Hi Larry
I've made about 6-7 magnet/coil assemblies for the verticals I have and =
I also had problems with speaker magnets(12" radio shack woofers) and =
their clearances. I used Sean Thomas' coil construction description with =
magnets purchased from Mcmaster. They work well with the coil =
construction description STM describes. I used a homemade balance beam =
with razor blade fulcrum to determine the coil factor (G).
Regards
Barry
----- Original Message -----=20
From: Larry Conklin=20
To: psn-l@.................
Sent: Thursday, February 05, 2004 2:18 PM
Subject: Re: Another STM-8 question
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
Hi Larry
I've made about 6-7 magnet/coil assemblies for =
the=20
verticals I have and I also had problems with speaker magnets(12" =
radio=20
shack woofers) and their clearances. I used Sean Thomas' coil =
construction=20
description with magnets purchased from Mcmaster. They work well with =
the coil=20
construction description STM describes. I used a homemade balance =
beam with=20
razor blade fulcrum to determine the coil factor (G).
Regards
Barry
----- Original Message -----
From:=20
Larry=20
Conklin
Sent: Thursday, February 05, =
2004 2:18=20
PM
Subject: Re: Another STM-8 =
question
Chris,
Thanks for the info. Kieth Payea =
suggested a=20
simple test that I can run to evaluate the adequacy of the speaker =
magnet that=20
I have. If it does prove to be too wimpy, I appreciate your =
suggestions=20
for 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=20
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=20
a magnet is needed or whether I would have to make adjustments =
elsewhere 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=20
much mass you are planning to use. Check on the values given in =
Sean's=20
description? The usual speaker systems use external Alnico cylinder =
magnets=20
with a pole piece of 1.0" to 1.5" dia., maybe 4" outside =
diameter. The=20
original speakers were 12" to 15" dia or even larger. You can =
measure the=20
push well enough with one of the small 100 gm electronic scales, or =
by=20
lifting a coin vertically. The cheaper speakers use Ferrite magnets, =
which=20
were much more bulky, have lower fields, high stray fields and are =
much more=20
temperature dependant.
One =
of the=20
problems with speaker magnet systems is that the space for the coil =
only=20
allows a few thou radial clearance. Ideally, you would find =
construction /=20
setup much easier if you could turn out the pole gap a bit on a =
lathe to=20
give maybe 15 thou clear! ance, inside and outside. The older =
speakers were=20
bolted together, which made them easy to disassemble. Turned pole =
faces need=20
a coat of rust resistant priming paint / electro-plating. The =
central column=20
and the backplate of a speaker are usually one item. 16 ohm speakers =
are=20
preferable, if you can get =
them.
=20
Another alternative is to use four rectangular NdFeB magnets, maybe =
5/16" to=20
1/2" thick, mounted on two vertical mild steel backing plates held =
apart=20
with soft iron spacers. You then wind yourself a rectangular flat =
coil to=20
fit in the central pole gap. The movement is parallel to the pole =
gap.=20
www.Wondermagnet.com sometimes have magnet pairs already mounted =
onto a mild=20
steel backing plate, Ref 0031. They also sell magnet wire. It is =
easy to=20
visually check the magnet to pole face clearance in flat=20
systems.
It is also quite =
possible=20
to use 1"~1.5" dia NdFeB disk magnets as the centre column an! d add =
a=20
circular end polepiece, a circular backing plate and a mild st eel =
outer=20
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=20
dip it in candle wax, turn it down to size and mount up the end =
plate and=20
the foil tube with epoxy. You then wind and varnish the coil. You =
warm the=20
wax with a hair dryer and slide off the finished=20
coil.
=20
Regards,
Chris Chapman=20