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=20Hi LarryI'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).RegardsBarry----- Original Message -----From:=20 Larry=20 ConklinSent: Thursday, February 05, = 2004 2:18=20 PMSubject: Re: Another STM-8 = questionChris,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 LambTo: psn-lSent: Thursday, February 05, = 2004 12:08=20 AMSubject: 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