lol i plucked that out of my head, i dont pretend to know that much about superconductors. If u say its zero R then ok. Withov voltage or charge, what then motivates the electrons? Ill read up lol.
Carbon brushes, i think im correct in saying have a negative temp coefficient.
I guess graphite would need to be hot and the resistance way too large.
I dont know if carbon fibre conducts...
Maybe in the future, Graphene? Who knows.
Would a purely reactive loudspeaker be advantageous?
Way OT. im shutting up now.
Carbon brushes, i think im correct in saying have a negative temp coefficient.
I guess graphite would need to be hot and the resistance way too large.
I dont know if carbon fibre conducts...
Maybe in the future, Graphene? Who knows.
Would a purely reactive loudspeaker be advantageous?
Way OT. im shutting up now.
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A good pep speech..
jn
edit: There will be an end terminal voltage when the current in a super is increasing, and the current will rise based on the total loop inductance and resistance of the charging circuit. The drop across the wire will simply be L dI/dt. Once dI/dt is zero, so is the voltage across the terminals, but the current doesn't necessarily stop.
If you have a loop of super with current flowing, then short the ends together, you get what is called a persistent current. It will continue unending as long as the short is super to super. MRI's work on this principle.
If you have a 10 henry magnet and put 10 volts across it, the current will rise at 1 ampere per second. As long as there is a voltage applied, the current will change. If you shorted the ends when the current reaches 1 kiloamp, that current will continue.
If you break the connection of a charged superconductor magnet, the magnet voltage will flip and attempt to jump the break like a flyback circuit, and that can be very nasty...this is what happened at the LHC. Of course, there the actual magnets tried to flip. They are about 15 meters long, 1 meter diameter, weighing about 55 tons apiece. They played leapfrog in the tunnel.
Cheers, john
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Expelled for proving the teacher wrong?
That is harsh and very bad behaviour by the teacher.
I used to have lots of arguments with my teachers which I usually won. My biology teacher once claimed that lungs evolved from swim bladders whilst I maintained it was the other way around. It got quite heated until he got a book from the library and tasked me to support my view which I duly did.
All it got me was top marks.
Super Conductors? For sure they all live to ninety years of age in one of those interesting and strange observations.
Furtwangler hated the Nazis. Whereas I thought Herbert Von was a Nazi, all that Wagner an' stuff...For all that Georg Solti was my favourite, and I did see him in action knocking a rough student orchestra (Imperial College, London) into shape on Vivaldi and was deeply impressed.
An impish and teasing sense of humour like Beethoven, who he did extremely well, and, of course, like our own learned sreten. 😀
On a change of topic, don't think that superconducting loudspeakers are going to solve the resistance problem. You merely run into the Quantum Hall Effect which quantises the music. I know, I've done all that Indium Antimonide and liquid helium stuff. Utter bore. 🙄
Furtwangler hated the Nazis. Whereas I thought Herbert Von was a Nazi, all that Wagner an' stuff...For all that Georg Solti was my favourite, and I did see him in action knocking a rough student orchestra (Imperial College, London) into shape on Vivaldi and was deeply impressed.
An externally hosted image should be here but it was not working when we last tested it.
An impish and teasing sense of humour like Beethoven, who he did extremely well, and, of course, like our own learned sreten. 😀
On a change of topic, don't think that superconducting loudspeakers are going to solve the resistance problem. You merely run into the Quantum Hall Effect which quantises the music. I know, I've done all that Indium Antimonide and liquid helium stuff. Utter bore. 🙄
A speaker driver is a current operated device just like any other type of electromagnet. Force is proportional to current, not to power as dave correctly points out.
Make the voice coil out of wire with lower resistance for the exact same volume of conductor (cutting the same number of lines of force in the magnetic gap) and the power dissipated will reduce, and less voltage will be required for the same current to achieve the same force. Less power, less voltage, same current, same force. The same force means the same acceleration if the mass is the same which means the same SPL.
Pretty basic stuff really.
Any static power dissipation due to resistance in an electromagnet is "wasted" power, hence the impetus to invent room temperature superconductors...
