How is magnet like transformer?

[Colin]

The size of the magnet on a driver seems to be important to the control of the cone excursion of a woofer, giving it better speed, sustain and decay. The size of a transformer on a tube amplifier also seems to be very important to the speed, sustain and decay of the bass driver. They are both related by the impedance of the woofer. How is the magnet of a woofer like the transformer of a tube amplifier?

[colterphoto1]

This is some type of trick question, and as such, I refuse to answer it.

Very tricky, Mr. Colin!

Michael

[Colin]

I trick you not.

Interested in plain English explantion so I can understand it better myself.

[Squigster]

Correct me if I'm wrong, but don't subs use vibrating quartz to generate a certain frequency? If this is correct, then you need specific quantities of electricity to resonate the quartz at a certain frequency. Since woofers are metallic, Neodymium magnets are used to vibrate the thingy. (The magnets are moved via an induced magnetic field created from electricity?)

[D-MAN]

Don't confuse MOTORS with transformers. Transformers are PASSIVE and MOTORS are active, of course.

The transformer is a winding around a non-magnetic core - that is - it has nothing to do with magnets, although it creates a magnetic field of its own when current flows through its primary winding much like a coil. The flow of current induces a different current flow in the secondary winding which is close in proximity but is a separate isolated winding. Transformers in there simplest form are two different coils placed in close proximity to each other so that they "couple" electrically, although they do not touch or otherwise complete an electrical circuit.

It changes current to either higher voltage/less current (step-up) or lower voltage/higher current (step-down) and can be used to match impedance for different circuits. Hence the name "transformer" because it transforms the current flow in some way dependent on its design.

A motor has moving parts and a transformer doesn't.

The motor uses a magnetic field to generate current flow when a winding (armature) passes through the field, inducing current flow in the winding. A Dynamo is a motor that produces current from rotational movement (i.e., a generator). Dynamos can be DC or AC, dependent on the magnet structures employed which causes induced current flow to reverse (the speed of rotation determines the frequency, such as the case for 60Hz AC) some more than once in a single rotation.

If current is applied to a dynamo, then it results in rotational movement of the armature (i.e., a motor).

The dynamic loudspeaker motor does not use or produce rotational movement - it is simple bi-polar movement based on the magnetic attraction or opposition of the voice coil armature inside a two-pole magnet structure. It uses an electrical circuit that either attracts or repels based on the polarity of the electrical signal in close proximity to a bi-polar magnet structure. However, since it produces movement from current (signal) it is referred to as a motor.

{edit} additionally, a dynamic loudspeaker can PRODUCE current exactly like a dynamo (it is then a microphone) when no signal is applied, and movement of the cone (soundwaves) are present. If the speaker cables where connected to a active preamp/amp then it could be recorded or broadcast in some way, just as if the speaker was a microphone (albeit a rather poor-quality one). This is the basis for the early horn-microphones used by Edison).

DM

[djk]

The size of an output transformer has little or nothing to do with the impedance of the driver. It is determined by the power required and the low frequency response desired.

Woofer performance is horsepower to weight ratio. Power is flux in the gap. A 2" coil can get by with a weenie magnet and still have high flux in the gap because a 4" coil has much higher gap volume to energize.

Like a car, you can have too much horsepower for the chassis.

Qts is the number we look at here. A Qts of .312 is optimum for efficency and minimum box size vs bass extension.

If the magnet is too big the speaker will have no bass.

If the magnet is too small the speaker will require a huge box.

Bass horns are similar, but yet different. A small magnet generally penalizes the HF response.

[WMcD]

There are similar issues.

SPEAKER as a motor:

In the speaker the magnet is part of the motor system. The other part is the voice coil winding. The quality of the motor system is established by the Bl product. The B is the magnet strenght and the l is the lenght of wire in the field.

When you put current through a wire it creates a magnetic field. If you coil the wire the magnetic field is concentrated.

Consider when you have ordinary permanent magnets. Opposite poles attract and like poles repel. Meaning you can create forces.

Ergo: When the changing current (music) in the coil changes, the magnetic field changes North and South.

The magnet gap as a fixed polarity and so there is an overall force in proportion to the current in the coil. This force depends on the strenght of both fields. One field is the magnet.

Bigger magnet, bigger permanent field. But also better, stronger motor.

Naturally, the coil is attached to the diaphragm and pushes it back and forth, in time to the music. Bigger magnet, better motor. More efficient.

SPEAKER as a generator:

There is reciprocity here. It is less well understood by the casual observer.

Just about any elecromagnetic motor is also a generator. If you move the coil of wire in the magnetic field it creates current in the coil. The other part of the issue is what the coil is attached to electrically. In the speaker it attached to the amp.

This takes some getting used to. But look at this as a two port device. You put current into one end and get force out the other. But if you put force in, you get current out the other, also.

Just how good is the generator? Same Bl issue. The bigger the magnet, the better the generator.

But . . . what happens when you try to crank any generator and there is a dead short as a load? Answer, it is hard to crank. Hard to crank means it applies a lot of force to resist cranking when you try to turn it.

