You are asking people to do calculation for you for something utterly unusable, that is absolutely rude.
Be a gentleman, read up the links people gave you.
You are right the values were out of scope. Sorry for that. I am Newby. Reading the links and can calculate the source and drain resistor. What I can't do is find the best values for lower THD and lower output Z. Should I use high idss and high Vg? Highest tramscond for sure. Please help me choosing the right jet of current production? J309 j310 bf862 Bf861 a b or c?
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Again, a misconception. Each will impart different signature. They are cheap enough to buy several pcs of each for trial. Use some kind of socket to avoid repeated soldering and desoldering of semiconductor parts. Remove socket and permanently solder once choice is made.
I don't know your reason for choosing the JBOZ against the BOZ, Gregg on the neighbor forum really like his adaptation of the BOZ with 24V supply he named MooseFET with really good sonic and measured performance.
As a very rough guide :
For a beginner, target supply voltage a bit less than jfet max Vds, 24V or less for J309/J310.
Choose operating current > your voltage swing requirement divided by intended output impedance. In case of 5Vrms and 1k, peak voltage is 5 * sqrt(2) = 7.1V so 7.1mA. Calculate and use suitable source resistor.
Target voltage at drain for 1/3 - 1/2 supply voltage, but it has to be higher than target max peak output voltage swing.
Pick drain resistor max 2x target output impedance say 1k8. Jfet drain will be at about 24V - (7.1mA * 1k8) ~ 11.2V (for 24V supply , 7.1mA bias and 1k8 drain resistor), close enough.
Use another 1k8 resistor to ground after output coupling cap.
Use 100 ohm from coupling cap to output terminal as accidental short protection. ~1k output impedance.
Once it play sound, listen for sometime, tweak as necessary to your taste.
Ioutpk = Voltage swing pk / load impedance, NOT source impedance..
If the circuit has feedback then the feedback string is in parallel with the load and may require an increase in device current.
Yes, use a grounding resistor to take any leakage current to audio ground/return. But try values from 100K to 2M2, (I use 1M0 or 2M2 at the output of the source and at the input of the Receiver, where I also add a 47pF for extra RF interference attenuation).
Yes, use a protection resistor, But don't put it too late in the circuit.
Attach the protecting resistor to the Output Node of the BOZ. Here it does two jobs:
a) isolates capacitive loading from the BoZ output
b.) acts as a current limiter when the output is abused.
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I believe working with Mosfet is easier for a beginner, almost plug and play while matching and biasing is unavoidable for the jfets. But it is your project, go with your choice.
All the jfets you mentioned will give you output impedance low enough for F4 and nice sound when biased correctly. However, each will have different sound signature. You will need to ask a very gifted oracle to tell you which will sound the best to you before a build.
I like the 2SK147 in my system, but others will say they prefer the J310 in their system and your system will most likely be different. Are you willing to match my system and listening room and try the 2SK147? What if you prefer the J310 even if you match my system? Would you be content if I tell you to use the 2SK147 because it is the goddamn best I have ever heard? What if your taste and mine are different?
the only thing that I don't understand fully is about the low output Z.
for low Z you need a low value of drain resistor, therefore high Id. in this case the J310 and BF861C should be more suited?
2SK147 is hard to get?
nothing against mosfets indeed, and easy bias via pot on the gate.
I could try it with irf610 à la moosefet
or use BS170 or BS270 like JUMA did? any advice on the sound?
for low Z you need a low value of drain resistor, therefore high Id. in this case the J310 and BF861C should be more suited?
2SK147 is hard to get?
nothing against mosfets indeed, and easy bias via pot on the gate.
I could try it with irf610 à la moosefet
or use BS170 or BS270 like JUMA did? any advice on the sound?
Bias and impedance are requirement for the circuit to get sufficient output without letting the magic smoke out. The sound however is another matter, they are all different, and I regretfully only try very few of them in preamp duty before settling with DHT.
2SK147 is harder to get compared to the 2SK170. Be prepared to get fakes. Mouser stock Interfet substitute IFN147 which sell for $7.65.
In a Moosefet, the IRF610 is a bit more polite while the IRF510 has more slam and zing. My business partner once compared a ECC88 tube preamp and a BS170 preamp ala BOZ a customer brought in and can not pick which sounds better to him.
2SK147 is harder to get compared to the 2SK170. Be prepared to get fakes. Mouser stock Interfet substitute IFN147 which sell for $7.65.
In a Moosefet, the IRF610 is a bit more polite while the IRF510 has more slam and zing. My business partner once compared a ECC88 tube preamp and a BS170 preamp ala BOZ a customer brought in and can not pick which sounds better to him.
I have not heard the BS170 myself. I think if you want really strong tubish you would prefer the rare small signal Sony SIT jfet like 2SK79 or 2SK63. Go ahead and obtain some before they become unobtainium, liubincalvin sell matched pairs for $34. But I strongly suggest learning to build and tweak with more available jfets first to reduce the chance of wasting extremely rare parts on beginner's mistakes. Build separate good shunt regulated PSU for each channel with quiet soft recovery rectifiers to LC filtered raw supply.
Once you think you are ready for the SITs, I think you would understand your preferences a little better. I suggest loaded tube circuit style with transformer or choke (but nice finemet ones start north of $200 each), or at least gyrators like Ale Moglia's Gyrator Boards (calls for good quality caps like teflon or polystyrene). Most of us start with what our budget allows and save for the upgrades.
Once you think you are ready for the SITs, I think you would understand your preferences a little better. I suggest loaded tube circuit style with transformer or choke (but nice finemet ones start north of $200 each), or at least gyrators like Ale Moglia's Gyrator Boards (calls for good quality caps like teflon or polystyrene). Most of us start with what our budget allows and save for the upgrades.