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-   -   Demagnetize MM cartridge body??? (http://www.diyaudio.com/forums/analogue-source/85313-demagnetize-mm-cartridge-body.html)

fastcat95 24th August 2006 11:22 PM

Demagnitize MM cartridge Body???
 
Hello to All!

I have a Shure V15 IV cartridge. Would it be of value to
demagnitize it (body only!!!) with a tape head demagnitizer,
or perhaps a 1 KHz demagnitizing tone fed into each channel
at no more that 5 mV per channel? Does this work to benefit,
or are there problems or cautions?

Many Thanks!

phn 24th August 2006 11:28 PM

NEVER demagnetize a M.M. cartridge with the replaceable stylus still inserted. It will ruin the mini-magnet at the end of the cantilever in its magnetic power. With the stylus insert removed you can consider it, but first read the rest of this answer:

From a more theoretical point of view, I think it is not wise to demagnetize. It is a short term effect with more negative side effects on the long-term. Your first experience will be very positive as also was mine. But after 2 record sides you have to do this again and it never stops. So you have become a serious victim. And keep in mind: the original cartridge magnet already puts the tiny armature in such a strong magnetic field that its magnetization is unavoidable, so why should you demagnetize? Also, since the magnet never is perfectly at center position but always has some offset, the mu-metal made centre of the coil system always holds residual magnetism and cant become demagnetized. And if this is not enough: The current through the 20 micron thin coil wire is so small that there is no chance to build up any serious magnetic field to cancel the existing residual magnetism. What it only does is breaking down the magnetic domain boundaries in the mu-metal. This improves the magnetic resolution of the music. But after a short while these boundaries will be there again and the number of atoms per magnetic domain has only got bigger. So the magnetic resolution is worsening instead of getting better. So from the point of view: I bought the thing and now I will use it, you have to use it. The only advantage is that some crystal boundaries inside the coil wires are welded. But as soon as you move the armature by playing your records that effect is over again. So save the money and buy some good records.

http://www.vandenhul.com/artpap/phono_faq.htm

bulgin 25th August 2006 09:28 AM

Demagnetise MM Cart. Body???
 
I don't believe in this voodoo!

bulgin

EC8010 25th August 2006 09:49 AM

I have a hypothesis that cartridge demagnetisers don't work as they say at all. Firstly, to demagnetise something you have to be able to take it to saturation in both directions. That requires a powerful demagnetising force. I have a demagnetising coil for dealing with colour CRTs. It's a 10" diameter coil with about 800 turns of 0.67mm wire that I connect (briefly) across the 240V mains. It draws 3A, so it has a demagnetising force of 2400 Ampere turns. That's the sort of brute force required for demagnetisation, not an itsy-bitsy little signal that can be applied to the delicate wires of a cartridge without melting them.

However... A cartridge is a transducer, so if you apply a signal to the coils, the cantilever will move. And that means it will flex the suspension and because the suspension is lossy, the losses will cause heating. We all know that cartridges sound better when warm, so my hypothesis is that if cartridge demagnetisers work at all, it's by warming the suspension.

sreten 25th August 2006 10:34 AM

Quote:

Originally posted by EC8010
Firstly, to demagnetise something you have to be able to take it to saturation in both directions.


Hi,

Not quite true. The field has to be high enough to reverse the
current level of magnetism - saturation is not an issue unless
you want to be able to remove any level of magnetism.

AFAIK many MMs (including Shure I think) are induced
magnet designs, the body contains magnets and a high
power demagnetiser will stop them working rather promptly.

Applying low level signals to an MM won't do anything.

:)/sreten.

EC8010 25th August 2006 10:39 AM

Quote:

Originally posted by sreten
Not quite true. The field has to be high enough to reverse the current level of magnetism - saturation is not an issue unless you want to be able to remove any level of magnetism.
Good point. Thanks for that. Having just worked out how much force my coil applies, I'm about to work out how much could be applied by a moving magnet cartridge.

EC8010 25th August 2006 10:56 AM

Quote:

Originally posted by sreten
Applying low level signals to an MM won't do anything.
The body of a typical Shure was about 15mm long and about 8mm wide. Now, you need some room at one end to connect to the cartridge pins and room at both ends for the cores to circle round and complete the magnetic circuit. A back of the envelope (literally) sketch suggests that each channel has a pair of coils (one on each leg of the core) about 8mm long and 3mm in diameter. The core and former will take up part of that diameter, so I estimate that the depth of each coil from outside to the core to be about 0.8mm. If we assume 0.1mm wire, then we can have 8 layers of 80 turns, giving 640 turns. For each channel, there are two coils in series, so that makes 1280 turns.

Compared to 0.67mm wire, 0.1mm wire has 1/4500 of the cross-sectional area, so we can only safely pass 1/4500 of the current, that's 0.67mA. (Actually, it's probably rather less because the thickness of the enamel is probably disproportionately thicker on very fine wire.) Anyway, together with our 1280 turns, that implies that we can apply 0.9 Ampere turns of demagnetising force compared to the 2400 Ampere turns achieved by my CRT degaussing coil.

The previous suppositions were based on a moving magnet design with weak magnets (and therefore large coils). But the Ortofon OM series of cartridges is rather newer and has much smaller coils with much lower inductance, implying fewer turns and a more powerful magnet and/or more efficient magnetic circuit (they're induced magnet). Not only would such coils have reduced degaussing ability, but you wouldn't want them to be able to degauss the magnet within them anyway.

Consider your comment proven.

EC8010 25th August 2006 11:20 AM

Finally, to directly answer the original poster's question...

I don't know about the V15/4, but previous Shures typically had an inductance of about 600mH. Let's assume that Shure have improved things for the mk4 version, and got it down to 100mH. At 1kHz, 100mH has a reactance of 628 Ohms. Applying your suggested 5mV at 1kHz would result in a current of 8uA. I think we can safely say it's not going to do anything.

Just for fun, we can also work out how much heat would be dissipated in the coils if we applied my projected 0.67mA of current. Assuming a DC resistance of 1000 Ohms (correct me if I'm wildly out, sreten), that works out at 45mW. I suspect that in such a small case, that might actually noticeably heat the suspension.

sreten 25th August 2006 02:06 PM

Hi,

Ball park figures for a MM are 500R and 500mH.

Applying 5mV is the same as the cartridge producing 5mV, ~ 10uA.

0.5mA on the other hand needs 0.25V = 0.125mW = 1/8000 W.

(correct my maths if I'm wrong ......)

A small lamp is the option for heating up the cartridge.

:)/sreten.

EC8010 25th August 2006 02:16 PM

Your maths is correct, but I was considering the rather beefier 0.67mA that's just under what might damage the cartridge. As you say, a small lamp would be a safer option.


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