You can quibble over the exact wording of what was said but at the end of the day what was meant was force is directly proportional to current, not voltage. Since a speaker is not a flat resistance but in fact has a complex impedance curve, this is a very significant and pertinent fact.
No, we do drive speakers with low impedance sources by convention, but only since the transistor era.
Before that amplifiers had much higher output impedances. One only needs to look back to 50's/60's era black and white valve (tube) TV's some of which including Philips had speakers of 600 ohms or so impedance so that they could be driven directly by a high impedance cathode follower without a step-down transformer... even with a step down transformer a typical valve amplifier designed to drive an 8 ohm speaker would have an output impedance of several ohms...
Low impedance drive became popular because it could be easily done with transistor amplifiers and is a natural result of high levels of voltage feedback. Having a "low" (close to zero) impedance drive as a convention makes a lot more sense than picking an arbitrary figure such as 4 ohms, and gives speaker designers a target to aim for when utilising drivers that are inherently current driven.
Low impedance drive does indeed give increased electrical damping at resonance but its not necessarily inherently superior as all it is doing is altering the Qts, which can also be done electronically with a notch filter in an active system.
Whether you use a voltage drive with Qes providing the target Qts, or you use a current drive with an appropriate resonance compensator (conjugate notch) in the transfer function feeding the speaker doesn't really make much difference at the end of the day, although the former is far easier to implement in a passive design.
As for less distortion, well, you might want to revisit that theory. Current drive can increase the distortion at some bass frequencies somewhat, but it also typically decreases the distortion at midrange frequencies. The change in distortion when going from voltage to current drive depends on the frequency, increasing at some frequencies and decreasing at others, so no such blanket statement like yours can be made.
No, it makes the speaker as a whole including the crossover a device which has the desired transfer function when driven by a voltage source. The drivers themselves are still fundamentally current driven devices. And this is only because the overall speaker has been designed "by convention" to expect to be driven by a voltage source at the input of its crossover.
You could equally and successfully re-design the crossover to expect a current source for a flat response, although in a passive design it would require many more components as you would need some resonance compensators...but it could be done. There isn't much point though when there would be very few amplifiers capable of driving such a speaker correctly, so we're back to the "by convention" argument again, since amplifiers of the last 40 years are by convention voltage output devices, it then follows that speakers will be designed to such a target.
With fully active designs there are actually many potential advantages to using current drive instead of voltage drive including reduced midrange distortion due to independence from Le modulation and elimination of thermal compression due to independence from changes in Re with temperature.
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More pure nonsense, and a further exercise in hand-waving and smokescreens.
Even more high-school level pseudo pop-science at its best.
Or....just maybe....
Thanks,
David
Even more high-school level pseudo pop-science at its best.
Or....just maybe....
Thanks,
David
Hi,
Quite true. Any speaker can be modeled as a zero R voicecoil with an
external resistor = to Re, but in this case driver response to voltage
drive would completely unworkable, due to very low Qe and then Qts.
With a perfect conductor in in the voicecoil you have to add external
resistance to make it work, so simply a perfect conductor is pointless.
rgds, sreten.
If you could somehow build a room temperature superconducting voice coil with zero resistance, it wouldn't be quite as useless as you suggest.
No, it wouldn't be compatible with a standard voltage source amplifier because you'd have to add some external resistance which would just dissipate power that the voice coil otherwise would have. (although keeping that dissipation away from the driver itself is still beneficial...)
But why can't you drive it with a current source amplifier ? The external resistance doesn't have to be an actual physical resistor, it can simply be an extremely high (near infinite) source impedance provided by a current source amplifier. (or somewhere in between using a mixture of current and voltage feedback in the amplifier...)
Furthermore if the voice coil is super conducting, (and presumably the speaker cables...) you could achieve the necessary current to get the necessary drive levels from VERY low supply rail voltages of maybe just a few volts, instead of requiring +/- 50 volts or more as is common now.
That brings the efficiency of the whole system way up compared to a high voltage rail, voltage source amplifier with an extra series resistance added.
A solid state amp with a low voltage rail, high output current capability with current feedback to make it a current source would in many ways be a lot easier to build since its far easier to operate high power transistors at low voltages. There is no shortage of low voltage very high current transistors that would be ideal for the task of driving high current into a zero ohm load from low supply rail voltages.
