Speaker coil for dummy load?

[Axtman]

I have a 300W 15" EVM-15B bass speaker that has a large rip in the cone. I bought a recone kit and am going to recone it. Can I salvage the good voice coil and use it for a dummy load?

[pdf64]

There was a thread over on fenderforum.com about this, a guy did it very successfully, had the coil suspended in a jar of oil for cooling, and a steel cylinder in the coil for inductance, I think. Have a search. Peter.

[Dave Curtis, dB AudioTech]

It can be done, but I wouldn't use it for testing purposes; you want a non-inductive load for that.

[Chuck H]

Dave, doesn't it depend on what and why your testing? A purely resistive dummy load can't tell you what the amp is doing with a speaker plugged into it. Which is how most people play amps. I have both a resistive and a reactive dummy load and find them both useful.

As to the original question... Sure you can make a dummy load out of the old coil. But it won't make the amp think it's playing into a speaker. The coil seperated from the rest of the speaker will no longer present a complex impedance as a speaker does. And without knowing some very specific details about the original core structure (steel post and magnet) it may not even represent anything like the original average impedance, let alone the complexities. Even with a steel core in your coil you will only have a basic inductor with a single inductance that may also be unlike anything represented by the original speaker.

I've always been suspect of the Weber idea of using a speaker motor as a load for the same reasons. I don't think you can get the same complex impedance without having the coil mobile as it is when attached to a cone.

It's easy enough to build an acurate reactive load if you want one. But it takes a combination of resistors, inductors and capacitors. It can't be done with a single inductor. Even if it was part of a speaker once.

Chuck

[hasserl]

I understood the Weber MASS motor does move thru a range just as a speaker does, which would give it the same reactance as a speaker. I do know that the MASS attenuator vibrates as you play, indicating the motor is indeed moving inside the case. I haven't opened one up and checked how it is constructed.

[Enzo]

A speaker has the coil moving in a magnetic field. And as the voice coil moves in that field, it can generate back voltages. (CLip your voltmeter to the terminals of a speaker and push on the cone.) From the light bulbs and magnets chapter of your electronics book, remember a wire moving through a magnetic field generates a voltage. if you have a voice coil just sitting there it doesn't do that. It is just an inductor. Adding an iron piece in the center just changes the inductance. To emulate a speaker you need that interaction with a magnetic field.

[Chuck H]

Hasserl, thanks for the info on the MASS. I had wondered about that. If Ted does have the motor set up to move (even sorta) like a speaker then I think it's a great idea. Years ago I had thought of using a speaker motor surrounded with silicone goop or something, but I never tried it. I thought it would be too difficult to align well without a cone and I'd end up with coil rub. I wonder how Ted does it.

Chuck

[Don Moose]

Think of Weber's MASS as a coneless speaker with (nearly) no basket.

You have the coil, magnet and spider - that assembly is the 'motor' and provides the spring-backed-coil-moving-in-a-magnetic-gap reactive complexities of a speaker.

You don't have the cone, surround or most of the frame, so you don't get the mechanical-reflected-in-electrical-reactance-of-pushing-a-cone-against-air-in- and-around-a-box-with-its-own-frequency-response-characteristics complexities of a real speaker/cab system.

'Better' than a resistor, 'better' than an inductor, still not a real speaker.

Hope this helps!

[Dave Curtis, dB AudioTech]

Dave, doesn't it depend on what and why your testing? A purely resistive dummy load can't tell you what the amp is doing with a speaker plugged into it. Which is how most people play amps. I have both a resistive and a reactive dummy load and find them both useful.

Chuck

Of course! I wasn't trying to discourage the OP from doing it. Any testing I do with a dummy load involves the 800w resistive unit I built.

Don't take this the wrong way, Chuck, but when I want to know what the amp is doing with a speaker plugged into it, I use a speaker. That's the only reactive load I have kickin' around, and while it may not be optimal, it does the job nicely.

Dave

[Chuck H]

It's all good Dave. I don't have an actual "shop" since I don't do repairs professionally. I work in the spare room in my home. Since I have a wife and daughter I need to minimize full blast testing with an actual speaker so we don't all end up in a newspaper article... 'Man bludgeoned to death with oscilloscope while he slept.'

Chuck

[Dave Curtis, dB AudioTech]