Any experience with negative impedance?

Google werner steiger "negative output impedance"

Back in the '60's a popular term was "motional feedback" I built a speaker system for my truck using negative output impedance to damp the speaker/cab resonance and boost the lows. I can't find any documentation on it other than the paper referenced above. I never had oscillation problems, but this was ~40 years ago when transistors weren't very fast.

On a sunny day (Mon, 28 Jan 2013 10:38:23 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

Been doing quite a bit of power amp stuff, and did ye no-weh that those creatures cannot hear the diffrerence between things?

It almost sounded like you were contemplating some kind of current drive, like the old toob amps.. had a rather high Zi.

Some creatures like that sound.

Not high impedance; the exact opposite.

No audience, it will be the subject of tests by a group of audio historians, museum curators and researchers.

On a sunny day (Mon, 28 Jan 2013 12:56:25 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

Na, sorrry, I can remember the oldest 'loudspeaker' I ever had, in a Philips radio from before WW2... The sound quality was *shit*, the efficiency was not much better, was driven from a single tube amp, so with a relative high impedance. Those speakers were designed to be driven like that. Not sure if it does not make it sound totally different when driving from low impedance. Had a HUGE magnet, but magnets in these days were not that strong. Was a moving coil type however :-)

Just burn the * thing and give them some Senheiser earbuds for HiFi.

Really you should drive it with wat it was suppoded to be driven from?

This loudspeaker is much older than that, it is one of the first ever moving coil types.

Those old Philips sets gave much better quality than many of the British ones of the same age and price range. I used to listen to symphony concerts on one in the 1950s and to Radio Netherland's shortwave programmes in English during the 1960s. I still have one and, the last time I used it, the quality was far better than any modern self-contained A.M. radio.

The servicemen used to hate them because the circuits were too 'clever' and managed to squeeze 5-valve performance out of only 4 valves. Nobody seemed to know how to mend them properly.

Phiips was one of the first companies to produce magnets strong enough and cheaply enough to make permanent-magnet loudspeakers available in domestic sets. That initially made a separate smoothing choke necessary (previously the loudspeaker field coil had served as a choke) but then Philips started tapping the HT into the primary of the output transformer and drawing off a current to feed the RF stages which balanced the output valve current. This reduced the hum and removed the DC magnetisation from the output transformer core.

Philips were a very innovative company right from the late 1920s onwards. They were technically years ahead of most of the other manufacturers, sometime to their cost.

We have a bit of a problem with that, we know the output stages of the 'modern' amplifier were push-pull UV845s with 1Kv on the anodes, capable of giving about 40 watts; but there was a response-correction network between the output transformer and the loudspeaker which threw most of it away. The earlier amplifier was far worse.

We are talking about very early experimental technology here - and my job is to allow it to be tested without damage.

Can you elaborate a little?

Wouldn't this modulate the RF stages with audio, and lead to instabilities?

All the more reason not to have a negative impedance amplifier.

My only knowledge of their use was in place of a transmission line repeater, and even then would only manage modest gain of 10dB or so.

A negative impedance amplifier is like to hoot and destroy your speakers whilst being set up.

Without any further knowledge of how the amplifier truly worked, I'm not sure how anyone here can give you a way forward. Do you have any schematics of the original circuitry?

So voltage driven. Even valve output stages can have low impedance though negative feedback.

The ratio of the two halves of the output transformer primary was chosen so that the current in one half produced equal core magnetisation to the current in the other half. The value of R was chosen so that the ripple it passed was the same as the ripple through the output pentode, so there was effectively a push-pull action at audio frequency. There was inevitably some audio power lost in R.

The transformer inductance and the resistor also assisted with smoothing the H.T. supply to the earlier stages. The H.T. smoothing capacitors were around 16 microfarads, so no A.F. found its way into the early stages.

You can imagine why the traditional English serviceman was baffled when he met a fault on this sort of circuit.

Why bother? With your numbers, the acoustic effects of the cable will be inaudible. Just adjust the volume control until it sounds right.

There are many protection circuits to make sure that any excessive power, no matter what the cause, will shut down the whole system.

We have enough information to know that reproducing the original circuit will only complicate the problem of measuring the performance of the loudspeaker. The idea is to drive it from a well-controlled modern source and make the necessary measurements, not to try to replicate the original operating conditions (however much fun that would be - with mercury arc rectifiers and a motor-generator set in the power supply).

This pre-dates Black's patent, there was no external feedback around the valves (only the unavoidable internal feedback due to the anode-cathode potential gradient of a triode).

We are trying to make objective measurements in addition to subjective listening tests, so it is worth going to some trouble to remove the effects of any external equipment from the equation.

Well, he should have had a cap of T...

On a sunny day (Mon, 28 Jan 2013 14:27:58 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

Yes, that was much later, after the war. AD9710 was sort of the standard for a while in the sities?

OK, well hope it works.

Maybe damgae can ooccur iof yo udrive one of them ancient ones from a low impedance, as it may not want to move a lot at some frequencies. Roll-surround came later ? (rubber hinged cone).

Been looking for a couple of Quad electrostats last year, ebay has some, refurbished. Expensive. I liked the sound, we had them in the TV studio audio as monitor too. And the amps, has soem tricks I copied for a RF design :-)

With all the things you are saying, why don't you just use a graphic equaliser. Are there any other forms of distortion you are trying to recreate? Such as 2nd harmonic? How will an negative impedance help you?

Be sure to add a soft limiter, so that the oscillation is sinusoidal

...Jim Thompson

Two reasans:

2) A 'graphic equaliser' doesn't equalise anything, it is a sound effects unit which has no place in objective audio measurement.

I am trying to measure how a loudspeaker responds to a known voltage across its voice coil, unfortunately this one cannot be taken anywhere for testing and the only practical way of making a connection to it is by means of a very long piece of wire. I want to remove the effect of the wire on the source impedance.

Thanks, that is an excellent idea because, even if I don't make it oscillate accidentally, someone else might feed in signals at an excessive level and cause damage before the protection circuits can operate. Two back-to-back yellow LEDs work a treat across a 600-ohm circuit at about 0dBm.

It is hard to tell without knowing your setup; but probably the impedance at main resonance is going to be several times of rated resistance. With this kind of coil resistance, the inductance of the coil is probably in mH range; so at kHz frequencies the impedance would be dominated by inductance.

Why 4-wire system can't be used?

You may need R-C and parallel C compensation to the feedback resistor.

VLV

The important thing is whether the impedance pf the external circuit drops below the negative impedance of the amplifier at any frequency. I agree with your assessment that the loudspeaker impedance will always be more than the rated resistance, so I should have nothing to worry about from that cause.

I have heard of acoustic reflections and resonances giving lower than normal impedances at some frequencies and the possibility of stored energy being dumped back into the amplifier. For this reason I intend to make the feedback adjustable, so that it is only sufficient to compensate for the circuit resistance and not for the resistance of the voice coil.

There are practical difficulties with getting the wires to where the loudspeaker is located. Ordinary house-wiring cable will have to be used for both the voice coil and for the field coil supplies because it may share a trunking with other mains wires. If unsreened and untwisted wire is use for the sensing circuit, it could pick up a lot of interference and I have to supply this system so that it can be installed and made to work immediately without any further alterations.

That is a possibility. I must try to ensure that there is only one dominant pole in the response.