SemiSouth power Jfets .......... FW J2

[generg]

thanks Zen Mod

[tazzz]

Hello,I upper Jfet functions as an "impedance multiplier" John broskie explains the workings of this circuit in detail. (there are more discussions on it on his site)http://www.tubecad.com/2009/10/blog0174.htmIn short the reason to reduce the upper "sense" resistor is that in effect the inverse of the upper jfets transconductance is in series with already.

[sp300b]

Hello,I upper Jfet functions as an "impedance multiplier" John broskie explains the workings of this circuit in detail. (there are more discussions on it on his site)http://www.tubecad.c...10/blog0174.htmIn short the reason to reduce the upper "sense" resistor is that in effect the inverse of the upper jfets transconductance is in series with already.

Hi tazzz - :welcome02:FYI BJ2 Schematic is to be found here and we all agreed not to be posted on Big Diyaudio forum.edit: schematic

[sp300b]

So, from Your experience we should try to lower our 0,5 Ohm (upper) and increase in same absolute amount (lower) 0,5 Ohm - correct ?

[Tea-Bag]

Tea-Bag ...just for curiosity..You are talking about F2 J with SS, not BJ2 - correct ?

I was talking about the BJ2. The F2J sounds a little 'cleaner and sweeter' on the top end, same with Mini-A with SemiSouth output.The BJ2 sounds a bit darker to me. So it doesn't have the 'house' SemiSouth sound I was expecting. I am of course wondering if mine is 'tuned' correctly. It certainly doesn't sound bad, just has a whiskey tone, not champagne.

[generg]

hi Zen Mod, I am ready to implement the second pair of SemiS, but I have to reduce the current first I have now 0,91V over the drain resistor and the R25 is at minimum position. I want to get less current let´s say 1,4 A per pair. the problem is I cannot lower the current because of minimum position already. Should I change R 26 in what direction to get less current?

[sp300b]

hi Zen Mod, I am ready to implement the second pair of SemiS, but I have to reduce the current first I have now 0,91V over the drain resistor and the R25 is at minimum position. I want to get less current let´s say 1,4 A per pair. the problem is I cannot lower the current because of minimum position already. Should I change R 26 in what direction to get less current?

Start putting additional R26 over existing R26 (I mean in parallel) - You have to lower it in order to have less voltage over it - which means less voltage over Vgs of the upper fet...which means less current....I mean You can play by means of soldering additional R over R26 and fine tune...No need this way to de-solder R26 that is already in place. Once You satisfied and finish testing, You'll easy remove R over R26, without R26 touched from PCB side. Hope it's clear...

[Zen Mod]

hi Zen Mod, I am ready to implement the second pair of SemiS, but I have to reduce the current first I have now 0,91V over the drain resistor and the R25 is at minimum position. I want to get less current let´s say 1,4 A per pair. the problem is I cannot lower the current because of minimum position already. Should I change R 26 in what direction to get less current?

BIAS SCHMIAS:you should decrease value of R35 to reduce bias ( if R12 is already on minimal )then - if you are having positive offset and you can't decrease it - decrease R26 ( if R25 is already on minimum)easy peasy , isn't it .....

[sp300b]

R12, R35 - Biasing lower JFetR25, R26 - Biasing upper JFet:character13:Correct ?

[Zen Mod]

matter of nomenclatureI'm taking it as - biasing goes from input LTP ......... other setting is chasing offset

[AR2]

Vlado - sp300b is interested ( and I'm asking just because he's busy all day long ) - can you make direct comparison of sound of FW J2 and your Babelfish J2 , considering all differences - you heard FW at Papa's , while you are listening yours at your place .....

we already have Tea Bag's opinion - " The BJ-2 sounds a lot like I remember the FW J2 sounding like at Burning Amp 2009 "

tnx

Guys sorry for the late response, but here it is. I would not feel comfortable to give you a definitive answer since my last listening of J2 was at Nelson over a year ago. So I really cannot compare in detail. Plus we listen it on completely different speakers, diffrent room... There is only one expression I would share. I was really excited about how it sounded then. And I am equaly excited how it sounds on BJ2. I love it, it is completely new beast and it is a good one. Other than that I really do not remember the detaiils, besides that when we placed that amp in, I knew that was it. I also remember that it sounded just right and very close to how tube sounds, and likewise that is how I would describe the BJ2 sound.

I hope this answers your question.

