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Sometimes, Things Aren't How They Appear....

Michael Fremer  |  Mar 16, 2020

After watching the video showing the Revolv VTF gauge react to the Anna D stylus, SAT's Marc Gomez sent me this analysis:

"Looking at your video of the scale reacting to the Anna I feel compelled to comment. I believe the metal platform is aluminum and non-magnetic. Nevertheless, as the magnet of the Anna is very strong, the movement (velocity) of the magnet getting closer or away from the aluminum platform creates electromagnetic induction in the platform - pushing it down when getting closer and pulling it up when moving away (negative reading in scale). It only happens when you have relative velocity between the magnet and the conductive part (Lenz law). It is not due to the magnetic attraction. It will not affect the correct reading of static VTF once the stylus rests on the platform."

He added: "The reading of 2,6-2,7g in the Revolv scale compared to the 2,4g in your scale is due to the higher position of the platform on the Revolv, which makes the arm exert a bit more VTF as stated on the instructions."

I think what he posits makes sense. More than do I sometimes......

COMMENTS
miguelito's picture
Submitted by miguelito on Tue, 2020-03-17 05:47

What he says about the currents produced by a VARYING magnetic field is correct. However, once the stylus is resting on the aluminum plate, those currents have immediately dissipated and there is nothing to be concerned with. Even in motion, I have trouble seeing an impact of any meaningful magnitude. As for the VTF vs arm angle, for sure, but this is something you have mentioned as well with any arm where the VTF is done by balancing rather than dynamic VTF.

miguelito's picture
Submitted by miguelito on Tue, 2020-03-17 05:47

What he says about the currents produced by a VARYING magnetic field is correct. However, once the stylus is resting on the aluminum plate, those currents have immediately dissipated and there is nothing to be concerned with. Even in motion, I have trouble seeing an impact of any meaningful magnitude. As for the VTF vs arm angle, for sure, but this is something you have mentioned as well with any arm where the VTF is done by balancing rather than dynamic VTF.

Hummer's picture
Submitted by Hummer on Tue, 2020-03-17 05:50

I have one of these digital balances but although it looks exactly the same it has no brand name on it.
I use it sat on the turntable to set up VTF and have noticed it does weigh slightly different from my other digital balance but the platforms are at different heights so I presume this is the main reason for different results?

Does this also mean because the weighing platform is slightly raised when the stylus actually sits on the LP the actual weight could be different again?

13DoW's picture
Submitted by 13DoW on Tue, 2020-03-17 11:21

... Lenz's law tells us that any induced emf will be such as to oppose the varying magnetic field that created it. Any current induced in the aluminum plate by the cartridge moving towards it will create its own field that will repel the cartridge. Hence, the VTF reading should get smaller, but settle once movement stops.

13DoW's picture
Submitted by 13DoW on Tue, 2020-03-17 18:55

Lenz also comes into play with current input phono amps. Such amps present, ideally, a short circuit across the coils of a MC cartridge. In doing so the current the flows in the coils due the induced emf will create a field to oppose the field that created it, i.e. the stylus movement. Taking things to the extreme if the coils had no resistance at all and the input resistance of the phono amp was zero then the induced current would exactly oppose the stylus movement and nothing would happen! Practically the resistive losses are enough so the back emf is smaller than the applied field from the stylus movement. Sadly, I am not smart enough to give some idea of how low the resistance must be for this effect to be noticeable - some cartridges are in the low-ohm range and they work fine into current inputs.

Regards,
13DoW

warpig's picture
Submitted by warpig on Wed, 2020-03-25 13:49

If I were to unplug the cables rca or in my case balanced would there be no magnetic force on the cart. Is the cartridge acting like an electromagnet.

13DoW's picture
Submitted by 13DoW on Wed, 2020-03-25 18:14

messed up my reply - it should have gone here but I ended up posting it at the bottom

OldschoolE's picture
Submitted by OldschoolE on Tue, 2020-03-17 12:33

I have a very similar looking scale (well, actually, exact same one) without the branding I picked up on Amazon years ago for less than $20. MC/MM doesn't matter, I have had no strange magnetic issues with it. I have used it on different tables and arms as well (mine are all vintage and stock). I take the mat off the platter, secure the platter and then set the scale on the platter because it is designed to be the thickness of an average record. If it is off at all (which I doubt)it wold not be enough to make any difference.

13DoW's picture
Submitted by 13DoW on Wed, 2020-03-25 18:11

An MC cartridge contains a fixed magnet and moveable coils. The coil is waggled by the cantilever and when a coil of wire is moved within a fixed magnetic field a voltage is induced across the coil outputs. A MM cartridge has fixed coils and a moveable magnet, that is waggled by the cantilever. When a fixed coil is in a changing magnetic field a voltage is induced in the coil.

In this case, as the arm is lowered towards the stylus force gauge the cantilever is not moving so no voltage is induced in the coils. But, Gomez (arm designer, not the band) postulated that the aluminum plate of the stylus gauge behaves like a single turn of wire shorted back on itself and it feels a changing magnetic field as the cartridge descends towards it. If it did act as a single turn coil then the changing magnetic field induces a current to circulate in the aluminum plate (an open circuit coil would see a voltage across it but a shorted coil sees a current flow through it).

As already noted, when a current is induced in a coil that current creates its own magnetic field. Gomez suggested that the induced magnetic field would attract the cartridge more and make the tracking force to appear higher until it has stopped moving. But, as I also noted, the induced magnetic field has a polarity that always opposes the changing field that created it. So, if this is really having an appreciable effect the induced field should slightly repel the approaching cartridge.

A simple thought experiment might help - if the induced field did attract the cartridge then it would accelerate toward the aluminum plate slightly. As it accelerates the magnetic field from the cartridge magnet changes more, inducing a larger current in the aluminum plate that then accelerates the cartridge more creating a bigger field that induces more current etc. and we've created energy from nothing. Sadly not.

warpig's picture
Submitted by warpig on Wed, 2020-04-29 22:24

Thanks for the info.
I have a Benz Micro Ruby 3 that I thought I was going to be smart and try something new. I have a few digital scales that will weigh 2 grams easily. I took my platter off got the height correct for where a record would be and lowered the tonearm down to the scale. I saw the numbers going negative then all of a sudden the scale jumped up into the cartridge. I freaked out to say the lease and it was stuck pretty good. I pulled it off straight down and luckily there was no damage to the cart. After that for a while I used a Shure tool. I did finally purchase one like above but still check it with the Shure.

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