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BF862 Preamp
BF862 Preamp
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Old 9th December 2017, 02:57 PM   #541
xrk971 is online now xrk971  United States
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BF862 Preamp
Update to the above Aksa Lender pre FFT, when implemented in real PCB as designed by JPS64, distortion went down to below 0.001% THD.

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Old 11th December 2017, 08:43 AM   #542
sarathssca is offline sarathssca  India
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1. Do we need the single BF862 in gain stage to be matching with the one in the other channel of the stereo preamp?
2. For getting 4 matched BF862 for the buffer stage, approx how many BF862s we need to procure and do test matching?
3. Why are we using asymmetric power supply here ie positive and negative rails of different voltages, unlike more commonly used +/-10v or +/-12v (where both +ive and -ive rails have same voltage with opposite polarity)?
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Old 11th December 2017, 09:56 AM   #543
AndrewT is offline AndrewT  Scotland
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I bought 600 lsk170.
I found many matched twins, in various grades of matching accuracy from 0.25% to 5%.
I did not find any quads.

BTW, that is matching over a range of Id, not selecting by measuring Idss.
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Old 11th December 2017, 12:22 PM   #544
juma is offline juma  Germany
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Quote:
Originally Posted by sarathssca View Post
1. Do we need the single BF862 in gain stage to be matching with the one in the other channel of the stereo preamp?
2. For getting 4 matched BF862 for the buffer stage, approx how many BF862s we need to procure and do test matching?
3. Why are we using asymmetric power supply here ie positive and negative rails of different voltages, unlike more commonly used +/-10v or +/-12v (where both +ive and -ive rails have same voltage with opposite polarity)?
1. Yes
2. In 500 pcs. of BF862 I have bought at Mouser almost all have the Idss in range of 13-14mA so matches were abundant in that quantity (all JFETs delivered to me were on the same production stripe). Others have different experiences.
3. Already explained - please read the thread from the beginning. Actually, you can do it without negative rail - see post #346. The price to be payed is introduction of input coupling cap.
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Last edited by juma; 11th December 2017 at 12:30 PM.
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Old 12th December 2017, 01:54 PM   #545
xrk971 is online now xrk971  United States
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BF862 Preamp
I made a small quick and dirty mockup of the actual circuit the BF862 will be used in and then measured the current through a 1k drain resistor (part of actual circuit) at the voltage it will be used at (16.4v) and can get pretty amount of pairs (40%) with better than 1%. Trick is to push down on JFET with enough force to make contact yet not make it fly off with a pop. The eraser of a pencil is good for this.

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