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Is the at 150mlx a bad match on the jmw9 tonearm

I am asking because I just installed the at150mlx on my vpi scout 1.1 and am getting woofer pumping. I had a ortofon 2m blue before this and had no pumping. The table is on a flexy style stand with 4 butcher block shelves and a laminated top shelf of 1/2" acrylic ,1/2" aluminum, 1/2" acrylic. The table sits on the top shelf on top of a sand box. The stand is spiked into carpet on a concrete slab. It is located about 12' away from the right speaker behind the listening position. I am not sure what other information would be helpful but it seems to happen at normal listening levels( slightly louder than conversation).If it is a bad match I would like to figure it out ASAP and return it for something else. Thank you for any help.
wilson667

Showing 3 responses by johnnyb53

johnnyb53
3,510 posts
 
Effective mass of the JWM 9 is 7.7g. The AT150MLX weighs 8.3g and the mounting hardware add 0.5g for a total effective mass of 16.5g. AT150MLX dynamic compliance is 10. If you run those figures through the resonance frequency calculator, you get a resonance frequency of 12.29 Hz, which is at the upper end of the 8-12 Hz recommended frequency range. If you can see the woofers pumping, that may be a damping issue of some sort, the kind moderated by a fluid-damped tonearm.

You can also lower the resonance frequency into the 10's by adding headshell weights to bring effective mass up to around 20-25 Hz.
johnnyb53
3,510 posts
 

09-19-15: Wilson667
Johnnyb53 I am not sure that I want to add a headshell weight. It sounds like you are saying adding a weight of about 13 to 18 grams.that seem like an awful lot of weight to add.
No. The effective mass of the tonearm alone is 7.7g. The mass of the cartridge plus hardware adds 8.8g for a total effective mass of 16.5g. This is an unmodified effective mass, as VPI would certainly know that the cartridge is going to weigh something in that range.

So I'm talking about adding a headshell weight of 3 to 5g. VPI even makes a 3g headshell weight for JMW tonearms, so it must be OK with them too.
johnnyb53
3,510 posts
 
I knew somebody used a different frequency for its compliance calculation,
but forgot that it's AT. For that you might ask a customer service rep at
LpGear how the 100Hz spec translates at 10 Hz. It could be that your
compliance is spot on and something else is amiss.

When you think about it, a high amplitude cart/arm resonance in the 12 Hz
range would cause the woofer to flutter rather than pump, as the woofer
excursions would be 1/12th of a second apart. Visible woofer pumping
would occur at a few Hz (e.g., 5) or less, more like 1 or 2 Hz.

So maybe this is related to 33-1/3 rpm, which, if eccentric, would be about
1.8 Hz, ideal for visible pumping.

So what could cause eccentric tracking? I could guess a few:

o Antiskate out of adjustment
o Record with off-center hole
o Cartridge azimuth out of alignment
o Tracking weight too high
o Belt stretched and needs replacing
o Platter bearing needs lubrication
o Turntable not absolutely level
o Maybe even VTA or SRA setting

Something along those lines.
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