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Old 25th June 2012, 11:38 AM   #1
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Default reducing thd

here is part of a schematic i have seen where is a few words the author claims that tuning of the trimmer ( marked in blue ) by inputting a 20kHz sine wave and measuring the harmonic distortion on the output. When turning the pot you should see an increase or decrease in harmonic distortion, simply turn the pot until you find the minimum value obtainable.

would you like to comment the above practice ?

Kind regards
sakis
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Old 25th June 2012, 11:55 AM   #2
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R34 / C10 will inject high frequency signals from the supply rails into the input. I cannot say I am familiar with this but would suggest that the consequences will be heavily dependent upon
- Your specific rail decoupling
- The load you are driving, i.e. how much of the signal appears at the points driving R34
- layout and grounding

On this basis I would consider it somewhat hit and miss.

I would be interested in other interpretations as to it's use.

As an aside, R4 looks like it is there for balancing the differential pair - I wonder if the circuit would be better off with a current mirror as a load for the differential pair...
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Old 25th June 2012, 03:11 PM   #3
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interesting ....

Since this is a part of a big amplifier 300+ watts i wonder if such a circuit will add any goodies to a simply made amp ..
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Old 25th June 2012, 04:23 PM   #4
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Quote:
Originally Posted by rabbitz View Post
I'm not a fan of tube sound but do like the AKSA where I can dial in how much harmonics I like. The amount I use is much less than the genuine AKSA and heads towards the lower levels in the LF55. I'll be doing this with the B-AKSA as well.
Is this what rabbitz referred to?
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Old 25th June 2012, 04:36 PM   #5
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post a link ...i am not familiar with that
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Old 25th June 2012, 08:51 PM   #6
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odd almost 200 ppl seen that only one opinion ? I was expecting more ...
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Old 26th June 2012, 12:46 AM   #7
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You can click the > button after the quoted's name

Anyway, here's the link.
Based on Hugh Dean's AKSA 55

Last edited by jlester87; 26th June 2012 at 12:48 AM.
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Old 26th June 2012, 05:44 AM   #8
srh is offline srh  New Zealand
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Hi Sakis, Perreaux used that technique to null distortion in poweramps they manufactured in the 1980s.
Steve.
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Old 26th June 2012, 07:05 AM   #9
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I would be somewhat concerned about increased susceptibility to RF injection, since it's likely the same balance won't occur for all frequencies.

'Agree with googlyone that without a full understanding of the grounding and supply decoupling, the effectiveness would be uncertain.
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Old 26th June 2012, 07:13 AM   #10
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I completely agree with googlyone & Ian; IMHO far too many drawbacks for what might be a tiny improvment for a very specific working point.
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