Slow, sluggish cloudy sound.....I usually do it after 20 to 50 hours. BTW MC are the only cartridge you can demagnetize. You will permanently damage MM or MI cartridge if you use a demagnetizer on them.
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Using the demag doesn't demag the magnet, but does demag the pole pieces and other stuff around the magnet that does become magnetized over time. Never demag a moving magnet cartridge, only moving coils, and even with that, some manufacturers recommend avoiding this process. Benz, and many others do recommend demag.
I have done demags on well over 100 cartridges without any problems. It has always worked. I did once accidentally demag a MM cartridge and it completely ruined it, so be careful.
The tell-tale signs that it needs to be done is a loss of high frequencies, soft, flat overall sound with a lack of dynamics. No sparkle. Demaging will usually bring all of this back.
Safest way to degauss cartridges (including MM cartridges) is passively as suggested by Terry9. I happen to use the Cardas/Stan Ricker test record, but any good (meaning as high a level as possible from a vinyl groove!) frequency sweep test track will do.
The only "drawback" is you should run it through the frequency sweep track two or three times for a 'thorough' job ;~) But you cannot do ANY damage; not even to MM or MI cartridges, and you will get benefits with both types of cartridges which you will never get by running an active degaussing signal through the cartridge's coils -- for the following reasons:
1. MM cartridges actually CAN benefit from degaussing. They have coils with hundreds of windings of insulated copper wire. The problem is if you run an active degaussing current through those big coils, you will create a strong enough electromagnetic field to most likely completely demagnetize the tiny/weak little moving magnet! However, running a GROOVE-GENERATED frequency sweep signal through those big coils (via the little moving magnet or moving iron element) will be sufficient to "burn in" the coil windings -- actually you're burning-in the interface between the copper coil wire and its insulation.
2. Additionally, the frequency sweep signal will help "re-form" any capacitors in the phonoamp's input circuit -- I'm thinking primarily about the RIAA equalization circuit.
These are both "maintenence benefits" you WILL hear; and which you will never realize if you (rightly!) avoid degaussing your MM or MI cartridge with an active demagnetizer!
Using a frequency sweep with MC cartridges gets you the same two additional benefits listed above (plus there's no chance of damaging those delicate coils which just seem to get finer and more fragile every year!) There is very little ferrous material in most modern day MC cartridges, so not much (if anything) to degauss anyway! And of course, you are doing nothing to 'clean up' the input circuitry of your phonostage.
Enjoying vinyl is all about taking the time necessary to get the most out of the medium. Playing a frequency sweep track (even half a dozen times, every six months!) will take all of 15 minutes. So what's the big deal?
In my beginning of my audiophile career, you know, young, no money but enthusiastic and hungry for every tweak which was able to lift my system to a higher lever without investment in serious electronics...to make a long story short...I grilled the coils from my Lyra, my Dealer smiled and sold me another one. Super.
In all the following years I never used the Demag grill again and guess what...I never even thought about it until I read this thread. I can live very well without that but others may think different. My first question to a cartridge seller always was "Did you demag it with a battery unit" and when he said "yes", I stopped any further discussion at once. One of my golden rules ...
Shorting the MC outputs to GND as recommended by J.Carr (as demonstrated on YouTube) would yield the strongest current i.e. maximising magnetic flux, which is proportional to current.
This would produce the strongest and most "coercive" effect while keeping current at a sensible level which the coils can withstand.
That current would also be "normalised" (no DC offsets caused by connecting biased circuitry).
I know some folk are sceptical about the methodology but it seems reasonable that if the MC can magnetise the coil cores to a slight degree then it is just as capable of reversing the effect. How long and how often to achieve that result is the only remaining question. Since we are working ostensibly within the limits of the cart's operation, there seems little reason for A.J VdH to be concerned about damaging long-term effects(?)
Combining that method with e.g. the Cardas demag signal might be the ultimate "safe" deflux? It also might prove to be a fractionally quicker process than just using the shorted MC method with a normal LP?
I've not tried either method BTW but plan to test at least one of them.
I should mention for the benefit of anyone who's just tuned in to the demag argument that Lyra carts such as the Delos use a paramagnetic grade of iron for the coil formers, i.e. will "conduct" magnetism but resist storing it, so residual magnetism should be minimal anyway?
Exactly how much effect any small degree of permanent magnetism will have on Lyra carts and certain other makes of MC is a matter of debate.
I'm sure JC will correct any misinterpretations that I'm guilty of.