This weekend I have completed F2J. The sound reminds me of my EL84 with no NFB, but with more details and tighter bass. Really sweet sound, the midrange is magical, female voices are perfectly clear and natural. I tried the amp with Karlsonator, but F2J waits for Alpair 12P in some fancy cabinet 🙂 I also tried with ordinary speakers with XO but as expected the sound was rather dull, too dark.
First Watt F2J with Karlsonator 0.53 3FE25 - YouTube
One problem is that - and it was also the problem in the simulation - that the pot is out of the range, so I can't set a perfect clipping - photo attached. So I think I need to reduce the value of R5 to 1.8k or... increase R4. Also, my JFET has the gate reverse leakage current below 0.3mA (at 25 °C, Vgs=-15V, Id=0), so I am thinking about increasing the value of R6.
Mr. Pass, thank you very much for this amp.
First Watt F2J with Karlsonator 0.53 3FE25 - YouTube
One problem is that - and it was also the problem in the simulation - that the pot is out of the range, so I can't set a perfect clipping - photo attached. So I think I need to reduce the value of R5 to 1.8k or... increase R4. Also, my JFET has the gate reverse leakage current below 0.3mA (at 25 °C, Vgs=-15V, Id=0), so I am thinking about increasing the value of R6.
Mr. Pass, thank you very much for this amp.
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decrease R5 to 1K, increase R4 to 3K3, if you need less positive Ugs
the other way, if you need more positive Ugs
increase R6 to 47K, value per Pa's recipe, if your JFet gate isn't hungry
you're using Semisouth, or?
the other way, if you need more positive Ugs
increase R6 to 47K, value per Pa's recipe, if your JFet gate isn't hungry
you're using Semisouth, or?
Thanks for that, of course Semisouth R100, purchased in 2015 from Dr. Michael Mazzola 🙂
BTW... why Pippi Langstrumpf? 🙂
BTW... why Pippi Langstrumpf? 🙂
Do you know what the input voltage is for you to get it into clipping? Have you tried adjusting P1 while watching the FFT THD% and adjusting for minimum THD - does it run out of pot adjustment in that case as well?
I am asking because I haven't done FFT on my experiment (forgot how to - need to refresh my memory) but I was able to find an inflection point where THD was minimum.
I am seeing 2.55A through clamp meter and it's stable as the amp warms up. Voltage at the front of C2/C3 is a bit higher than 1/2 rail voltage for me.
I am very interested to see what your THD is for the Semisouth F2J.
Great work Ripson and your amp is so clean!!!
Cheers!
Twitchie
I am asking because I haven't done FFT on my experiment (forgot how to - need to refresh my memory) but I was able to find an inflection point where THD was minimum.
I am seeing 2.55A through clamp meter and it's stable as the amp warms up. Voltage at the front of C2/C3 is a bit higher than 1/2 rail voltage for me.
I am very interested to see what your THD is for the Semisouth F2J.
Great work Ripson and your amp is so clean!!!
Cheers!
Twitchie
thanks Twitchie, I haven't measured the input voltage at clipping, but as you can see on the FFT the gain is about 16.1 dB, the Vpp on the clipping is 18.8V so the input voltage must be 1.04Vrms. I haven't looked at the THD when changing the pot value, but I will, I got 2.57A at cold start and 2.41A when warmed up on both channels. With matched (Vbe) PNP and MOSFET source 0R47 resistors.