Any experience with negative impedance?

Don't do it and say you did.

RL

Snide shot? About alnico? It's just true.

I discuss electronics and you whine and cluck. I prefer my "raison d'être"

Geez, what old-hen blather. Talk about negative output impedance amps.

I like that solution.

Yes, but I'm still concerned about cable inductance. The three conductors will have (mumble mumble) tricky mutual inductances, so a purely ohmic correction may get wonky at high frequencies. As might a simple negative-resistance amplifier.

How long is the cable run?

Thanks to John Larkin's suggestion to use the spare conductor as a sensing wire, I can do it with one op-amp, two resistors and a capacitor:

- for analogue problems use analogue solutions.

Somewhere between 40 and 100 metres.

If the inductance is significant, it should only be the compensation which suffers, not the stability.

Above 10 Kc/s it is most unlikely that there would be any significant audio output from this unit and the voice coil impedance would be a lot higher than the nominal 15 ohms, so I am not particularly concerned about failure of the cable compensation at higher frequencies.

This is not an ancient audiophile project, this is an attempt to measure one thing at a time. That is the way to do objective research: measure each parameter with as little interference as possible from all the others.

like

I thought you'd already discounted additional (sense) wires. Didn't you object to the Kelvin connection idea?

You have far more control in the digital domain. I thought you had already rejected the obvious feedback solutions.

As long as it doesn't oscillate.

I had some US style 14/2 Romex handy, so I measured a meter of it.

As someone predicted, inductance is minimum if you use the paralleled pair against the bare ground wire. In the 3-wire remote sense config, there is probably a best permutation. This could be, and should be, Spiced.

Is your T&E similar geometry to Romex? Will it be in a conduit? That would change things a little.

How come you guys can say Kc/s and we have to use KHz? Not fair.

There were objections to using a full four-wire sensing system, however, the three-wire system is relatively easy to impliment and should be adequate.

I have one month to get this built and working. I can't afford to mess about experimenting with digital 'solutions' and any use of digital will immediately involve myself and the client in a long bureaucratic rigmarole of EMC testing.

Thanks for that info. It looks as though I don't have much to worry about there.

That looks very similar. T&E doesn't have the paper insulation and the colours are different, but the geometry is almost identical. I was proposing to use 1.5 sq.mm load conductor size (the earth is slightly smaller) which has a resistance of 12.10 milliohms per metre of single conductor. If the inductance doesn't prove to be a problem, I would prefer to use the load cables to feed the voice coil and the earth wire as the sensing wire - mainly because that is how the installer will wire it up, no matter how many times I instruct him to the contrary.

As the client will be needing a drum of cable, I will buy it and make the necessary tests and measurements before sending it on to be installed on the premises. That way I shall have the opportunity to sort out any problems 'in house' before arriving on site.

I can claim to be old enough to have always used it. :-)

Fortunately, help is available:

For the benefit of us non-audio guys, what are dBu referred to?

Romex is a brand name. Not all brands of cable have the paper. It's not intended as insulation, it is there to make it easier to strip the outer jacket without damaging the insulation on each wire. You make a cut next to the ground wire, then pull it to the side till the tear is long enough. Then use dikes or an electrician's knife to trim the unwanted jacket. The brands without the paper are cheaper, but require more time to prep, and sometimes the outer jacket is stuck to the insulation on the wires, which wastes a lot of time to carefully peel them apart.

That is planned, but not drawn yet.

I tend to get involved in projects where the hours are less important than getting the job done to my satisfaction - it's called "being self employed".

Very good point, I shall have to add those when I am satisfied that the circuit does what is required of it. (This drawing is only a prototype at this stage.)

I am surprised that you didn't get noises from the traction current too. If I remember correctly, Amsterdam trams have a carbon strip on their current colectors, so that probably helps to reduce contact noise. It is much easier to fit on-board filters to a tram than to a trolleybus, because the tram has a well-earthed chassis.

The old UK trams used a brass wheel for current collection and had no suppressors. On rainy days, people at the outer end of a tram route would switch on their wireless sets to listen for a tram approaching, so as to avoid standing a long time in the rain waiting for it.

The real reason was to give me some control over the peak power of the amplifier. If I find it is capable of delivering dangerous transients to the loudspeaker, I can change the zener diodes and bring down the power rails to a lower value. There are reservoir capacitors on the pins of the power amplifier chip (as recommended in the data sheet), so the speed of the regulating device is not important.

The dBm I use are relative 1mW in 50 ohms.

On a sunny day (Wed, 30 Jan 2013 21:23:18 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

No, but in the TV studio in Hilversum they had strange earth currents in the video cables every time a train went past it. There is a special train stop across from the studio complex.

When working in Amsterdam next to the railway central station I had 400 Hz or so pickup from the trains there.