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Old 12th February 2018, 06:52 PM   #211
Michael Rothacher is offline Michael Rothacher  United States
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Luminaria?
Wow! Super cool.
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Old 13th February 2018, 03:48 AM   #212
Ben Mah is offline Ben Mah  Canada
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Thanks, Michael.

The more I listen, the more I like.

The L'Amp and Luminaria is a great sounding combination.
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Old 15th March 2018, 12:35 AM   #213
Ben Mah is offline Ben Mah  Canada
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Default Fixed Bias Luminaria

Michael had written that although fixed bias is not necessary to enjoy the sound, it is worth the effort. I have been enjoying my Luminaria with a source resistor but hey, this is diy so effort is no charge. I decided to redo my Luminaria with fixed bias.

For the bias supply I used one transformer and rectifier bridge to supply both channels, and provided each channel with a LM337 regulated supply.

I removed the source resistor and capacitor and re-positioned the resistor to the drain side in front of the load resistor, with a decoupling capacitor added. I decided to try a slightly higher Id at 55mA this time, with Vds at 83volts. The Vbias ended up at -16.1v and -17.9v for right and left channels respectively.

It was worth the effort. At this level the gains are sometimes small yet large. To me, fixed bias added that extra little bit of clarity that made the music seem that much closer to reality. I noticed the effect especially on recorded live performances in that applause and audience sounds seemed more realistic and alive.

So far I have just been ball-parking the operating points based on Michael's general recommended operating range. I have a Focusrite Scarlett 2i2 coming to me in the mail so soon I will be able to try to see if I can select some operating points quantitatively. So maybe more performance gains to come.
Attached Images
File Type: jpg Luminaria Fixed Bias (1).jpg (899.7 KB, 318 views)
File Type: jpg Luminaria Fixed Bias right ch (1).jpg (884.1 KB, 315 views)
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Old 15th March 2018, 01:14 PM   #214
Michael Rothacher is offline Michael Rothacher  United States
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Luminaria?
Nice work!
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Old 17th March 2018, 04:47 PM   #215
bvtrinh is offline bvtrinh  United States
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Hello Mike,

Do you think I can use this 1pc THF-51S TOKIN V-FET(SIT) (similar 2SK77,2SK180) NOS Original, Japan Made | eBay for the luminaria?

Bao
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Old 17th March 2018, 05:05 PM   #216
Michael Rothacher is offline Michael Rothacher  United States
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Luminaria?
No, not for LuminAria.
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Old 17th March 2018, 05:51 PM   #217
bvtrinh is offline bvtrinh  United States
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Thank you Mike! I will wait for Indra’s update on using the 2sk79.
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Old 18th March 2018, 06:41 AM   #218
Ben Mah is offline Ben Mah  Canada
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Quote:
Originally Posted by Michael Rothacher View Post
Nice work!
Thanks, Michael.

It was a fun build and the point-to-point wiring made changes easy to do.

I received my Focusrite Scarlett 2i2 the other day so I did some spectrum analysis today. I used the Focusrite and Right Mark Audio Analysis software to produce the 1kHz input signal for the analysis. The signal was sent through the whole preamp, from TVC to buffer stage to VFET output.

Initially I found a huge 60Hz bump and harmonics. It was due to my diy unshielded cable that was going from the Focusrite output to the preamp input. I moved the cable as far away from the preamp AC line as feasible and that dropped the 60Hz and harmonics considerably.

I found that the right channel was noisier and had higher distortion than the left. I played with the drain load resistance and the bias but that only made a small change in the second harmonic.

I will also make a shielded cable and try again to see if that will lower the noise some more.

Hopefully the results are somewhat meaningful. This is my first attempt at spectrum analysis and my test rig is hobbyist grade, plus there could also be user error. But I think the results do give a ballpark look at the preamp's performance.

Left and right channel spectra and Focusrite loopback spectrum below:
Attached Images
File Type: png Left Ch 84v7 56mA Spectrum.png (14.7 KB, 203 views)
File Type: png Right Ch 84v8 56mA Spectrum.png (14.5 KB, 190 views)
File Type: png Focusrite 2i2 24 96.png (12.4 KB, 185 views)

Last edited by Ben Mah; 18th March 2018 at 06:48 AM.
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Old 20th March 2018, 01:01 PM   #219
bvtrinh is offline bvtrinh  United States
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Hello Mike,

If I use the 2sk79 instead, what values would I have to change? Thank you!

Bao
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Old 20th March 2018, 01:17 PM   #220
Michael Rothacher is offline Michael Rothacher  United States
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Luminaria?
You should track down PSz. for some insight into the SRPP he built.
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