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Old 25th November 2012, 03:57 PM   #3101
lhquam is offline lhquam  United States
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That will not work properly because the two primary windings are seeing different voltages across them and will "fight" each other. Assume that the feedback node has voltage Vfb. The voltage across the upper primary is In+ - Vfb. The voltage across the lower primary is Vfb - In-.

If In- = - In+, then clearly the voltages across the windings are different unless Vfb=0.

If we believe that the total flux in the transformer is the sum of the contributions of the two primary windings, then the total flux will be proportional to (In+ - Vfb) + (Vfb - In-) = In+ - In- and the feedback has no effect.

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coffee is brewing , now ... so take this with grain of salt
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Old 25th November 2012, 04:31 PM   #3102
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I can't give you exact recipe , not having experience with exact Jensen type ;

whatever - speaking strictly of xformer - loading each secondary with nominal impedance (150R) will result in better bandwidth

be aware that Jensen guys made it exactly that way

don't think even for a sec that Pa doesn't know that - he just took different approach - to correct entire amplifier , killin' seven flies with one blow

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Mr. Pass and Mr. Zen Mod. Is there a transformer which will present a 20 KHz bandwidth in OLG F6, and will it improve performance of CLG F6?. The OLG F6 circuit is a specific and useful test method to find such a transformer. Plot OLG versus Frequency for each TUT.
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Old 25th November 2012, 04:34 PM   #3103
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No doubt there is such, but you are likely to find that some other parameter
has been traded for it.

Did you read the transcript of the BAF talk on the F6?

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Old 25th November 2012, 04:48 PM   #3104
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Quote:
Originally Posted by Nelson Pass View Post
No doubt there is such, but you are likely to find that some other parameter
has been traded for it.

Did you read the transcript of the BAF talk on the F6?

Thank you Mr. Pass. I'll read it slowly and carefully on your FW site. I have the weakness of speed reading.

N.B. Got the point. The Jensen in F6 is already a rarity per its writeup. Others which may push a higher OLG bandwidth will be unobtainium and prohibitively expensive.

Last edited by Antoinel; 25th November 2012 at 04:58 PM. Reason: Tweak
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Old 25th November 2012, 05:05 PM   #3105
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6moons audio reviews: FirstWatt F6
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Old 25th November 2012, 05:23 PM   #3106
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Thanks Zen Mod. I also found it on the FW site. I will have to read it many more times to absorb it and fully appreciate its content. It is a very well written seminar/article. The writing style of Mr. Pass like his past articles flowed smoothly, was clear, and he loaded it with valuable present and future information. As written, it is a technical feast for the mind of DIYers.
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Old 25th November 2012, 09:16 PM   #3107
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The transformer coupling in the F6 feedback loop cause the phase of the 2nd harmonic to vary with frequency. Here are THD vs Freq and Phase vs. Freq plots for the F6 and the F5. Not how much greater the phase shift is for the F6, particularly in the mid frequencies.

I suspect that this changes the sonic signature of the amplifier.
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File Type: jpg F6-thd-phase-sweep-p01.jpg (109.3 KB, 593 views)
File Type: jpg F5-thd-phase.jpg (107.5 KB, 569 views)
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Old 26th November 2012, 02:03 PM   #3108
lhquam is offline lhquam  United States
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In my previous post, the F5 THD was so low that the phase change was totally determined by the MOSFET gate capacitances.

Here is the same F5 circuit with P3 adjusted for 0.025% 2nd harmonic. As you can see, the phase is essentially constant to 20kHz.
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File Type: jpg F5-thd-phase-2.jpg (109.3 KB, 495 views)
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Old 26th November 2012, 02:38 PM   #3109
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Interesting thing would be to listen to both circuits adjusted for similar harmonic structure and see if they sound the same.
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Old 26th November 2012, 03:27 PM   #3110
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It would be hard to adjust the F5 to have 3rd harmonic high enough or
to have the H2/H3 vs frequency at all similar to the F6.

What would be interesting is to see H2, H3 plots with phase for some vacuum tube amps with sonic signatures that are at all similar to the F6.

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Interesting thing would be to listen to both circuits adjusted for similar harmonic structure and see if they sound the same.
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