Nice result flancfuster!
Can you tell me why you are hybridizing tube and SK170 JFET? Sort of like a cascode to get the 50Vpp? Why not all tube front end?
That’s a nice result though - can you also share FFT plot for 8v and 25v case?
Thanks for sharing. Will you pursue this to prototype?
Can you tell me why you are hybridizing tube and SK170 JFET? Sort of like a cascode to get the 50Vpp? Why not all tube front end?
That’s a nice result though - can you also share FFT plot for 8v and 25v case?
Thanks for sharing. Will you pursue this to prototype?
Indeed it is a cascode and I'm getting better distortion numbers. One can go with tube front end without cascode, but the distortion numbers will almost double.
I have some LU1014D in hand and I was thinking for some time now to try a hybrid amp with a source follower in the output stage. This might me the prefect choise.
8Vpp FFT
25Vpp FFT
I have some LU1014D in hand and I was thinking for some time now to try a hybrid amp with a source follower in the output stage. This might me the prefect choise.
8Vpp FFT
25Vpp FFT
Beautiful harmonic profile! This looks excellent. Dominant 2nd order and less 3rd and nothing else at 8V.
Another simulation, now without cascode, simpler, SE with CCS, higher distortion, more of H2.
I prefer the previous schematic.
Schematic
8Vpp FFT
25Vpp FFT
I prefer the previous schematic.
Schematic
8Vpp FFT
25Vpp FFT
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I like the previous schematic and profile more also. But the latter would still sound quite good and sweet like honey. 🙂
A simple version of that hybrid cascode has been used by Audio Research. The LSK170 operates in a stable voltage and current controlled environment.
Quick update - Vunce is enjoying stereo from his dual monoblock single chassis LuFo! Way to go Vunce! 
I love the LuFo’s built in mood lighting! You will have to fill us in on listening impressions when you get more time to listen during the day without waking up the while house.
I love the LuFo’s built in mood lighting! You will have to fill us in on listening impressions when you get more time to listen during the day without waking up the while house.
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Yup, listening in stereo now. Low volume listening so far, over the weekend I’ll swap speakers for more bass impact and flex LuFo’s muscle. The amp is silent with out music playing. A single Antek AS-4225 provides both channels with 29.5vdc with SLB psu’s. I want to listen for some time without any onboard FE to get a feel of the sound, but it is very tempting to stuff the ACP+ or Hakuin daughter boards onto LuFo 😉
Attachments
We look forward to hearing your full report tomorrow then. In the meantime, congrats and have fun listening to it play quietly.
Wohoo, a tube front end! Great!
Would it be possible to get the B+ running off the 28 VDC PSU using a voltage booster?
Also, I recently learned about the 3S4, 3C4, and 3V4 directly heated tubes. They can be hooked up as triodes, they are cheap, can be biased at low B+, and are super linear. See here and here.
It would be great if it would be possible to make a tube front end PCB! Maybe also for the SuSyLu!?
We have made DC/DC boosters for the B+ of an E88CC before (also filament 6.3v was also DC/DC). It completely eliminates mains 60/120/180Hz hum or noise and nothing more than 12v (or 28v) DC input to power. This is a hybrid solid state / tube buffer with SE/Bal input and SE/Bal output. Gains selectable (0dB/6dB/14dB/20dB). I am actually using this as part of the line stage to drive either the LuFo and planned for SuSyLu as I need balanced drive.
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Nice, the front end from first schematic requires 160V on B+. It will make the build easier if we can use the DC booster. The only concern is that tubes will be somewhat close to power transformer. It has to be tested inside the case to see if they pick up noise/hum.
A version vith one tube and one k170 (and maybe some mosfet) would perform similar?
Geeze - I take a 3-month break from amp building and you guys come up with this! I am itching to try a LuFo now. Great job, Vunce.
Difficult to say what the relative performance would be.
Here is the article where I saw the write-up of the circuit and similar, as used in the AR SP-11 and SP-8:
The Audio Research SP-11
For SuSyLu, you can consider the first front end. I did a simulation and can easily drive SuSyLu. See below.
8Vpp FFT
25Vpp FFT, 3rd is rising
50Vpp FFT, 3rd is rising more, it will have more "bite" at higher power
I you want more 2nd harmonic distortion, just create an imbalance between R16 and R17.
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I've tried that, but no unfortunately. I can't get the same numbers. The balanced LTP version result in distortion cancelation, hence the low numbers.