What can you find that has a negative temperature coefficient and enough conductivity to make a useful voice coil?
Possibly it could be invented?
It's not about what one can find but more like what the benefits could be.
Just an idea.
It's not about what one can find but more like what the benefits could be.
Just an idea.
Benefit (theoretical): No power compression.
Reality: Known NTC materials are metal-oxide polymers, which are not good conductors, especially not for voice coils wires.
Copper is pretty hard to beat for voice coil use. Silver is a tad better in conductivity, but not worth the expense. I don't know how the tempos of the two metals compare, though. Silver's tendency to tarnish could also be an issue.
Metals have a positive tempco, (intrinsic) semiconductors have a negative tempco across the range of temperatures where charges are being thermally generated. Trouble is semiconductors don't conduct well, hence the name... Heavily doped semiconductors behave more like metals (and have positive tempco for the same reasons, higher temperature means more collisions of charge carriers with the lattice and phonons.
In practice a negative tempco material that was good enough for voice-coil would raise the problem of thermal-runaway, since speakers are voltage driven.
Should be mentioned that intrisic semiconductors can be many orders of magnitude more resistive than any metal.
The other possibility is superconductors...
In practice a negative tempco material that was good enough for voice-coil would raise the problem of thermal-runaway, since speakers are voltage driven.
Should be mentioned that intrisic semiconductors can be many orders of magnitude more resistive than any metal.
The other possibility is superconductors...
Just forcing ice water through the VC gap would probably go a long way, even with the normal tempco of copper. Not near as good as cryo-cooling but probably a lot closer to being practical.
What materials do you have in mind?
Better yet, use a superconductor! (And a custom amplifier capable of pushing extremely low impedance). I can envision it now: Each speaker comes with its own liquid nitrogen cooling system (pumps would need to be in another sound isolated room). Build it, and somebody will buy it, which I presume based on the number of 6 figure speakers on the market.🙂
LN2 not cold enough for superconductors that can carry enough current. Need liquid He. And that will give you enough field strength to yank earrings out of your ears, if you wanted. Fine line between not enough and too much when it comes to superconductors.
Current drive also can do this.
But of course you lose the protection effect of thermal compression.
Correct! I was thinking of midranges and tweeters.
Woofers also tend to have heavier, more durable voice coils.
You can use just enough positive current feedback, which gives the amplifier a negative output resistance, to cancel the voice-coil resistance.
Then you get a very damped resonance.
When the coil heats up, and the resistance increases, it will be less well damped.
Then you get a very damped resonance.
When the coil heats up, and the resistance increases, it will be less well damped.
Do not attempt to use negative tempco materials or use mechanisms to cancel the Re of the voice coil, as this could lead to catastrophic damage by means of thermal runaway, especially at PA power.
However, at small power levels, the steady state (heat gained = heat lost) is reached much before all that happens and it maybe OK provided the resulting steady state temperature (which is constant) is well within the operating limits of the driver / magnet.
However, at small power levels, the steady state (heat gained = heat lost) is reached much before all that happens and it maybe OK provided the resulting steady state temperature (which is constant) is well within the operating limits of the driver / magnet.
Last edited: