It is likely that the PCB was made for 2SJ103, but in that case the partner device is the 2SK246 - which also uses reversed pins, so for the other case, there would be a mistake as well. It's an error, no two ways about it.
I don't believe Jim is the guy designing the PCB anyway. He has a good stock of genuine J74/K170 in various Idss grades and they are all 100% original. I picked up a few myself. He's probably assigned the job to someone else because I don't think he could make such a blooper. Fact is that in DIY it remains our responsibility to find and fix mistakes, and to properly verify that what we buy is correct. However I think seller also needs to have good understanding and response to these kind of issues.
Good Luck!
I don't believe Jim is the guy designing the PCB anyway. He has a good stock of genuine J74/K170 in various Idss grades and they are all 100% original. I picked up a few myself. He's probably assigned the job to someone else because I don't think he could make such a blooper. Fact is that in DIY it remains our responsibility to find and fix mistakes, and to properly verify that what we buy is correct. However I think seller also needs to have good understanding and response to these kind of issues.
Good Luck!
So the 2SJ 74 and SK 170 I bought from him is not fakes and I just have to reverse the 2 SJ 74. So the ordered Lsk can be kept for something else?
Eyvind it seems you have a problem with your F 5 .
Eyvind it seems you have a problem with your F 5 .
Really would help too see some pictures (close up) of the devices you bought. Mine were the V grade, and were genuine.
2 new enclosure arrived and I am working on an amp with 4 other modules ( Current Feedback). I will take the F 5 apart tomorrow I think.
Hi guys,
My F5v2 is up and running now...still in the setup phase, build with:
- Diyaudio pcb and fets
- Hifi2000 5u case,
- 25V x-former
- 2*1ohm resistors like in the orginal schematic,
- no cascode
Well...when the bias of the fets was at zero, I got +-36.6V, with 300mV over 0.5 ohm I have now 34,6 V...still much more than the desired 32V...
your advise ? I really would hate to cascode as I like to have a jfet only input stage...will this cook the little creatures ? Heatsinking ? or changes in the psu to get down to 32V in any case ?
My F5v2 is up and running now...still in the setup phase, build with:
- Diyaudio pcb and fets
- Hifi2000 5u case,
- 25V x-former
- 2*1ohm resistors like in the orginal schematic,
- no cascode
Well...when the bias of the fets was at zero, I got +-36.6V, with 300mV over 0.5 ohm I have now 34,6 V...still much more than the desired 32V...
your advise ? I really would hate to cascode as I like to have a jfet only input stage...will this cook the little creatures ? Heatsinking ? or changes in the psu to get down to 32V in any case ?
lol...
Well, it is a melted into his case...no chance. If I have to go down to 32V in any case, I would start with a smaller c before the crc, so have a crcrc, a mixture of pi and r-input basically or have a choke in between the c...but the question is, if the 32V is indeed to absolute max without cascode ?
Well, it is a melted into his case...no chance. If I have to go down to 32V in any case, I would start with a smaller c before the crc, so have a crcrc, a mixture of pi and r-input basically or have a choke in between the c...but the question is, if the 32V is indeed to absolute max without cascode ?
It will depend on the Idss and hence the operating current in the Jfet since dissipated power=vxI Measure the voltage across the source resistor of the Jfet and divide by its ohm value. This should give you the operating current flowing thru the Jfet. Voltage will be about 4V less than supply ie 31V in your case
nash
Thx...so my values are:
R3: 72mV, 75mV
R4: 62mV, 64mV
...the voltage is now down to 33.6V with 280mV measured across the 0.5ohm source resistors...it is now warming up, once it is warm, I will increase bias to 330mV/660mA and see were V+/V- goes, I guess it will go further down below 33V, which I guess would be overall good enough to settle down with 350mV/700mA in the end...
any comments /hints ?
THX a lot
R3: 72mV, 75mV
R4: 62mV, 64mV
...the voltage is now down to 33.6V with 280mV measured across the 0.5ohm source resistors...it is now warming up, once it is warm, I will increase bias to 330mV/660mA and see were V+/V- goes, I guess it will go further down below 33V, which I guess would be overall good enough to settle down with 350mV/700mA in the end...
any comments /hints ?
THX a lot
This makes no sense at all. The cascode circuit have nothing to do with the input stage in that manner. Its only a way to cut down the operating voltage for the J-fets.
...So they are not adding any noise at all ?
Nevertheless, I am now at 375mV/ 750mA, 50 degree celsius with open case, 33V...
..the x-former is a 600VA type, but gets only 44 degree warm, so everything is sweet. PSU Board is the Diyaudio with MUR3020.
I guess I can live with 33V instead of 32V...
So, I will let it burn in further today, its running now for four hours , close the enclosure, repeat the fine tuning of the bias...and finally enjoy some music...
But overall this was a joy to build and setup, thx for this wonderful nice project !
Nevertheless, I am now at 375mV/ 750mA, 50 degree celsius with open case, 33V...
..the x-former is a 600VA type, but gets only 44 degree warm, so everything is sweet. PSU Board is the Diyaudio with MUR3020.
I guess I can live with 33V instead of 32V...
So, I will let it burn in further today, its running now for four hours , close the enclosure, repeat the fine tuning of the bias...and finally enjoy some music...
But overall this was a joy to build and setup, thx for this wonderful nice project !
Close the box. Repeatedly cold start it and warm it up while periodically monitoring output offset.
Once you are convinced it will stay good, then attach your speakers.
Once you are convinced it will stay good, then attach your speakers.
Great that things are working out. I would follow the advice of Audiosan and install the cascode if your boards allow them. If you will read NP he says that they act like a voltage umbrella for the Jfets. Also read page 3 of the F5 Turbo V1 article where NP discusses your concerns of Jfet dissipation.
Setting the final bias without the lid is a waste of time just like Andrew suggests. In my personal experience with my F5TV3 cold start up bias is about 70-75% of final bias which will keep increasing until the temperature stabilizes.
nash
It's a low impedance node looking into the emitter, so no, not much noise at all.
Anyway it's probably more bearable than the sound of JFETs breaking down after prolonged use at close to 30V, which is significantly higher than their rating.
YMMV, though. I have never heard that sound. Though I had a lot of fun rebuilding an amp (that disregarded cascodes without crunching the numbers or caring for consequences), I wouldn't want anyone to go through the agony of a pair of burnt woofer voice coils and a charred amplifier.
I listen to you, guys...good advise, thanks a lot...
...and I have been reading Nelson's article again and he talks as well about better noise isolation from the PSU with the cascode...which raises a few questions:
- Why not using this always when it helps to reduce noise and gives better reliability ?
- Has anyone compared cascode vs. non-cascode sonically as well as measurement-wise ?
...and I have been reading Nelson's article again and he talks as well about better noise isolation from the PSU with the cascode...which raises a few questions:
- Why not using this always when it helps to reduce noise and gives better reliability ?
- Has anyone compared cascode vs. non-cascode sonically as well as measurement-wise ?
Ok...enclosure is closed and I am burning in...a couple of observations which I would like to x-check with your experience:
- Offset at power-on and cold amp: 60mV Dc on both channels
- After a minute at 25mV
- Goes to +- 5mV Dc in 15 min and stays there when warming up to 52 degree C. at 740mA
- When switching off: DC offset goes up to 0.7V, I gues that will become a nice audible "thumb"...and than slowly migrates to zero
- Offset at power-on and cold amp: 60mV Dc on both channels
- After a minute at 25mV
- Goes to +- 5mV Dc in 15 min and stays there when warming up to 52 degree C. at 740mA
- When switching off: DC offset goes up to 0.7V, I gues that will become a nice audible "thumb"...and than slowly migrates to zero
1) can't speak for anyone else, because I never use any part out of its limits. Which means at voltages exceeding 25V I always implement a cascode.
2) No, and the reason for this is 1)
The F5 usually has no output thump. What you see is normal as the power supply voltages drain and you have no output connected. Once you put a load in there there should be no thump at all.
The rest of your offset, temperature and bias readings seem normal.
The Jfets are rated at 25V so in theory anything above that and you should use a cascode.
Nelson has stated in the F5turbo article and in other places that the Jfets shouldn't have a problem up to 32V, but you should choose lower Idss ones and if needed use some sort of heatsink.