[Zen Mod]

:air_kiss:idi spavaj

[generg]

hm, hm dear brothers in Babelfisch J2 :Viannen_loungelizard: :Viannen_loungelizard: :Viannen_loungelizard: on the way to a fish with two pairs of SemiS I could not resist testing the possibility of doubling the number of drain and source resistors so that a value of 0,235 Ohm results. With the method Zen showed I reduced the voltage over the new drain resistors to 0,35V, giving in the sum 1,4 A, that we suppose Nelson uses. (did I do maths correct?)R26 was 100 Ohm, to get the lower current...... the sound seems to have a bit more body and more crazy details what tea-bag called "dirty sound", the same ability I always admired with Pumpkin!But I have to say the resistors put over the originals are other type ..... sound influence?I compared strictly with Aleph 3 only mono, one channel changing. I choose Aleph 3 because he has much body, may be too much (9610 mosfet bloom).The sound of BabelJ2 had, before the changes, always more room around the box and always the better highs, but voices ( Ella and Louis) sounded better with aleph3, more chest, more dynamics. After the changes I have the feeling Babelfish articulates better, has more resolution, still not the chest Aleph 3 has, but a quality in forming the shades of voices that competes the "chest".You all know the seduction of the new, and the disillusion some days later, but I hope my impression persists the next days ....... I can encourage try the lower resistor values and 1,4 A. I am eager to hear your impressions!

[Tea-Bag]

Guys,I got lost on the upper lower source resistor thing, is that idea beaten down by offset problems?

[generg]

I managed to install the second pair of SemiS, and don´t ask how it sounds, I am too exhausted...... now all other things of life, no more electronics........

[sp300b]

Good Job Generg !!!Today seems to be "day for good news"On other hands, I have just tried what tazzz was proposing and I can say :matrix002: :matrix002:So, in short, I put additional 1 Ohm over the upper parallel 1Ohms resistors making now 3 of them in parallel (which equals to 0,33 Ohm).The rest of the system was not touched.Sure, offset setting procedure after that, and I have 0,721 V over upper and 1,020 V over lower resistor.Sounds seems to improve in good way - Much sweeter than it was yesterday.So this is really a way to go.Eager and thinking would I try 4 Resistor in parallel for upper making 0,25 Ohms of it.According to what generg is reporting - seems - why not

[Zen Mod]

.... is that idea beaten down by offset problems?

nope ;feel free to experiment with ratio of these resistors ...... meaning ratio of lower vs. uppertake care to re-check both bias and offset after changeeverything is OK , as long you have overall sum of 1R - and at least 1A5 across them - optocoupler is functional

[sp300b]

generg, did You changed also a resistor over the bottom JFet Sourse (R37/R31) ?

[Tea-Bag]

nope ;feel free to experiment with ratio of these resistors ...... meaning ratio of lower vs. uppertake care to re-check both bias and offset after changeeverything is OK , as long you have overall sum of 1R - and at least 1A5 across them - optocoupler is functional

Excellent, so leave bottom Q14 Sources alone?

[Zen Mod]

sp300b just ended conversation over the phone with mehe'll obviously (restless spirit ! ) try lowering source resistor of lower SS .....anyway - what he tried - now we have dilemma what's more influental in his test - change in overall (sum) value of these two resistors in middle of totem pole , or change in their ratiogenerg ( and all ) - be careful when decreasing voltage for optocoupler ( led part) - you can shut off that LED , and circ will be still biased fine , but you will loose current tracking , for which optocoupler is put there , in first place

[generg]

thank you Zen Mod, I will observe the tracking over a longer time and look if it stays stable ....... next days! it´s a changing bazar....... if it would only be easier to keep the sound in memory.......

[sp300b]

So, I was playing a bit more for Today..So upper R is 0,33 Ohms, Lower is 0,5 Ohms and bottom one (connected to Sourse of bottom Jfet) is 0,33 Ohm.Sound improved in the right direction, too. Maybe a bit less than the previous noticable change.I measured Voltage over the Totem resistors and it is 1,77 V which means bias is set to 2,1 Amp.1,77V seems enough for CNY-17 functioning well.All in all,. seems that winning combination is to be:Upper R - 0,235 OhmLower R - 0,5 OhmBottom R - 0,235 Ohmor...Upper R - 0,165 Ohm ( 2 x 0,33 in parallel)Lower R - 0,68 Ohm or 0,5 OhmsBottom R - 0,165 Ohm Still this is to be checked, but definitely not tonight.