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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 27th October 2012, 12:12 AM   #28501
jan.didden is offline jan.didden  Europe
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Actually in a way that phase modulation is quite easy to measure for all of us:
Measure the phase shift between input and output with zero Vout.
Then insert a DC offset of a few volts, and again measure it; you'll find it most probably has changed.
One typical table from his paper:

DC offset at the output Phase shift @ 15kHz
-2V -25 degr
-1V -24 degr
0V -23 degr
+1V -22 degr
+2V -21 degr

So there's you varying phase shift with output level.
Fascinating, isn't it?

jan
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Old 27th October 2012, 12:21 AM   #28502
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John Curl's Blowtorch preamplifier part II
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Originally Posted by janneman View Post
Measure the phase shift between input and output with zero Vout.
How do you accomplish that at zero volts out?
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Old 27th October 2012, 12:22 AM   #28503
Joachim Gerhard is offline Joachim Gerhard  Germany
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Old 27th October 2012, 12:39 AM   #28504
Wavebourn is offline Wavebourn  United States
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John Curl's Blowtorch preamplifier part II
Jan;

I can tel you the similar way to catch a mice. When it is under the stool take a saw and quickly cut stool's legs.

1. Cut the 1'st leg 0.5 centimeters
2. Cut the 2'nd leg 0.5 centimeters
...
5. Go to 1 and repeat until the space is too small for the mice to escape.
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Old 27th October 2012, 12:44 AM   #28505
RNMarsh is offline RNMarsh  United States
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John Curl's Blowtorch preamplifier part II
Quote:
Originally Posted by janneman View Post
One typical table from his paper:

DC offset at the output Phase shift @ 15kHz
-2V -25 degr
-1V -24 degr
0V -23 degr
+1V -22 degr
+2V -21 degr

So there's your varying phase shift with output level.
Fascinating, isn't it?

jan
Facinating! OMG. Maybe what M.Otala and JC etal has been talking about is true? And, what would that affect do to the clarity of the sound?

I wouldn't be surprised to find that this affect has also been heard and described by careful listeners without knowing what it was, exactly. And, why certain designs sound better and thus contributes to why JC does design the way he does? OMG.
 
Old 27th October 2012, 12:50 AM   #28506
Wavebourn is offline Wavebourn  United States
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John Curl's Blowtorch preamplifier part II
What is fascinating?

Did you hear SSB station on AM radio? How does it sound?

Similarly, very small amount of phase intermodulation causes "harsh" highs and fuzzed stereo image. It's one of reasons why measurements of THD and IMD don't tell how good will be the sound.
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Old 27th October 2012, 12:51 AM   #28507
kevinkr is offline kevinkr  United States
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John Curl's Blowtorch preamplifier part II
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Originally Posted by SY View Post
How do you accomplish that at zero volts out?
Phase shift is referred to changing levels of DC offset at the output. (Not changing levels of AC signal amplitude) This apparently is pretty typical behavior for op-amps operating on relatively low supply voltages, shifts internal operating points, etc. (Look at GBWP and phase margin at different supply voltages for a hint)
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Old 27th October 2012, 12:57 AM   #28508
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by SY View Post
How do you accomplish that at zero volts out?
Zero V DC. You need to have a small signal riding on the DC offset to be able to measure the phase shift.
Of course.

jan
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Old 27th October 2012, 01:00 AM   #28509
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by kevinkr View Post
Phase shift is referred to changing levels of DC offset at the output. (Not changing levels of AC signal amplitude) This apparently is pretty typical behavior for op-amps operating on relatively low supply voltages, shifts internal operating points, etc. (Look at GBP at different supply voltages)
Except that the opamps all measured essentially the measurement floor. Even the lowly TL074. Low ol bandwidth, very high feedback factor, go figure.

jan
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Old 27th October 2012, 01:01 AM   #28510
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by RNMarsh View Post
Facinating! OMG. Maybe what M.Otala and JC etal has been talking about is true? And, what would that affect do to the clarity of the sound?

I wouldn't be surprised to find that this affect has also been heard and described by careful listeners without knowing what it was, exactly. And, why certain designs sound better and thus contributes to why JC does design the way he does? OMG.
Ohh yes of course it is true.
But not where we thought it was.

jan
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