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Default Null speaker test

Years ago I read about a test where 2 amps could be connected to a single
speaker and if there were any differences the speaker would generate sound.

Does anyone know of this test and can you give details?


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sane54
 
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What do you mean by a 4 ohm series resistor? Is it a box I plug the
speaker wire into on it's way to the speaker cabinet or part of the
speaker itself? sorry for my ignorance!

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Phil Allison
 
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"

Years ago I read about a test where 2 amps could be connected to a single
speaker and if there were any differences the speaker would generate
sound.

Does anyone know of this test and can you give details?



** If you drive the left and right channels of a stereo amp with the same
signal, then ideally there should be no difference in the outputs at the
speaker terminals. A speaker connected across the two "hot" terminals
should be *silent*.

In fact, that outcome is impossible do to component * tolerances* in the
resistors and capacitors that set the gain and LF and HF roll offs in each
channel.

Even a 1% gain mismatch will mean a difference signal appears across the
outputs that is 40 dB down from the normal one.




.......... Phil


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William Sommerwerck
 
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Phil Allison" wrote in message
...

"


Years ago I read about a test where 2 amps could be connected to a single
speaker and if there were any differences the speaker would generate
sound.


Does anyone know of this test and can you give details?


** If you drive the left and right channels of a stereo amp with the same
signal, then ideally there should be no difference in the outputs at the
speaker terminals. A speaker connected across the two "hot" terminals
should be *silent*.


In fact, that outcome is impossible do to component * tolerances* in the
resistors and capacitors that set the gain and LF and HF roll offs in each
channel.


Even a 1% gain mismatch will mean a difference signal appears across
the outputs that is 40 dB down from the normal one.


Phil Allison's answer is correct as far as it goes. However, I believe the
test was intended to see if there were any differences between two different
amplifiers -- a Crown and an ARC, say -- not the channels of a single stereo
amplifier.




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Scott Dorsey
 
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In article .com,
sane54 wrote:
What do you mean by a 4 ohm series resistor? Is it a box I plug the
speaker wire into on it's way to the speaker cabinet or part of the
speaker itself? sorry for my ignorance!


It's a power resistor, in series with the speaker, to keep the two
amps from fighting one another too badly.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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anahata
 
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Scott Dorsey wrote:
Sure. You take the output of amp A, and you run it through a 4 ohm
series resistor into the speaker. Then you take the output of amp B
and you run it into the speaker the same way. Then you drive the two
with opposite polarity sources.


Surely you get the same result much more simply by connecting the (+)
terminal of the speaker to one amplifier and the (-) terminal to the
other, and drive the apmlifiers with same polarity signal (allowing for
gain differences).

That's assuming the amp outputs are single ended and the the cold sides
are grounded or connected to each other to complete the circuit.

--
Anahata
-+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
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anahata
 
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Ther impedance would vary by the amount of level difference.
If the amps were totally equal, there would be infinate impedance,
and thats not the way to test an amplifier to promote its
shorcommings.
greg


What you're saying is that the amplifier should be driving a realistic
load for a test to be maningful, and of course you're quite right.

This kind of test has been done before, by potting down the amplifier
output to the same level as the input and using a differencing amplifier
to compare the result with the input signal. You put the "real load"
speaker in another room where you can't hear it, and listen to the
output of the differencing amplifier.

Once HF phase shifts and gain had been balanced out, the result in the
tests I've read about was silence. It was also instructive to
investigate the dramatic sudden noises that would occur in the
difference signal if the amplifier under test was driven to clipping, or
if it was class B and the quiescent current reduced to produce crossover
distortion.

Anahata
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My Last Sigh
 
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I would use an oscope at the amp outputs.....the variances suggested here
(cables, resistors, stray capacitances) would be tot great to really
determine if the amp was truly balanced....

" wrote in message
link.net...
Years ago I read about a test where 2 amps could be connected to a single
speaker and if there were any differences the speaker would generate
sound.

Does anyone know of this test and can you give details?





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Phil Allison
 
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"anahata"


What you're saying is that the amplifier should be driving a realistic
load for a test to be maningful, and of course you're quite right.

This kind of test has been done before, by potting down the amplifier
output to the same level as the input and using a differencing amplifier
to compare the result with the input signal. You put the "real load"
speaker in another room where you can't hear it, and listen to the output
of the differencing amplifier.

Once HF phase shifts and gain had been balanced out, the result in the
tests I've read about was silence. It was also instructive to investigate
the dramatic sudden noises that would occur in the difference signal if
the amplifier under test was driven to clipping, or if it was class B and
the quiescent current reduced to produce crossover distortion.




** The only folk to ever carry out the above ( it is very tedious to do )
and publicly demonstrate the result was the UK Quad firm. The test
procedure was suggested by Peter Baxanadall:

http://www.dself.dsl.pipex.com/ampin...o/subjectv.htm

Quad demonstrated their model 303 amp as passing the test of "no audible
distortion".



.......... Phil







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anahata
 
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Phil Allison wrote:

** The only folk to ever carry out the above ( it is very tedious to do )
and publicly demonstrate the result was the UK Quad firm. The test
procedure was suggested by Peter Baxanadall:

http://www.dself.dsl.pipex.com/ampin...o/subjectv.htm


I had a feeling Baxandall was involved in that. Thanks for the link.

--
Anahata
-+- http://www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827
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Arny Krueger
 
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"anahata" wrote in message

Ther impedance would vary by the amount of level
difference. If the amps were totally equal, there would
be infinate
impedance, and thats not the way to test an amplifier to
promote its shorcommings.
greg


What you're saying is that the amplifier should be
driving a realistic load for a test to be maningful, and
of course you're quite right.


As appropriate test load can be an electrical network, but
it shouldn't be just a resistor or an open circuit.

This kind of test has been done before, by potting down
the amplifier output to the same level as the input and
using a differencing amplifier to compare the result with
the input signal.


An appropriate pot can be very linear. Standard high-grade
5K 2-watt composition pot still costs less than $10, is
readily available and can really work! Comparisons involving
a lack of inversion can put a premium on the sensing
amplifier's common-mode rejection.

You put the "real load" speaker in
another room where you can't hear it, and listen to the
output of the differencing amplifier.
Once HF phase shifts and gain had been balanced out, the
result in the tests I've read about was silence.


It's only silence until you amplify the difference enough.
Our ability to amplify small signals exceeds our ability to
balance out real-world equipment.

It was also instructive to investigate the dramatic
sudden
noises that would occur in the difference signal if the
amplifier under test was driven to clipping, or if it was
class B and the quiescent current reduced to produce
crossover distortion.


The whole Class B crossover distortion issue disappeared
once people figured out how to make thermally-compensated
bias networks do what they should do. A lot of simple and
complex things have been tried over the years, but it turns
out that what works is pretty simple. You just have to get
the parameters right.


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John O
 
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Default Null speaker test

What do you mean by a 4 ohm series resistor? Is it a box I plug the
speaker wire into on it's way to the speaker cabinet or part of the
speaker itself? sorry for my ignorance!


You would have to wire this circuit with bare components, or some other
resistor. See this: http://www.doctronics.co.uk/resistor.htm

-John O


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