R
Robert Baer
- Jan 1, 1970
- 0
Come now...let us not be practical..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..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.
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.
[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.
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.
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.
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?
Wire spacing makes inductance. Capacitance is probably negligable
here.
Right, an extra series field winding can make a DC generator have
negative impedance.
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.
Fred Bartoli said:Adrian Tuddenham a écrit :
Hmmm, Western Electric 15A folded horn by chance?
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.
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.
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.
[/QUOTE]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.
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.
I really don't understand the need to monitor the end point of theAdrian 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.
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.
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?
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.
Michael A. Terrell said:Probably, but who knows for sure with a better description?
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.