Even though there is still obviously some power dissipation in the amplifier, (proportional to the supply rail voltages) having none in the voice coil is obviously a huge advantage as it would eliminate the thermal limitations of the speaker...
So no, a superconducting speaker would not slot right into the current paradigm of voltage source amplifiers and passive crossovers, but with a fully active crossover and a suitably designed current source amp it could have some massive advantages.
I will wait patiently for my room temperature superconductors to arrive. 😀
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I agree. Not only could an amplifier be derived that was happy putting current into a short, but don't forget that even with no DCR there would still be motional impedance. Away from resonance the motional impedance is typically swamped by DCR. In his case it would not be, hence the high efficiency of the approach.
Some interesting hand waving going on now!
David
Some interesting hand waving going on now!
David
Wouldn't the motional impedance do nothing if the amplifier was a true current source with infinite source impedance though ?
Even with a SC coil the voltages will be as high as dictated by the generation via BL*v(t), v(t) being the coil velocity. OK, L can be reduced to basically a one-turn winding then voltages would in fact be low.
But, since the impedance needed for correct (whatever that is) damping can be synthesized with special feedback for the amplifier without lossy series resistors, the total electrical efficiency could easly reach 90%...95% when using a class-D amp. In fact all losses would be in the amp/supply, and close to none in the driver (except for mechanical losses). Class-D is important as it can handle reactive loads as efficient as any load.
For this and other reasons I try to use drivers with the biggest amount of conducting material volume in the gap (it is the volume of the conductor in the gap vs the volume outside the gap that determines an important factor for efficiency, of course scaling with conductance) ==> slightly overhung VC's with copper-clad aluminum rectangular wire to fill the gap as much as possible. And the required damping vs. frequency is established by the synthesized output impedance profile of the amp.
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Yes, the generated microphonic voltage then goes into the voids having zero effect. Exactly what you want to have once you are outside of the system resonance region (IMHO and IME). Current transfers into force working against the acoustic load without being disturbed by the generated voltage (from internal or external exitation of the cone).
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Ah, very good point which I had not considered. Even though the DCR is zero the motional impedance will generate voltage across the voice coil, and the amplifier has to have enough available voltage swing to continue to provide the correct current through the voice coil with that instantaneous voltage offset present.
Whether its a big problem or not in practice I don't know. How many volts of motional feedback would we expect ? Somehow I think it would be a lot less than the +/- 50-60 volts that amplifier supply rails typically are today.
If we could get the supply rails down to +/- 10 volts that would still be a huge reduction in power dissipation for the same current drive levels, and allow us to avoid worrying about secondary breakdown in bipolar devices as well.
As you say you could reduce the number of turns of the voice coil but then you need more current, so I'm not sure if we gain anything overall going that route.
Yep agreed. At least from an electrical efficiency point of view, this does seem to be the way to go if a super conducting voice coil were possible.
One problem with super conductors in this application though is that all superconductors have both a magnetic field density and current density above which they stop superconducting and snap suddenly back to having resistance.
Whether this would manifest as a sudden and gross clipping like distortion if driven too hard is an interesting question.
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It only defines the voltage across the driver. If we use a true constant current amp then the driving force is proportional to current which doesn't change whether we have zero ohms, an external current limiter resistance, or some motional impedance function.
I only mentioned it to calm those worried about driving a total short. A zero ohms superconducting coil would still have motional impedance.
As an aside, this is one of the methods of defining or measuring efficiency. Power into motional impedance results in acoustical radiation. Power into DCR results in heat. If efficiency is low then DCR is much greater than any reflected radiation reistance. In a horn where efficiency is very high the the reflected radiation resistance is very significant. A compression driver with 4 ohms DCR may look like an 8 ohm driver because of it.
David
Yep, B squared x Volume of Copper determines magnetic "shove". Calculate the volume of copper in a gap an the number is pretty dissapointing, 10 to 15% of the available volume if I recall. Air gap clearance, insulation coating and packing factor take a toll. Overhang adds non contributing resistance.
David
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