Chris,
Thanks for the info. Kieth Payea suggested a simple test that I can run =
to evaluate the adequacy of the speaker magnet that I have. If it does =
prove to be too wimpy, I appreciate your suggestions for alternatives.
Larry
----- Original Message -----=20
From: Meredith Lamb=20
To: psn-l=20
Sent: Thursday, February 05, 2004 12:08 AM
Subject: Re: Another STM-8 question
(This is a forwarded email reply from Chris Chapman)
I am starting to collect materials for a vertical seismo based on the =
STM-8 design. I have a rather small (7") speaker that I am considering =
for using for the magnet and feedback coil, but I'm wondering if the =
magnet is strong enough. Can anybody give me some insight into how big =
a magnet is needed or whether I would have to make adjustments =
elsewhere in the design to compensate for a smaller magnet?
Larry Conklin
Liverpool, NY
lconklin@............
Hi Larry,
This 'bit of string' sounds quite a bit too short! But it =
depends on how much mass you are planning to use. Check on the values =
given in Sean's description? The usual speaker systems use external =
Alnico cylinder magnets with a pole piece of 1.0" to 1.5" dia., maybe 4" =
outside diameter. The original speakers were 12" to 15" dia or even =
larger. You can measure the push well enough with one of the small 100 =
gm electronic scales, or by lifting a coin vertically. The cheaper =
speakers use Ferrite magnets, which were much more bulky, have lower =
fields, high stray fields and are much more temperature dependant.=20
One of the problems with speaker magnet systems is that the =
space for the coil only allows a few thou radial clearance. Ideally, you =
would find construction / setup much easier if you could turn out the =
pole gap a bit on a lathe to give maybe 15 thou clear! ance, inside and =
outside. The older speakers were bolted together, which made them easy =
to disassemble. Turned pole faces need a coat of rust resistant priming =
paint / electro-plating. The central column and the backplate of a =
speaker are usually one item. 16 ohm speakers are preferable, if you can =
get them.
Another alternative is to use four rectangular NdFeB magnets, =
maybe 5/16" to 1/2" thick, mounted on two vertical mild steel backing =
plates held apart with soft iron spacers. You then wind yourself a =
rectangular flat coil to fit in the central pole gap. The movement is =
parallel to the pole gap. www.Wondermagnet.com sometimes have magnet =
pairs already mounted onto a mild steel backing plate, Ref 0031. They =
also sell magnet wire. It is easy to visually check the magnet to pole =
face clearance in flat systems.
It is also quite possible to use 1"~1.5" dia NdFeB disk magnets =
as the centre column an! d add a circular end polepiece, a circular =
backing plate and a mild st eel outer tube. It very much depends on what =
you can make / have made. I have used magnet columns about 1" long - two =
1/2" thick magnets. I make the former out of 3.5 thou Al foil with a =
layer of tissue paper insulation, stuck onto an 1/8" Al end disk with =
air holes in it. I first machine a wood former, then dip it in candle =
wax, turn it down to size and mount up the end plate and the foil tube =
with epoxy. You then wind and varnish the coil. You warm the wax with a =
hair dryer and slide off the finished coil.
Regards,
Chris Chapman=20
Chris,
Thanks for the info. Kieth Payea suggested =
a simple=20
test that I can run to evaluate the adequacy of the speaker magnet that =
I=20
have. If it does prove to be too wimpy, I appreciate your =
suggestions for=20
alternatives.
Larry
----- Original Message -----
From:=20
Meredith Lamb
To: psn-l
Sent: Thursday, February 05, =
2004 12:08=20
AM
Subject: Re: Another STM-8 =
question
(This is a forwarded email reply from Chris Chapman)
I am starting to collect materials for a vertical seismo based on =
the=20
STM-8 design. I have a rather small (7") speaker that I am =
considering=20
for using for the magnet and feedback coil, but I'm wondering if =
the=20
magnet is strong enough. Can anybody give me some insight into =
how big a=20
magnet is needed or whether I would have to make adjustments =
elsewhere=20
in the design to compensate for a smaller magnet?
Larry=20
Conklin
Liverpool, NY
lconklin@............
Hi =
Larry,
=20
This 'bit of string' sounds quite a bit too short! But it depends on =
how much=20
mass you are planning to use. Check on the values given in Sean's =
description?=20
The usual speaker systems use external Alnico cylinder magnets with a =
pole=20
piece of 1.0" to 1.5" dia., maybe 4" outside diameter. The =
original=20
speakers were 12" to 15" dia or even larger. You can measure the push =
well=20
enough with one of the small 100 gm electronic scales, or by lifting a =
coin=20
vertically. The cheaper speakers use Ferrite magnets, which were much =
more=20
bulky, have lower fields, high stray fields and are much more =
temperature=20
dependant.
One of the =
problems with=20
speaker magnet systems is that the space for the coil only allows a =
few thou=20
radial clearance. Ideally, you would find construction / setup much =
easier if=20
you could turn out the pole gap a bit on a lathe to give maybe 15 thou =
clear!=20
ance, inside and outside. The older speakers were bolted together, =
which made=20
them easy to disassemble. Turned pole faces need a coat of rust =
resistant=20
priming paint / electro-plating. The central column and the backplate =
of a=20
speaker are usually one item. 16 ohm speakers are preferable, if you =
can get=20
them.
Another alternative is =
to use=20
four rectangular NdFeB magnets, maybe 5/16" to 1/2" thick, mounted on =
two=20
vertical mild steel backing plates held apart with soft iron spacers. =
You then=20
wind yourself a rectangular flat coil to fit in the central pole gap. =
The=20
movement is parallel to the pole gap. www.Wondermagnet.com sometimes =
have=20
magnet pairs already mounted onto a mild steel backing plate, Ref =
0031. They=20
also sell magnet wire. It is easy to visually check the magnet to pole =
face=20
clearance in flat systems.
It =
is also=20
quite possible to use 1"~1.5" dia NdFeB disk magnets as the centre =
column an!=20
d add a circular end polepiece, a circular backing plate and a mild st =
eel=20
outer tube. It very much depends on what you can make / have made. I =
have used=20
magnet columns about 1" long - two 1/2" thick magnets. I make the =
former out=20
of 3.5 thou Al foil with a layer of tissue paper insulation, stuck =
onto an=20
1/8" Al end disk with air holes in it. I first machine a wood former, =
then dip=20
it in candle wax, turn it down to size and mount up the end plate and =
the foil=20
tube with epoxy. You then wind and varnish the coil. You warm the wax =
with a=20
hair dryer and slide off the finished=20
coil.
=20
Regards,
Chris Chapman=20