had a MM board. I was going to swap the boards over but found out that they are a different fitting, the 502 MC being the same as fitted to newer G0 series.

Getting back to the cartridge, after some research and due to price constraints I actually bought an Audio-Technica AT33PTG which from reviews was quite tolerant to mass/resonance and was better or at least as good as the AT-OC9ML/II. I was originally going to purchase the OC9ML. However I managed to buy the AT33PTG for £240 all in from a highly rated eBay seller in Japan.

It sounds superb and after being burned in sounds even better with great depth and separation, however I have now installed a spacer under the cartridge seat which adds weight (5g) as I found that there was a slight bit of distortion/resonance which I presumed was due to lack of arm mass which has now been eradicated so maybe the ATs aren't quite so tolerant as first thought. This increases the arm mass to 16 grams and still leaves plenty of movement on the counter balance weight.

As a matter of interest, in your opinion what would be the next upgrade step? If you say speaker cable, please note that they have to turn a few corners and measure 9 metres each and are bi-wire.

Hmm; the notion that arm / cartridge matching is just about where low frequency resonance falls is overly simple and very outdated. Your Roksan Tabriz, at 11 gms effective mass is actually just below the 12 gms benchmark 'normal'' value nowadays and would suit an AT OC9 MLII well enough. Moving coil cartridges have lower compliance, higher tracking force and greater ability to tolerate massy arms, although nowadays even a large 12 incher like SME's 312S comes in at 14 gms.

Historically, cartridge manufacturers stopped pursuing high compliance in the cantilever hinge a long time ago, allowing arm effective mass to rise without arm/cartridge resonance sinking to dangerously low values (below 8Hz). This means the pairing ride over warps and do not try and trace them as a signal, something that would introduce excessive cantilever movement.

Some magazines measure arm/cartridge resonance and these days it commonly hovers around 10Hz, which is acceptable. To be specific, a compliant Ortofon 2M Black cartridge in a Rega RB301 arm resonate at exactly 10Hz (vertical modulation). With a Ortofon Cadenza Black moving coil this figure rises to 12.5Hz. Since the Rega has an effective mass of 12gms the Roksan would have given a value much like the Cadenza, with an AT OC9MLII.

Arms have a characteristic sound determined by many other factors, including arm wiring, vibration behaviour of the structure, geometry and so on. Silver wired arms can sound a bit zingy bright, whilst acrylic arms often sound well damped and neutral, and long arms just very smooth and easy going. I am generalising here of course, to make a point. Not everyone likes the Rega arms, mainly for a midband less smooth than is possible elsewhere, but I love their lower midband separation and dynamism. So you can be a bit more adventurous with your Roksan Tabriz, which is a decent arm

Thank you

Neil McCauley / editor in chief

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