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Pass B1 pre with BF862
Pass B1 pre with BF862
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Old 20th April 2017, 01:48 PM   #1
kinku is offline kinku  United States
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Default Pass B1 pre with BF862

hi,
Does anyone has experience using BF862 to replace 2SK170 /LSK170 in B1 pre amp?
Are they better than LSK170
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Old 20th April 2017, 01:56 PM   #2
juma is offline juma  Germany
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Slightly higher gm i.e. lower Z_out mostly. Noise performance might be considered irrelevant since B1 is a line level buffer (no voltage gain). Sonical differences are on the level of nuances - I prefer BF862 but you might like k170 more...
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Old 20th April 2017, 03:33 PM   #3
kinku is offline kinku  United States
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Thanks Juma. Do you have any designs with buffer stage and attenuator ?
Also is it necessary to use ferrite bead at the gate to reduce noise when using Bf862?
Is there any need to change other components when using BF862 instead of K170?
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Old 20th April 2017, 06:21 PM   #4
Nelson Pass is offline Nelson Pass  United States
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Pass B1 pre with BF862
You can drop the BF862 right into the original design, although you might
do a little better to put some resistance on the Source to Ground connection
of the current source transistor, connecting its Gate to Ground so as to
take the current down a bit, say to 15 mA. This will lower the dissipation
and (hopefully) bias the upper gain transistor to slightly less than its Idss.
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Old 20th April 2017, 06:36 PM   #5
kinku is offline kinku  United States
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Thanks Nelson, Q101 and Q201 are the current sources for each channel?
What should be the value of resistance I need to use ?
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Old 20th April 2017, 10:31 PM   #6
juma is offline juma  Germany
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Quote:
Originally Posted by kinku View Post
Thanks Juma. Do you have any designs with buffer stage and attenuator ?
Also is it necessary to use ferrite bead at the gate to reduce noise when using Bf862?
Is there any need to change other components when using BF862 instead of K170?
Kinku,
no ferrite beads are necessary, gate stopper (220R - 1k) is all you need.
What do you mean by "designs with buffer stage" ? B1 is a buffer stage.
Attenuator of your choice (10k - 25k) is best put before the B1.

Source resistors, mentioned above by Mr. Pass, should be 5R - 10R, depending on Idss of your particular BF862 JFETs (the higher the Idss, the higher value of source resistors) in order to keep the Id at 80 - 90% of Idss
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Old 20th April 2017, 11:30 PM   #7
kinku is offline kinku  United States
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Thanks Juma.
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Old 21st April 2017, 02:09 AM   #8
kinku is offline kinku  United States
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Attached the new schematic. I guess grid stop resistors only needs to be at the gates of Q1 and Q3?
Attached Images
File Type: png B1 BF862 V.png (57.4 KB, 769 views)
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Old 21st April 2017, 08:00 AM   #9
juma is offline juma  Germany
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Grid stoppers are used at the grids of tubes - only gate stoppers here
Add them at the gates of Q2/Q4 too.
R102/R202 are superfluous. Value of R104/204 should be in range of 15R - 22R (just to isolate capacitance of the next stage/cable and not to dramatically increase the Z_out of the buffer).
I didn't check the input switch contraption - it's drawn confusingly. R201/202 and R100/101 can be left out - just make sure that input signal comes undisturbed at pin 1 of the P100/P200.
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Old 21st April 2017, 10:42 AM   #10
kinku is offline kinku  United States
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Juma , why the value of R104/R204 is significantly different from original B1 design?they are 1K in this schematic
http://www.firstwatt.com/pdf/prod_b1_man.pdf
Also wondering the reason behind removing the R102 and 202? Did you mean to say R100 and R101?

Last edited by kinku; 21st April 2017 at 10:48 AM.
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