WW resistor inductance

[boater sam]

Liquid nitrogen cooling a la Cray? A possible winner, selling renewables to audiophools? As an aside would the average power Rs stand cooling to this degree or would they become superconductors with no resistance?

[Mike Watterson]

The resistance would certainly be much lower.
Very few materials are superconducting with liquid nitrogen (if indeed more than one?). Most need liquid helium. MRI maybe uses liquid helium.

[Refugee]

There is always the option of a "berco" to boil all those harsh electrons off. This method also works on bench PSUs after they have been repaired.
You end up with clean power and fresh audio.
Dummy loads must not contain animal products

[Valvebloke]

Superconductors for use in hi-fi are still in the realm of speculation AFAIK (although people are speculating about it). In areas with enough money, and hi-fi can be one of these, technology does penetrate though. The use of graphene in loudspeaker cones/diaphragms has been a hot topic for a little while.

Getting back to(wards) the original topic, Mark's wirewound resistors have an L/R current risetime of 0.73us, so a 10%-90% risetime of a little under a couple of microseconds. This is insignificant for audio signals, but not by a huge amount given the ludicrously small imperfections that some hi-fi folks worry about. It is actually within an order of magnitude of being an issue. If the current risetime was 20us rather than 2us then that would significantly round the leading edge of a 10kHz square wave.

Of course while having a perfect resistor might be useful for checking the performance of an amp into a well-defined load, it will matter relatively little when it comes to actual use in service. Practical loudspeakers are a very, very long way from being perfect resistors.

VB

[Mike Watterson]

Loudspeakers are very strange loads ...
if you  lock voice coil, then it's a rubbish sort of inductor, as the metal will have eddy currents.
If the cone can move, then there is back EMF. This can be quite out of phase depending on baffle, cabinet design / filling, room size / shape / reflectiveness. Oh and crossover network too! A large cabinet loudspeaker is a different load on an open hill top, stage in large hall and a small room. All varies with frequency.
Mostly the assumption is that you should drive from a very low impedance relative to speaker dynamic load. I've wondered what difference it makes:
a) Constant current (i.e. infinite impedance drive). Probably terrible idea.
b) A transducer measuring speech coil movement and putting speaker in a fast enough analogue servo loop (I think this has been tried)
c) Measure sound near listener (decent electret?) and apply DSP. Initialise for room and occupants using maybe white noise and then apply feed forward dynamic transformation. I've a feeling something like this has been tried.
One problem is non-linear behaviour of the cone vs frequency, transients and level.

The basics of the electro-mechanical-acoustic system can in theory be modelled* and stuck in spice as a purely electrical model. You can then examine what is happening on the amplifier output or if you are very expert, at any location in your chosen room.

*Deriving an accurate model may be difficult.

It's amazing what folk with slide rules and hand made parts achieved in 1930s. 

[Valvebloke]

...
b) A transducer measuring speech coil movement and putting speaker in a fast enough analogue servo loop (I think this has been tried)
c) Measure sound near listener (decent electret?) and apply DSP. Initialise for room and occupants using maybe white noise and then apply feed forward dynamic transformation. I've a feeling something like this has been tried ...

The best early example of b) was the Philips Motional Feedback system. When it worked, it worked well. When it went wrong though ... gorblimey !

There are a few suppliers of room correction systems of the c) type https://en.wikipedia.org/wiki/Digital_room_correction. It's a tough job to do well though. As with all feedback control systems the best results are achieved by getting the uncorrected system to behave as well as possible before starting to apply the correction. The instruction manual for the Quad II amplifier made a point of encouraging the listener to spend a few evenings tinkering with the position of his speaker (this was the early 1950's so there would only be one speaker of course). The benefits of doing this are just as large today and, it seems, just as often ignored in place of putting the speakers somewhere convenient or visually pleasing from a room layout point of view. Durrrrrr !

VB

[ppppenguin]

VB, do you really mean to say that some things can be more important than hifi?