Maker Pro
Maker Pro

Any experience with negative impedance?

R

Robert Baer

Jan 1, 1970
0
John said:
Why bother? With your numbers, the acoustic effects of the cable will be
inaudible. Just adjust the volume control until it sounds right.
Come now...let us not be practical..
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jan Panteltje said:
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.

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.

[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.

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.
 
F

Fred Bartoli

Jan 1, 1970
0
Adrian Tuddenham a écrit :
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.

Hmmm, Western Electric 15A folded horn by chance?
 
T

Tauno Voipio

Jan 1, 1970
0
On Mon, 28 Jan 2013 18:54:26 +0000,


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


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.
 
A

Adrian Tuddenham

Jan 1, 1970
0
John Larkin said:
[...]
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).

What is T&E?

"Twin and Earth" (wiring contractors' shorthand - sorry I didn't
explain).
Wire spacing makes inductance. Capacitance is probably negligable
here.

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

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.
 
A

Adrian Tuddenham

Jan 1, 1970
0
John Larkin said:
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.

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

This loudspeaker was made a long time before that. That's why they need
custom-built equipment and a lot of protection circuits to make sure it
cannot be damaged.
 
A

Adrian Tuddenham

Jan 1, 1970
0
Fred Bartoli said:
Adrian Tuddenham a écrit :

Hmmm, Western Electric 15A folded horn by chance?

Very nearly - it's a WE 555 driver but with a different horn.
 
G

Gerhard Hoffmann

Jan 1, 1970
0
Am 28.01.2013 11:38, schrieb Adrian Tuddenham:
The requirement is to design a specialist one-off audio amplifier to
drive a loudspeaker at the end of a long cable for experimental
purposes. The voice coil is nominally 15 ohms with a pure resistance of
10 ohms. The loop resistance of the cable will be somewhere between 2
and 4 ohms. It would be desirable to have a damping factor of 10 or
better, so I need to reduce the effect of the cable resistance in some
way.

Actively damping the bass chassis by removing all R can
produce a ridiculously low corner frequency for small speakers,
at the cost of efficiency and SPL.

I once have calculated a little bit around such a construction
but gave up when I noticed that the system is not even time-invariant.
I.e. after a drum kick the voice coil is really hot and its resistance
is _much_ higher than b4. A moving target if you want to zero it out,
and the combination of small chassis and low efficiency calls for lots
of power.

IIRC, one Mr. Stal brought the idea up. With a small circle above
the a in Stal.

regards, Gerhard
 
G

Gerhard Hoffmann

Jan 1, 1970
0
Am 28.01.2013 19:43, schrieb Adrian Tuddenham:
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.

Is that any different from a Kelvin connection?

Gerhard
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jon Elson said:
Adrian Tuddenham wrote:


WHAAAT? You are worried about damaging the historic speaker, and you are
going to be running the speaker wires in the same conduit as 220 V AC
wires? That would be illegal in the US on safety concerns, at the least.

It is legal in the UK as long as the low voltage circuit is insulated to
the same standard as the high voltage one.

I hope I shan't have to do it that way, but I must allow for the
possibility. The biggest danger would be an electrician muddling up the
power and signal circuits, but I shall be terminating the signal
circuits well away from any mains equipment, so only the central parts
of the run might be sharing a trunking with the mains.
 
A

Adrian Tuddenham

Jan 1, 1970
0
John Fields said:
On a sunny day (Mon, 28 Jan 2013 18:54:26 +0000) it happened
[email protected] (Adrian Tuddenham) wrote in
<1kxf84x.hhpl70mxxqt2N%[email protected]>:


On Jan 28, 2:38 am, [email protected] (Adrian
Tuddenham) wrote:
The requirement is to design a specialist one-off audio amplifier to
drive a loudspeaker at the end of a long cable for experimental
purposes.

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.

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.

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.

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.

[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.

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.
[/QUOTE]

http://www.poppyrecords.co.uk/other/images/Circuit1.gif

....but bear in mind that this is the first draft. It is bound to
contain mistakes and quite a few of the component values are still
unspecified.

The link for inserting an equaliser is between Rb and the 2k2 LIN gain
control, there will also be power connections for any op-amps which may
be necessary. The signal level at that point is nominally 0dBu.
 
R

Ralph Barone

Jan 1, 1970
0
Mike Perkins said:
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.


Would going 4-wire to the speaker and pulling the NFB from the speaker
terminals (essentially a Kelvin connection) help you any?
 
J

Jamie

Jan 1, 1970
0
Adrian said:
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 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
 
T

Tim Williams

Jan 1, 1970
0
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
 
A

Archimedes' Lever

Jan 1, 1970
0
2) is not an option: This is an historic loudspeaker and there will
have to be several extra safety circuits to integrate the amount of
power delivered in various frequency bands and trip out the amplifier
immediately if there is any danger of causing damage. We are being very
careful indeed - any mistake will be the last.



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.


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.
 
A

Archimedes' Lever

Jan 1, 1970
0
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.


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

Adrian Tuddenham

Jan 1, 1970
0
Ralph Barone said:
Would going 4-wire to the speaker and pulling the NFB from the speaker
terminals (essentially a Kelvin connection) help you any?

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.
 
A

Adrian Tuddenham

Jan 1, 1970
0
Jamie said:
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 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.
 
A

Adrian Tuddenham

Jan 1, 1970
0
Michael A. Terrell said:
Probably, but who knows for sure with a better description?

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.
 
A

Adrian Tuddenham

Jan 1, 1970
0
John Larkin said:
On Mon, 28 Jan 2013 10:38:23 +0000,


If you are running 3-wire T&E, you can do remote sense, analogous to a
3-wire RTD. Signal, return, sense. The sense wire gives you half of
the total wire drop, so gain up by 2 and add that to the outgoing high
side signal.

The return wire *is* the sense shunt.

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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