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AKSA's Lender Preamp with 40Vpp Ouput GB
AKSA's Lender Preamp with 40Vpp Ouput GB
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Old 13th August 2019, 01:30 PM   #451
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Ouput GB
The gain is set by the ratio of the resistor sum above the JFET divided by the sum value below the JFET (not including the feedback R102). R102 is a feedback resistor to the gate, but the current to the gate can be assumed to be close to zero in a JFET so it doesn't really affect the gain calculation.

From the tabular values for the 3 different JFETs given in the schematic:
Click the image to open in full size.

We can see gain for case BF862, we have G=(940)/(47+270)=2.97 or 9.45dB
for the 2SK209, we have G=(2700)/(220+680)=2.97 or 9.54dB
for the 2SK170, we have G=(930)/(22+330)=2.64 or 8.43dB

Why did I not set all the gains the same exact value? The gain value was secondary next to optimizing the correct bias current and harmonic distortion profile via the LTSpice sim. If you want to change the gain, you can try reducing R106 a bit for higher gain, and increasing it a bit to reduce gain, but don't change it too much.

If using 2SK209, you can safely run at higher Vcc (up to 32v even - just adjust R112 to R115 accordingly not to exceed about 125mA and not to exceed 1.5w dissipation over the MOSFET).
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Last edited by xrk971; 13th August 2019 at 01:34 PM.
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Old 13th August 2019, 05:16 PM   #452
adason is offline adason  United States
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Nice preamp...tempted to try it.
You have typo in your name on the schematics.
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Old 13th August 2019, 06:22 PM   #453
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Ouput GB
The Cat in the Hat suggested that since I am so busy, I should ask my mini-me to make the schematics.

It is perhaps one of the best bang-for-the-buck preamps in DIY. No fancy, expensive, or unobtanium parts, ultra low noise, great sonic character. Easy to build.
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Old 14th August 2019, 01:12 PM   #454
xrk971 is offline xrk971  United States
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Vunce and I did some work on tweaking the resistor values last night for 2SK209 "GR" grade JFETs and 2SK170 "BL" grade JFETs to get closer to an ideal quiescent bias current (~40%) relative to the Idss. Values to be posted shortly but circa 2x current values for resistors both above and below the JFET (not including R102 feedback resistor).

Vunce reports that the soundstage really opened up on the 2SK209GR once that fix was in place.

If you have 2SK209BL, the current values are good.
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Old 28th August 2019, 04:41 PM   #455
xrk971 is offline xrk971  United States
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For Aksa Lender PCA daughterboard builders, here are some good resistor settings to use for 2SK209GR and 2SK170BL JFETs.

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Old 20th September 2019, 12:26 PM   #456
twocents is offline twocents  South Africa
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AKSA's Lender Preamp with 40Vpp Ouput GB
Default PCA board R102

Quote:
Originally Posted by xrk971 View Post
R102 is a feedback resistor to the gate
X, looking at your build pictures in post #440 of the PCA daughter boards, I see that resistor R102 (47k) is different is it carbon film? I know that somewhere in the thread you mentioned the feedback resistor is best to be carbon film.

I received my BOM from Mouser and hope to get my Lender singing with BF862s this weekend.
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Old 20th September 2019, 12:56 PM   #457
xrk971 is offline xrk971  United States
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Twocents,
Yes, that is CF and it’s not as critical here but that is indeed the local feedback resistor. When you think there is not a critical need for a precise value obtained by metal thin film, and sound quality is important, a CF can’t hurt.
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