Any experience with negative impedance?

It really wouldn't be hard to do the negative resistance thing, to nearly compensate out the resistance of the cable. It will just make the impedance look lower at the far end, at the speaker load. A long speaker cable can have a lot of inductance, too, and that could be more work to try to null out.

I've designed negative-output-resistance voltage regulators to make up for wire and PCB trace losses, but remote sense is better. For some reason, buck switchers sometimes have negative output resistances all by themselves.

So what you really want is a matching current which flows through your speaker circuit to also flow in a copy of your "very long piece of wire" and then subtracting the voltage differences?

Do you really mean "remove the effect of the wire from the terminating impedance"?

In the past I've run with the amplifier right at the speaker, and sent signal via a current loop. ...Jim Thompson

After reading the other responses and your added details ("legacy" speaker, wiring constraints, etc.) I'm wondering why you don't install a suitable wireless amplifier at the speaker and feed it power with the available wiring.

Mark L. Fergerson

The loudspeaker circuit contains three major sources of resistance, the resistance of the voice coil, the apparent output resistance of the amplifier and the actual resistance of the connecting wires. For the purposes of this application, I want the resistance of the voice coil to be the controlling factor, so I would like the other two to be negligible by comparison.

The apparent output resistance of the amplifier is very low, so low as to be negligible, because it has a large amount of local negative feedback. The wiring between the amplifier and loudspeaker has to be long (for reasons outside my control) and has a resistance which is not negligible compared with the drive coil resistance.

By using the amplifier in a feedback circuit which makes it appear to have a controlled degree of negative output impedance which is equal to the unwanted resistance of the wire, I can reduce the loop resistance of the whole output circuit loop to just that of the loudspeaker voice coil.

I am not trying to counteract the loudspeaker resistance, just that of the unavoidably long wiring.

I don't want to, but it is the least difficult of all the methods I have considered so far.

The loudspeaker is hanging from chains in the roof of a very tall building and the only possible wiring run to the control position is hundreds of feet long and must be insulated to mains standards.

I am hoping that the inductance of a long run of 1.5mm T&E will not be too high, but it is something to look out for. Perhaps it will need a small inductor in series with the current-sensing resistor to balance it (perhaps the current-sensing resistor should be made of copper, so as to counteract ambient temperature effects).

By earthing the central 'earth' conductor, I ought to be able to reduce the capacitive coupling between cables by a significant amount.

The idea goes back to the 1890s, when tramway generating stations used negative impedance boosters to counteract the I*R drop on long DC lines to remote parts of the system. It wouldn't surprise me to find that steam and hydraulic engineers were using the same principle in their domains fifty years before that.

Perhaps I should have mentioned earlier that this loudspeaker is hanging from chains in the roof of a tall building.

• Speakers NEVER have "pure resistance",except at one or a few points in the audio spectrum: Voice coil versions are inductive near the low end,tend to become resonant in the middle, with a widely varying _impedance_ thereafter.

• I suggest you read up on "damping"...

• Use of either line transformers or a four-wire feedback system is strongly recommended.

• Twisted wire can do wonders..

• No access? "Must work" first time? Forget it.

I didn't want to complicate my original question with lots of detail, but the reason for the long wires is because the loudspeaker is hanging from chains in the roof of a very tall building. As far as I know there are no commercially-available wireless amplifiers which will offer sufficient protection to the loudspeaker in the event of a fault or mis-use.

I also have to supply the field coil with power, preferably from the same unit as the amplifier, but that is another story.

(Adrian

Come now...let us not be practical..

On a sunny day (Mon, 28 Jan 2013 18:54:26 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

I think that was obvious, but do not DC couple a transistor amp. Use a high pass series cap. Speaker may not like to go against its frame. [Parametric] equalizer sell one to it.

If they want 'a golden touch of high timbre' just twiddle it in/. :-)

Equalizera are cool, I had a program on old PC that make acoustic measurements of the space, and then worked out some equalizer settings, and that was freeware. May still exist, used noise.

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What is T&E?

Wire spacing makes inductance. Capacitance is probably negligable here.

Right, an extra series field winding can make a DC generator have negative impedance.

(Adrian

Oh, I forgot it was audio. My bad.

Field coil? Do speakers still have them? I thought that went away when alnico was invented.

I am using an amplifier which is designed to be DC coupled, but with a blocking capacitor in the loudspeaker circuit to prevent damage if the output stage shorts to one of the power rails. Any remaining protection will be done through the muting and standby circuits using timed current-measuring circuits to keep within the dissipation and coil travel limits.

This is not an entertainment system, this is a research project - absolutely no equalisers of any kind are allowed at the start. When we have made the necessary measurements, there may be a possibility of incorporating a custom equaliser, so I am including a socket in the signal path which will be bridged-out, but can take an extra equaliser board if the need arises.

Some measurements made in the 1920s suggest that we might find a slope of 5dB per octave, which is a very strange value.

Hmmm, Western Electric 15A folded horn by chance?

It smells that Adrian has a speaker from the first half of last century in an anechoic chamber, and he's trying to measure it.

It may be a too heavy challenge to compensate for the cable resistance, but not for the back-EMF from the mechanical movement of the voice coil, in addition to the normal self-inductance effects.

"Twin and Earth" (wiring contractors' shorthand - sorry I didn't explain).

There's not a lot of space between the conductors, but I was worried that a long run might have enough capacitance to tip the system into instability somewhere above audio. It needs one dominant pole to damp it all down, but this must not intrude into the wanted audio band. [...]

These boosters were strange looking machines with huge commutators and banks of parallel brushgear to carry the full load current but small frames because they only needed to supply the boost/buck voltage (both, because some tramcars used regenerative braking) . The field was engergised by the load conductor wound a few turns around the pole pieces.

The strangest part of the machine was the sensitivity adjustment, which took the form of a hefty shunt across the field winding to divert a proportion of the load current. How they ever adjusted it accurately, when the total resistance was only a few micro-ohms, is a mystery to me.