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lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
The resistance of the pot isn't the main issue here because the impedance being driven is quite high: the 10K resistor (R5 or R6), the output FET gate capacitance, and the transformer interwinding capacitances. What you need to consider is the AC voltage between the FET source pin and the wiper of the pot. That voltage will be detemined by the AC current thru the source resistor times the source resistance times the wiper position, measured 0.0 at the source end to 1.0 and other end.

Quote:
 Originally Posted by flocchini OK Guys - I think I am being stupid here. I have been varying R13,R14 with fixed resistors in the range of .0125 to .1 ohms. I think I am getting a feel of what changes can happen and what I like. Then I think about paralleling a 10 ohm pot with a .05 ohm fixed resistor and using that to tune in system. Doing the parallel resistor calculation with pot at low extreme (0 ohms) parallel combo should be zero at high extreme (10 ohms) it should be .049 ohms . Now parallel 1 ohm with .05 and you get .047. The remaining numbers (3-9 ohms) are between .047 and .049. Doesn't that say that the major variation in R13 and R14 (paralleled) is in the first few turns of a 15 to 20 turn pot and wouldn't it be better to just use fixed resistors or fixed resistors with a rotary switch ??? What am I missing? Best Bob

 11th December 2012, 01:38 AM #3252 buzzforb   diyAudio Member     Join Date: Oct 2010 Location: Burlington, NC I was saying that a lower R pot would have more gradual change vs very sudden change of 10R pot. I dont even know if you can get 1R pots. __________________ ...Shape the sound , Man!
 11th December 2012, 01:51 AM #3253 flocchini   diyAudio Member   Join Date: Feb 2004 Location: Davis,CA thanks for the input lhquam and buzzforb - I will try to take that AC reading
 11th December 2012, 03:42 AM #3254 Zen Mod   Official Court Jester diyAudio Member     Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain if pot value is held in reasonable range , it's resistance is irrelevant - it can be , presumably,in range of 5-50R -without any practical difference it's here to take voltage sample of AC fluctuation across Rs of 0R1 , which can be set between 0 and 100% __________________ my Papa is smarter than your Nelson ! clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
 11th December 2012, 03:49 AM #3255 flocchini   diyAudio Member   Join Date: Feb 2004 Location: Davis,CA Thanks also to ZM - it is sinking in
 13th December 2012, 12:52 AM #3256 flocchini   diyAudio Member   Join Date: Feb 2004 Location: Davis,CA To see if I understand this correctly- if the 10 ohm pot ( variable 2nd harmonic) is set (totally CCW in my case) such that "all" AC current from cap will flow though the resistor and to the JFET source is this equivalent to the Output degeneration schematic - that is equivalent to having just the fixed resistor? Now if the pot is set to the opposite extreme (totally CW) so that the flow is directly to the source pin - is this equivalent to the naked F6? or is there degeneration just because a resistor is attached? If using the pot should both R13 and R14 be used. Latest report To my ears and in my system R14 =0 and R13 =.025 is the most listenable. Nice wide soundstage, good punch and excellent clarity Best
 13th December 2012, 01:35 AM #3257 buzzforb   diyAudio Member     Join Date: Oct 2010 Location: Burlington, NC Never got that low up top, but i stopped at bypasssing lower Rs and top Rs -.15R __________________ ...Shape the sound , Man!
lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Yes, that is essentially correct.

The simplest way to think about the role of the pot is as follows:
• Let X be the ratio of the wiper-to-source resistance to the end-to-end pot resistance.
• The effective AC degeneration resistance is X times the source resistance.
Quote:
 Originally Posted by flocchini To see if I understand this correctly- if the 10 ohm pot ( variable 2nd harmonic) is set (totally CCW in my case) such that "all" AC current from cap will flow though the resistor and to the JFET source is this equivalent to the Output degeneration schematic - that is equivalent to having just the fixed resistor? Now if the pot is set to the opposite extreme (totally CW) so that the flow is directly to the source pin - is this equivalent to the naked F6? or is there degeneration just because a resistor is attached? If using the pot should both R13 and R14 be used. Latest report To my ears and in my system R14 =0 and R13 =.025 is the most listenable. Nice wide soundstage, good punch and excellent clarity Best

 13th December 2012, 03:20 AM #3259 flocchini   diyAudio Member   Join Date: Feb 2004 Location: Davis,CA thanks lhquam
flocchini
diyAudio Member

Join Date: Feb 2004
Location: Davis,CA
Quote:
 Originally Posted by buzzforb Never got that low up top, but i stopped at bypasssing lower Rs and top Rs -.15R
to my ears and my experiments the .1 ohm and even the .05 ohm resistors made the sound flatter.

I really like this amp. I am using it to drive my midrange (250-5000Hz) with Aleph on low end and F3 on high end. I have also used the Aleph 30, the F3, the ZenLite, the interstage transformer F4 and the F4 to drive the midrange and the F6 stays there. Soundstage, punch, naturalness and clarity are all totally there. If I ever go to a Klipsch horn system I believe this is the amp that I would use. Of course clearly the other amps are fantastic and if I hadn't heard the F6 I would be happy with any of them. Side note - the F4 and the "M2"F4 are great headphone amps.

Best

Bob

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