You’re welcome X! It is a great and simple design. And 8 watts is more than enough for my speakers.
I still need to try the negative bias circuit. You’re telling me, about not having time! Life is ridiculously busy right now! Good luck finding that block of time. I’m off to listen to the LuFo Lite for a few minutes before bed.
Thanks Vunce! I was enjoying some great SACD’s tonight: Getz/Gilberto, Miles Davis - Kind of Blue, and Diana Krall. I noticed the imaging is really pinpoint with the LuFo Lite. I think X mentioned that should be the case, and IT IS! Soundstage depth is pretty good too. I do feel there is still some body missing compared to my other amps. That may also be due to speaker placement. I’ve been moving them around a bit. I’ll have to switch back to one of my other amps soon and do a comparison.
Good luck on your build Vunce! I might try out some MOT’s again too!
I just had a thought, I never flipped the polarity of the 10uF capacitor when we changed from the positive bias to negative bias schema , this might be why I was seeing a voltage reversal when the current started to flow! now to find some time to test this out ..dB
Well flipping the capacitor didn't work, same behaviour. No current flow till the pinch off threshold is reached then taps fullly open and my negative supply reads pos 0.9V and cannot be adjusted. Back to resistor and it works normally. Final Temps after an hour of playing music. JFET 45degC, resistor 55 deg C and heatsink is 42 deg C
Do you notice a ‘ligher’ bass presence with your normal music test tracks? If so, try paralleling another 4700uF cap to the existing cap in place at C124. Its a tight spot for a single 10,000uF cap.
No, bass doesn't seem to be impacted, I ran a frequency sweep (off the chesky test disc) and it behaves similar to my other amps on the test bench.
The natural bass test track I tend to use is "aeris" from the Legacy Sampler III (not sure it is good form to share a link), it is a solo cello piece that has a lot of low harmonic decay (beautiful composition). This track sounds deep and natural.
It would be good to get to the bottom of this negative bias issue. I'm just about to order some PCBs for a Tokin SIT follower project that has negative bias incorporated and the board could easily be repurposed for a LuFo Lite build so I'll probably give it a go and see how it works out.
I was wondering if adding a diode / zener to prevent the voltage flip. still not sure how this occurs given the independent negative supply - common ground perhaps.
I can tell you it sounds great, although not as spacious as a DIY F8 I have on the bench to repair. ( successful repair last night )
I think one problem with building these amps is the bar for "better" gets set exceptionally high. I would say that despite the lufo lite being optimized for headphones ( which I have yet to test ) , it offers a very delicate / responsive performance with a sense of ease and naturalness that is typical of all good class A amplifiers. It's soundstage ( on my test bench ) is focused but deep while the F8 offers a similar soundscape but throws a wider sound stage with a sense of more air.
an unfair comparison perhaps given the design intention for each.
I have a private DIY variant in a BOM box but not built yet called the “XFSeven”… I got scared because it requires simultaneously adjusting 3 trim pots while looking at 3 DMMs to set bias current and DC offset.
this amp got damaged in shipping when the transformer came loose and there was a little pin ball inside the chassis - so mechanical damage ( one of the mosfets snapped the bolt holding it on and all 3 pins broke off , traces on PCB broken etc )
It actually made me want to create a "how to ship a heavy amp" tutuorial. People forget that any amount of movement inside a box creates immense inertial forces which have the potential to destroy precious work.
I was fortunate enough that after managing to repair all necessary connections ( I missed one initially and had the full negative rail on the output ) but it was just a matter of resetting the DC offset and all worked ( at least thus far ).
I found this older FS thread this morning and am glad it was nothing more or else I may have been up the creek without a paddle. F7 clone kit and WHAMMY PCB
"One of the most difficult parts of building the F7 is biasing the JFETS. This is done for you via prematched and preadjusted pots, which must go in the correct position according to the instructions. You still need to adjust the bias current by ear or with measurement, so this is not "plug and play", but much closer than if you sourced parts yourself."
Thanks for letting us know. The 12ohm DCR will result in very low damping factor for speaker woofer duties. It would be fine as a headphone amp though. Has anyone tried to listen to it as a hard phone amp? That was really the point of LuFo Lite. Regular LuFo with a cascode would be the preferred speaker amp.