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Old 5th October 2012, 07:24 AM   #2251
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Increasing the Rs size definitely cleaned things up, but it sounded compressed, without any soundstage. Sweet spot is elusive
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Old 5th October 2012, 08:43 AM   #2252
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Quote:
Originally Posted by lhquam View Post
I am referring to AC voltage between the winding and ground (or other windings) not the AC voltage across the winding.

By Gain, I am referring to the closed-loop gain of the amplifier. Actually, the ratio of Vout to the voltage into the top of the driven primary winding.
Thx for precision.
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Old 5th October 2012, 10:36 AM   #2253
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Originally Posted by generg View Post
.....the problem could be that two perfect matched halves produce mostly k3 and no k2 and a slight mismatch will care for the k2 part. Of course absolute distortion is growing in this way, but the SemiSouth have so much less distortion compared to the IRF that you can risk some more k2 distortion to tailor the sound!

you must only find a way to get the offset of about 200mV back to zero but keep the distortion spectra.... or take a cap!
This could also be achieved by adding a small extra parallel [variable] current source that imbalances the balanced outputs to get just the trifle of H2 you like.
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Old 5th October 2012, 10:39 AM   #2254
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#2074 :

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why offset , when you can add SE bias?
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Old 5th October 2012, 04:58 PM   #2255
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Is running primary in series a dumb idea?
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Old 5th October 2012, 05:02 PM   #2256
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It reduces open-loop gain by a factor of 2 and changes the transformer impedances by a factor of 4. I cannot see any advantage for doing it.

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Is running primary in series a dumb idea?
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Old 5th October 2012, 06:16 PM   #2257
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Quote:
Originally Posted by lhquam View Post
After switching back to my F5 balanced with minimal THD, I am less happy with it now. High frequencies are too too sharp.

By first F5 build had badly mismatched FETs. But there was a satisfying sweetness to its sound that I have not been able to replicate. At that time I did not have equipment to measure the harmonics, so I do not know what the harmonic mix might have been. So far, my tweaking of the Teaser-6 does not come anywhere near to that sound.
lhquam: A method to tackle what you are doing is to start with an amp which already has an accredited sweet spot and then deny it [and yourself]; but of course reversibly, and at will. For example; you seem happy with your F5. I gather [implicitely] from your reports that it operates at its sweet spot. So here is a proposed approach to unravel the role of the Jensen transformer and loop feedback as potential variables affecting the sweet spot.

The attached file ModF5.pdf shows a proposed modification of the F5 schematic which I copied from the attendant article by Mr. Pass. It is not F5; but a tool to study the suspect variables above. I cut the pristine schematic of F5 at selected spots by using a razor blade. I then inserted the Jensen transformer plus other passive components, and finally spliced everything back together. It may need further tweaking; but I hope that my surgery was least intrusive so as to "preserve' the parent sweet spot. The mod circuit has a Zen Pot [P3; is it the sweet spot generator?], and the bias of the output stage is essentially unperturbed. I am not sure about the components in the NFB path; but they can be adjusted to suit.

The schematics [ConceptualF6#0,1,2.pdf] which I proposed in an earlier post several days ago can also be used as tools to study the effects of above suspect variables. The parent Zen amps are already accredited with "sweet spot" musicality.
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Old 5th October 2012, 06:39 PM   #2258
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Default Proposed alternate biasing scheme for Conceptual F6

Dear DIYers. The subject is attached. I borrowed the thought process to write it from the schematic of F5. May have DC servo-action; not that your current designs [Teaser-6, Franken6 etc.] need auto offest control.
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File Type: pdf ConceptualF6 Bias.pdf (36.3 KB, 62 views)
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Old 5th October 2012, 09:32 PM   #2259
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Quote:
Originally Posted by lhquam View Post
I am not sure that the ratio of H2 to H3 can be held constant across frequency with H2>H3.
You also will have difficulty trying to keep the ratio constant with different
amplitudes. Probably best to select for your listening level.

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Old 5th October 2012, 09:50 PM   #2260
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Default End the Agony

Nelson

How about ending our collective misery and releasing the F6 article, please. 2000 plus replies is surely enough agony.
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