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-   -   LuFo Lite - a 1 Transistor SE Class A Headphone Amp (https://www.diyaudio.com/forums/headphone-systems/374760-lufo-lite-1-transistor-se-class-headphone-amp.html)

jwjarch 24th August 2021 04:39 PM

Hi X. Where is a good value to start with for the pot? 100R?

xrk971 24th August 2021 05:12 PM

1 Attachment(s)
Try 10k series resistor from rail plus 10k pot to ground. We want the wiper voltage to go from zero to 10v max. Connect 10uF electrolytic to wiper and ground to smooth things out. Then connect that node to the gate via a 33k to 47k resistor. There should be very little current flow to the JFET gate. Disconnect the 100k from gate to ground. All this is in the gate side of the inout coupling cap. Please see sketch below.

dBel84 25th August 2021 03:22 AM

I added a power resistor this evening - wanted to make sure this worked

https://live.staticflickr.com/65535/...c77ec78d_c.jpg

Had it running for about an hour

https://live.staticflickr.com/65535/...813e5760_c.jpg

The bias is only 1.3A

The jfet gets to about 35 deg C , the resistor housing is just shy of 50 deg c and the heatsinks matched the jfet at 38 deg c

sounded good - driven directly from dac so no major volume but enough to hear the quality of sound and to make sure no gremlins

..dB

Vunce 25th August 2021 03:40 AM

dB,
What did you change in order to get the lower 1.3A bias from last power up when the bias was very high?

dBel84 25th August 2021 03:43 AM

used a 10ohm power resistor

Vunce 25th August 2021 03:45 AM

And removed the onboard resistor ladder?

dBel84 25th August 2021 03:47 AM

yes - only source resistor is now bolted to the heatsink - thanks to this board design, that was a very easy modification to make

Vunce 25th August 2021 03:51 AM

Yes, good move to cover all options.

12v or 24v input?

Nice job, The heat is very manageable now ;)

Running with lower bias, did you measure voltage before and after capMx? Curious if its still a 6v drop?

dBel84 25th August 2021 03:53 AM

24V into the capmx, 19.7V at the drain

what is curious , during all the manipulation, the gate connection was interrupted on one channel and when I fired it up the Capmx dropped 4V , so it is working as designed but with the larger current draw, it drops further

Vunce 25th August 2021 04:00 AM

Nice! You answered my question before I asked :D
4v drop is more in line.

Feed it a from a stout preamp and have a good listen!

:cheers:


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