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Old 23rd November 2012, 06:13 PM   #41
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That line looks smooth, but distortion is relevant in the 1/100 and up range, you cannot assess that visually on a scope. Actually I think I can see some Gm doubling there, it straightens out the sine curve near the crossover region so it looks more triangular.

Furthermore, say your VAS input has an impedance of 500k which is normal. Your miller cap is virtually multiplied by the voltage gain of your VAS, so your VAS gain rolloff corner is probably in the tens to hundreds of Hz. So for most of the audio band into RF, your input stage sees not what is in your scope shots, but that through a +6db/octave frequency filter. So the reality is worse. In LTSpice, you can plot this by entering D(R5) and you will probably see the crossover distortion now. Now you can compare it to an EF.
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Old 23rd November 2012, 06:21 PM   #42
bigwill is offline bigwill  United Kingdom
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keantoken,

The signal is across that 1k resistor that feeds the VAS buffer, which is after where the miller cap feeds back to the LTP - the voltage at that point would look very lowpassed I'm sure, since it's a current node, but this resistor should reflect to some degree the current being fed into the VAS transistor, I think?

I'm not speaking with any authority on this, I'm by no means an expert in solid state design.
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Old 23rd November 2012, 06:21 PM   #43
AndrewT is offline AndrewT  Scotland
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Bigwill,
how are you attaching your pics?
I can't see anything.
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Old 23rd November 2012, 06:24 PM   #44
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Bigwill,
how are you attaching your pics?
I can't see anything.
They're hosted on a popular image host - peculiar that they wont show for you - perhaps their domain isn't resolving for you?

I'll attach them directly to my post if necessary..

Last edited by bigwill; 23rd November 2012 at 06:29 PM.
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Old 23rd November 2012, 06:27 PM   #45
bigwill is offline bigwill  United Kingdom
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Here they are attached to diyaudio. It should be obvious which one is which
Attached Images
File Type: jpg 9cVsi.jpg (27.7 KB, 205 views)
File Type: jpg IZeLs.jpg (29.2 KB, 203 views)
File Type: png Screen Shot 2012-11-23 at 15.32.28.png (61.3 KB, 207 views)
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Old 23rd November 2012, 06:40 PM   #46
AndrewT is offline AndrewT  Scotland
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I can see these.
It will be my PC security blocking remote servers that are on an "avoid" list.
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Old 23rd November 2012, 06:42 PM   #47
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bigwill View Post
Note that the funny notches on the peaks of the sine wave are from the signal generator.
That's a common effect caused by synthesizing a triangular wave (that function generators produce as a base waveform) into a sine wave.
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Old 23rd November 2012, 06:44 PM   #48
AndrewT is offline AndrewT  Scotland
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why is the lower leg of the NFB taken to the -ve rail, rather than to the source return wire?
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Old 23rd November 2012, 06:47 PM   #49
bigwill is offline bigwill  United Kingdom
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why is the lower leg of the NFB taken to the -ve rail, rather than to the source return wire?
I used an electrolytic cap here - I just wanted to ensure it was charged to the correct polarity. I've always suspected there is a better way of doing this though!
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Old 23rd November 2012, 06:51 PM   #50
AndrewT is offline AndrewT  Scotland
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the -ve rail will inject all the rail rubbish into the -IN pin and it will get amplified to come out of the OUT PIN.
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