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#1
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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? |
#2
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#3
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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! |
#4
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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 |
#5
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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. |
#6
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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." |
#7
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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 |
#8
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#9
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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 |
#10
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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? |
#11
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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 |
#12
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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 |
#13
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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. |
#14
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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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