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Old 13th December 2012, 03:35 PM   #41
Merlinb is online now Merlinb  United Kingdom
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Quote:
Originally Posted by scott17 View Post
Here is an example at 1KHz of a push-pull 6CA7 in UL mode. The 2nd and 3rd are about equal, although pretty low.
Actually that is labeled wrong.
The top plot is without feedback, the bottom plot is with -5dB GNFB applied.
Odd that the 4th and 5th harmonic appear to be unaffected by GNFB?
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Old 13th December 2012, 03:43 PM   #42
rongon is offline rongon  United States
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Quote:
Originally Posted by scott17 View Post
Here is an example at 1KHz of a push-pull 6CA7 in UL mode. The 2nd and 3rd are about equal, although pretty low.
Actually that is labeled wrong.
The top plot is without feedback, the bottom plot is with -5dB GNFB applied.
That's pretty interesting!

In the top plot, I see the 2HD w/o GNFB at a little over -70dB.

With -5dB of GNFB the 2HD is lowered by about what, 3dB?
But the 3HD is barely touched.
I also notice that higher order harmonics go up a bit (way higher, like 7, 8).

In that particular amp, it looks like adding -5dB of GNFB is ' ton got'.

Interesting...
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Old 13th December 2012, 03:44 PM   #43
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Obviously 4th and 5th are already there at the input signal.
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Old 13th December 2012, 03:53 PM   #44
Cassiel is offline Cassiel  Libya
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Well, I will tell my experiences, even if it doesn't really add too much weight to the discussion. I don't like UL. Pentodes have their good things and their bad things, triodes have their good things and bad things. UL? Not great virtues or defects just....a compromise. A middle point. I'm just not that type of guy.

The amp I listen most is a PP triode amp with no global feedback loop; mind you, a little feedback from the output to the input doesn't 'destroy' the sound, it sounds great too with it but mine, for the moment, stays straight. No corrections. Bass is great (unlike my SE amps), it has a natural sound.... low power but it all depends on the speakers.

And now about pentode amps with global feedback. The ones I've built myself didn't sound that great but I have listened to commercial ones and liked the sound so I don't blame pentodes or feedback, I blame myself because, as it was said before, global feedback is tricky and I just can't get it right. No problem though becasuse triodes are easy and sound great.
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Old 13th December 2012, 03:57 PM   #45
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Quote:
Originally Posted by artosalo View Post
Obviously 4th and 5th are already there at the input signal.
You are probably correct. I did perform a baseline test and I saved the numerical data, but for some reason I did not save the plot. I'll have to do that now just to satisfy my curiosty.
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Old 13th December 2012, 04:01 PM   #46
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Quote:
Originally Posted by rongon View Post

...In the top plot, I see the 2HD w/o GNFB at a little over -70dB.

With -5dB of GNFB the 2HD is lowered by about what, 3dB?
But the 3HD is barely touched.
I also notice that higher order harmonics go up a bit (way higher, like 7, 8).

In that particular amp, it looks like adding -5dB of GNFB is ' ton got'.

Interesting...
I looked at the "white results"

Without feedback: 2nd -63 dB, 3rd -65 dB, 4th -85 dB 5th -80 dB
With 5 dB feedback: 2nd -73 dB, 3rd -68 dB, 4th -86 dB, 5th -79 dB

NFB improved: 2nd 10 dB and 3rd 3 dB.
There is practically no improvement at 4th and 5th and I assume that these harmonics are already at the input signal.
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Old 13th December 2012, 04:26 PM   #47
rongon is offline rongon  United States
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That is interesting. A just at the edge of noticeable improvement in 3H; -10dB improvement in 2H.

I see an increase in 9H, 10H (I think). Do you see that too?
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Old 13th December 2012, 04:32 PM   #48
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Those (9, 10) are at such a low level though. Could be just about anything.

I'll perform a second set of measurements, with the baseline, tomorrow.
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Old 13th December 2012, 04:35 PM   #49
rongon is offline rongon  United States
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The obsessive audiophool argument against gNFB is that it lowers lower order harmonics, but raises higher level harmonics, which are 'psychoacoustically audible' even at low levels, because they are 'so unnatural sounding.' Not that I buy that completely, but it would be interesting if the 9th and 10th harmonics came up in level as a result of adding only -5dB of gNFB.
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Old 14th December 2012, 05:20 AM   #50
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Quote:
Originally Posted by rongon View Post
The obsessive audiophool argument against gNFB is that it lowers lower order harmonics, but raises higher level harmonics, which are 'psychoacoustically audible' even at low levels, because they are 'so unnatural sounding.....
With tubes this is a myth.

Below are test results I got a few years ago with a 15 W UL amplifier.
The plot shows the level of the harmonics vs. GNFB


Click the image to open in full size.


Click the image to open in full size.
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