OT / Poor mans 'dummy load' [damped speaker coil?]

[petemoore]

  Edit...not 'stompbox'..I just realized I'm not posting this in OT.
  Anyone ever try this?
  I have a 15w bulldog w/Fender speaker [just to draw 1/2 the load off the 15w] which I run off my >15w 5e3 tube amp clones.
  I'd like to have only the Bulldog being heard, and draw a portion of the power to 'something else'...like a speaker...dummy load.
  I have old 12''s...has anyone ever heard of or modified a speaker to become a dummy load?
  Perhaps just double/triple doping the surround and weighting the cone would damp it enough so that being sealed inside an enclosure would sufficiently remove it's influence to the 15w speakers [in baffleboard] moving of the air outside the cabinet.
  Or...better yet, some type of damping system which removes say 80% of the cone and frame...re-fixing the remaining cone somehow where it can still move some but does'nt really put much sound out?
  Someone tell me it's safe to run a Celestion Alnico Blue 15w 12'' speaker on a 5e3 which probably produces <10w -12w [absul-max, the way I'm typically running it].

[Mark Hammer]

We've discussed the general topic of 'realistic' dummy loads at varying lengths many years back, perhaps on Ampage.  One of the suggestions I made was exactly what you describe; removing cone.  You want the coil to be able to move and create the assorted electronic changes a speaker normally does.  You just don't want it to move air efficiently and create audible sound.  Surgical removal of big segments of cone can accomplish that.  Just keep in mind that:
a) The mass of what's left should be in the neighbourhood of what you started out with.  The electronic parameters of the voice coil will depend partly on having the same acceleration/decelleration properties.
b) You don't want to compromise the structural integrity of the cone.  The spider at the base of the cone is only one of the mechanisms that lines up the voice coil so that it doesn't rub against the magnet and burn out.  The other is the cone's connection to the basket at the outer perimeter of the cone, so you need to leave "ribs" connecting the centre of the speaker where the coil is to the outside.

I don't know all that much about aerodynamics.  It may be that simply having a few select places for air to pass through, rather than be "cupped" and moved by the cone, will provide the necessary reduction in sound.

It this a dummy load intended for amp-measurement purposes or for using a dimed amp as an "effect" or for simply reducing the audible output by sticking unheard speakers in with heard ones?

[hairyandy]

Pete,

I just recently built a Ken Fischer designed Airbrake attenuator.  It's the same as the Dr.Z Airbrake.  It sounds amazing, or rather doesn't, it's much more transparent than my THD Hot Plate.  Like any attenuator it sounds best at low levels of attenuation which I think would be all that you'd need really in your case to keep your Bulldog happy and alive.  It's safe for any amp 50 watts or less and will work as is with 4-8-16-ohm impedance.

Here's a link to the layout that I used:

http://www.andyharrison.net/files/AirbrakeLayout.jpg

The parts aren't cheap, they were about $100 from Mouser, but it was well worth it.  This thing really makes my Trainwreck Express clone usable in any club situation.

Here's a link to a thread about my build on the Ampgarage.com forum:

http://ampgarage.com/forum/viewtopic.php?t=2402

Hope this helps,

Andy

[tommy.genes]

The Weber MASS Attenuator is what they call a "speaker motor", basically a voice coil without a cone. So apparently the concept is sound enough (um, maybe not the best words) for commercial products.

http://www.tedweber.com/atten.htm

-- T. G. --

[MKB]

The Weber MASS Attenuator is what they call a "speaker motor", basically a voice coil without a cone. So apparently the concept is sound enough (um, maybe not the best words) for commercial products.

http://www.tedweber.com/atten.htm

-- T. G. --

You can also buy the raw speaker motor from Weber: https://amptechtools.powweb.com/truload.htm  The motors are at the bottom.  Since a 50W motor is only $30, it may not be worth trying to build such an animal.  I've been meaning to buy a few but haven't gotten to it yet.

[Mark Hammer]

Where I live, surplus places sell subwoofers for $10.  Part of what makes them subwoofers are the bulky, heavy cones.  I would think that, once you have the cone itself factored out, a voice-coil is a voice-coil is a voice-coil (assuming suitable power dissipation and impedance specs), and there may be no need to have the voice coil of a 50hz-6khz guitar speaker as one's motor.

Naive or reasonable?

[petemoore]

  Back sounds loads better.
  The 'franken-coned' 15''er I further tore up, reduced diameter and height of, put 1/2 a HD paper cup over the dome, then flooring padding around the frame cutoffs I bend to 'exact' height..
  Works, but I don't trust having that old coil [the bottom half of can be seen moving] there well enough to dedicate that speaker to it.
  If I come across a high wattage handling but small cone sized speaker, I think I could work with dampening it some, mostly just cardoning it off inside the cabinet, and would feel ok about putting a defeat [short the dummy terminals] switch on it for volume changes.
  Anyway I found out what I was reading last night all about beam blocking certainly is what I'm going to try next, and with the GB celestion.
  The Bulldog stays in a 2x12'' cab.
  But I'm considering also making this a 'two fronted' 1 x G12M cabinet. It's a deep 1x12'' cabinet, the backs output is scads smoother, and is certain to be more directional [which I predict could be a very usable feature on certain stages].

[Pushtone]

Found this a while back.

http://www.amptechtools.com/truload.htm

Scroll down to bottom. Look for motors for mass attenuators.

Hope it helps.

[MKB]

Where I live, surplus places sell subwoofers for $10.  Part of what makes them subwoofers are the bulky, heavy cones.  I would think that, once you have the cone itself factored out, a voice-coil is a voice-coil is a voice-coil (assuming suitable power dissipation and impedance specs), and there may be no need to have the voice coil of a 50hz-6khz guitar speaker as one's motor.

Naive or reasonable?

That sounds reasonable, although subwoofers have a response cutoff that is lower than a guitar speaker; that might manifest itself in the impedance from the load.  But that might not be a big enough effect to worry about.

So the main problems with taking a woofer and whacking off the cone would be perhaps 1) reproducing the effect of the cone moving air and the cabinet load, and 2) replacing the stabilizing effect of the cone, which is a second mounting point along with the spider. 

Concerning point 1, you might be able to reproduce the air load by using foam instead of the cone; this actually could be quantitatively measured as the speaker parameters take that into account.  You could tweak the foam density and amount until the Thiele-Small parameters match the unmodified driver.

Not sure how to address the second point.  I bet the Weber motor has multiple spiders to stabilize the coil.  You might also be able to buy a second spider from Weber (individual speaker parts like that are pretty cheap), glue it on, and off you go.

[petemoore]

  If I were to start from scratch-, I think I'd get a 'pendulum' motor going, because speaker motors are necessarily more complicated...
  Instead of spiders and critical tolerances in all directions [coil is cylindrical and has magnet on both sides in all directiions, I think a simple 'spring stick' holding the coil and imparting increasing pressure as the swing range movement gets farther away from center [over-excursion or could more easily be controlled I thinks.
  Sprung and weighted correctly, I submit the 'spring-stick-pendulum' motor could be made to replicate the movement of a speaker coil motor without all the alignment problems, and the coil could probably be made to be easily swapped.
  No research that I know of exists, I wouldn't know where to start with the magnet/coil/assembly designs...perhaps a C shaped or concave magnet, or one magnet on either side of the coil.
  Something simple where the coil isn't buried, and the action can be easily worked with.
   

[aron]

I don't use an attenuator but when I did, I used the Weber Mass.  I have used the Airbrake and it's good too.

OTOH, sometimes simple resistors are good too.