PSN-L Email List Message

Subject: Re: Magnets
From: ChrisAtUpw@.......
Date: Tue, 29 May 2007 22:49:56 EDT


In a message dated 2007/05/30, rpratt@............. writes:

> I have found the combination of teenage boys and subwoofers has created an 
> abundant source of strong magnets for your type applicatiion.  We just had a 
> junk day in town and speakers were plentiful.  If no junk day or neighbor 
> boys then stop by an automotive sound shop and ask for some blown speakers.  The 
> ring magnets are generally much larger ID than the visible gap so the gap 
> can easily be cut out on a lathe or with a dremel tool and the post turned to 
> fit your coil if need be.  

Hi Randy,
       
       The problem that I have found with this approach is that most speakers 
these days seem to use ferrite magnets and they are glued with epoxy to both 
pole pieces. If you can get old speakers with cylindrical Alnico magnets, 
fine. Ferrite is rated for use up to about 250c and it looses it's magnetism at 
450 C. You need cook them to 150-180C, which is the temperature at which epoxies 
loose strength and you can lever them apart.
       If you have a lathe you may be able to turn out the flat top elect
rode, but cleaning out the turnings out can be a problem, if you try to do it with 
the mangets in place


> I have been having trouble recording any surface waves lately.  I used the 
> instrument several years ago and had no problem but now I do not seem to get 
> low frequency.  

       I suggest that you measure the capacity of any old electrolytic 
coupliing capacitors and maybe replace them. PSN amplifiers use polyester coupling, 
which should be reliable. Non polar electrolytic types are not expensive. 

       At what period is your sensor set? If you have a Lehman, but it is 
only set up for say 12 secs, it won't bring in 20 second waves very well. They 
resetting to 25 secs?

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/05/30, rpratt@............. writes:

I have found the combination of= teenage boys and subwoofers has created an abundant source of strong magnet= s for your type applicatiion.  We just had a junk day in town and speak= ers were plentiful.  If no junk day or neighbor boys then stop by an au= tomotive sound shop and ask for some blown speakers.  The ring magnets=20= are generally much larger ID than the visible gap so the gap can easily be c= ut out on a lathe or with a dremel tool and the post turned to fit your coil= if need be. 


Hi Randy,
      
       The problem that I have found with this= approach is that most speakers these days seem to use ferrite magnets and t= hey are glued with epoxy to both pole pieces. If you can get old speakers wi= th cylindrical Alnico magnets, fine. Ferrite is rated for use up to about 25= 0c and it looses it's magnetism at 450 C. You need cook them to 150-180C, wh= ich is the temperature at which epoxies loose strength and you can lever the= m apart.
       If you have a lathe you may be able to=20= turn out the flat top electrode, but cleaning out the turnings out can be a=20= problem, if you try to do it with the mangets in place


I have been having trouble reco= rding any surface waves lately.  I used the instrument several years ag= o and had no problem but now I do not seem to get low frequency. 

       I suggest that you measure the capacity= of any old electrolytic coupliing capacitors and maybe replace them. PSN am= plifiers use polyester coupling, which should be reliable. Non polar electro= lytic types are not expensive.

       At what period is your sensor set? If y= ou have a Lehman, but it is only set up for say 12 secs, it won't bring in 2= 0 second waves very well. They resetting to 25 secs?

       Regards,

       Chris Chapman

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