Any experience with negative impedance?

Sounds like Romex, a pair and a third ground.

--

John Larkin Highland Technology Inc

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Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

I really don't understand the need to monitor the end point of the transmission line? If you were to loop back a pair of wires only to be used to control feed back grain in the amp, it would not make any difference, the amp circuit would simply increase the gain to force the end point to match the desired set point at the source.

Simply put, if you just turn up the volume, you'll end up with the same results with out all the bull crap, also, doing that feed back like that is only going to introduce more problems.

The only part of this that I can see where you think you may need this kind of control is if the transmission line pairs for the speaker get hot and thus increase in R which will reduce the end point? If that is the case, then you are not using large enough wire or going at it totally wrong.

There is a reason why they make high voltage audio systems.

Jamie

Question: since you've established the goal is characterising an existing speaker and its acoustic environment, shouldn't the test be performed with period equipment? Or, given the availability and known limitations of that, then a modern equivalent designed to emulate key features?

If they're going for historical accuracy, I expect the original installation contended with the same wire length already, plus a tube amp, evidently one without the benefit of NFB by your description (typical triode amps achieve a damping factor of about 3, not counting the transmission line of course). You should be adding resistance, quite a bit by modern standards.

I'm sure you've already belabored all this to the client, so I'd just be curious to know the reasons behind it.

Tim

-- Deep Friar: a very philosophical monk. Website:

I had a 15" 'juke box' loudspeaker back in the seventies that had an extra coil in it to enhance the magnet's field.

I thought it only ever had a DC voltage on it when it was put to use. The speaker worked fine without the fixed coil being excited too.

Probably, but who knows for sure with a better description?

Forgot to mention that the speaker itself was from the 50s.

The impedance of the loudspeaker varies over the audio range, so the current will not be constant. This means that the volt-drop will vary with frequency and cannot be compensated by just turning up the gain.

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~ Adrian Tuddenham ~ 
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It would be very difficult to run an extra pair of sensing wires in these particular circumstances. I would like to contain the solution within the amplifier circuit if at all possible.

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~ Adrian Tuddenham ~ 
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T&E A pair of separately-insulated power wires sitting each side of a bare earthing wire, sheathed overall in (usually) heavily filled abrasion-resistant PVC. The overall shape is oval in cross section.

UK regulations specify brown for live, blue for neutral, and the earthed wire must be sleeved with green and yellow striped sleeving wherever the sheath is peeled back. Normal domestic grades have 1.0 sq.mm conductor cross sectional area for individual lighting circuits, 1.5 sq.mm for lighting mains and 2.5 sq.mm for power ring mains.

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~ Adrian Tuddenham ~ 
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I am kicking myself for not having considered that when I ran through the possibilities at the outset. It would solve the difficulty of having to set up the system again if the wire lengths were altered - and it could easily be strapped with a moderately low value resistor at the amplifier end so that it wouldn't run amok if the sensing wire became disconnected.

The only possible disadvantage would be if the connections became screwed up by the installer at the loudspeaker end of the circuit or if some busybody insisted that the 'earth' wire had to be earthed in order to comply with some obscure regulation.

Thank you for a very constructive suggestion.

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~ Adrian Tuddenham ~ 
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The performance of the loudspeaker is my client's main interest. We can investigate whether it changes with driving impedance at a later date, but extablishing the starting point, uncluttered by other variables, has to be the first move.

Although we know the circuits of two out of the three original amplifiers (different amplifiers were used at different times), we are missing key data such as output transformer ratios and construction. With triode output valves and no overall feedback, the transformer ratio is going to have a big influence on the output impedance and general performance of the amplifier.

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~ Adrian Tuddenham ~ 
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On a sunny day (Mon, 28 Jan 2013 23:30:07 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

An other thing, from experience: You are driving a high gain opamp differentially from an unknown impedance source (input connector).

Maybe an input transformer and a normal transistor input stage would be better. That stage should be designed so its maximum AC output can never send the TDA power amp into clipping. The idea is that with an 'unknown' impedance at the input you cannot guarantee the input circuit input will never oscillate if the input is part of the feedback circuit of the opamp. If you use transformers you need to shield against hum pickup (rotation, mu-metal).

I do not know this TDA7295, I have used the TDA7294, it is a very stable MOS amp that does not need the usual RC filter to ground at the input.

mm was year 2000, time flies, amp still working :-) I drive heavy inductive loads via a step up transformer like my cryo-cooler with it...

Best of luck:-)

I wote:

usual RC filter to ground at the input.

--------------------------------------------------------------------------------------------------------------^^^^^ Ooops, should be output

Something like this?

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~ Adrian Tuddenham ~ 
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Are you trying to null the speaker or the wiring?

I am trying to keep the price down, otherwise I would have preferred to ues an input transformer. This equipment is intended to be used with very short input connections, so balanced line isn't necessary. The sources will probably be a computer and a professional grade radio tuner.

I can't see anything in my input circuit which would lead me to expect oscillation with any source impedance from fully inductive to fully capacitive. Could you explain why you think this is possible?

The amplifier circuit has now been upgraded:

I have downloaded your data sheet and cannot see any difference between the TDA7294 and the TDA7295. I'm sure there must be a difference, but I cannot see it.

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~ Adrian Tuddenham ~ 
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Only the wiring, but the volt-drop of the wiring depends on the impedance of the loudspeaker, which varies with frequency. I have to find a way of continuously monitoring the volt drop* and adding it to the amplifier signal.

*Either by actual measurement or by eztrapolation from some other related parameter.

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~ Adrian Tuddenham ~ 
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On a sunny day (Tue, 29 Jan 2013 19:42:03 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

Well, how about a combination of those? And do not forget the usual mains plugged into it. Series resistors, limiting diodes?

What I expected to see was some relay that only connects the speaker after the output was found to be OK.

I don't know either, been 14 years, it is a good chip, the TDA7294.

As to 'as cheap as possible'? (was that what you mentioned?) seems to conflict with 'research project'. Component price usually is not a point with one of designs, it is the hours.

Other important thing: I do not see any RF filters in the input.

I had to decouple each and every input including the mains to my mixer in Amsterdam, as the electric tram drivers radio came right through (AM detection in input circuit). We had that happen in a hall a couple of times. Really is a distraction...

I see you stabilized the + and - 30 to the power amp, loop bandwidth big enough? Those are slow transistors... Or is it just over-voltage protection against high mains? Sometimes a simple emitter follower with cap and zener to ground can function as C multiplier and give extra filtering over a wide band, and limit voltage at the same time. OK I will shut up now ;-)

Can you measure the speaker/wiring? How about a DSP with the (complex?) inverse transfer function of the wiring, before the power amp?

Exactly. Do it in the digital domain. It won't be perfect but it might get you close enough.