The better the generator, the harder it is to crank. So speaker (or generators) with big magnets resist movement because they can create more current.

Now you ask: How the heck is my speaker a generator and what is causing it to run backwards this way. And why do we care.

The mechanical part of the speaker has mass and a spring. It will oscillate on its own at some frequency, which is Fs. That is the motion which we want to control in just the right amount.

The motion creates a force which drives the generator. So how much the oscillation (diaphragm movement in the bass) is controlled depends on how much the generator is hard to crank. See above. Bigger magnet, hard to crank, better damping.

How much damping is good? It depends. We want just the right amount. Too little and the speaker is boomy. Too much and we don't get the boost of bass caused by the resonance of the oscillation.

THE AMP OUTPUT AS A LOAD FOR THE SPEAKER GENERATOR:

Remember that we're pretty much looking at the system running backwards. The speaker is a generator too. The generator is hard to crank (damped) depending on how much current the speaker-generator is creating.

But what is the electical load on this generator? Answer: it is the amplifier. Yup, that amplifier which we are otherwise asking to create current to drive the speaker. Now we have to ask whether it is acting as a dead short (zero ohms), or something with some resistance (say 2 ohms). If there is higher resistance, there is less current. Less current, less damping force on the diaphragm. Generators putting out less current (because of less load) are easy to crank.

Lets give an example.

The amp has just put out a big bass note at speaker resonance and the diaphragm is moving like the dickens. Then the bass note ends. The diaphragm wants to keep oscillating. It now is a generator and how much force the voice coil creates does depend on the Bl . . . but it also depends on whether the amp (now the load) is a short circuit of zero ohms, or maybe 2 ohms.

This means the amount of damping of the system overall depends on the output impedance of the amp. This why output impedance is sometimes described in terms of damping factor. None the less, the Bl of the speaker is a big part.

Returning to the original subject, the bigger the magnet, the bigger the function as both a generator and as a motor. You can't have one without the other.

- - - -

TRANSFORMER:

There is a similar issue. But don't read too much into it. It is mostly power handling.

The transformer has two windings. The primary is the one which is connected to the power source. The secondary is connected to the load. Technically, it too is a two-port device. And it too runs backwards and forwards.

Let's take a technical aside here. An amplifier is a two port device. You put in a small input to one port and a big output comes out the other. But it doesn't run backwards. E.g. if you put power into the output, you don't get a small output at the input, normally. Now back to our transformer.

When you put current into the primary, it creates a magnetic field. If there is changing (note, changing) magnetic field and there is wire within its influence, then a current (technically, voltage) will be induced in the secondary winding. This is again a motor - generator set- up in a slighty more complex way.

Note that the magnetic field is the intermediary in this process of transferring power across the boundry between the primary and secondary windings. It is very important.

What can we do to focus the magnetic field so it is always close and intense to the windings and they are close to it? It is the iron core of the transformer.

(Another aside. Remember in grade school doing the d.c. battery and coiling wire around a nail to pick up paper clips. You could have wound the wire around a soda straw but it would not have worked nearly as well. This is because the field was not concentrated in the nail . . . which in our case is the transformer core.)

It is a difficult problem. One issue is just how much of the magnetic field can be focused and contained in the core. At some point it can't hold any more. It saturates. The bigger the core, the more it can hold. Again, bigger is better. Once it fills up and can't hold more, there is the same as amplifier clipping. You just run out of linearity.

= = =

I'm getting far afield (ahem) here. One issue in tube amps is that the primary coil is being asked to handle the full current though the tube even with no music. The secondary responds to changes but the primary winding is causing a strong field even with no music. So the transformer core is being filled up part way with even no music signal.

That is all for tonight.

Best,

Gil

[Hardhead]

Darn it, Gil, that's what I was gonna say!

[djk]

"Now we have to ask whether it is acting as a dead short (zero ohms), or something with some resistance (say 2 ohms)."

An 8 ohm woofer has about 5.8 ohms DC resistance in series with the back EMF. My McIntosh amps with their DF of only 14 sound 'tighter' in the bass and have more 'slam' than amps with more than twice the power and DF ratings above 400. DF is a red herring.

"So the transformer core is being filled up part way with even no music signal."

No net magnetizing current exists in push-pull or parafeed SET designs.

SETs with net magnetizing current are constructed with an air gap in the core to reduce core saturation.

[WMcD]

I quite agree.

If the DF of an amp is 10, the output impedance is 0.8 ohms.

The critical issue is the overall resistance (impedance) in the loop, which includes the impedance of the speaker. If that is 8 ohms, then the loop is 8.8 ohms. If you had an infintite DF the loop would go down to 8.0 ohms. Not much of an improvement.

I also totally agree that single ended amps have the most issues in saturation of the transformer core, and others less so. But generally the bigger the core, the better.

My main point was that there is a magnetic field in there.

Best,

Gil

[djk]

Absolutely correct.

[pauln]

You should know that magnetism does not really exist in the same way as electricity - magnetism is a relativistic side effect of electricity!

Paul