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Old 22nd January 2016, 06:39 AM   #1
donovas is offline donovas  South Korea
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Default jfet inplace of bjt for vssa

hello, i was told try here for help. so i try

PeeCeeBee

i am trying to use k246/j103 for vssa input. it works, sounds good but jfets rise temperature, offset increase then amp eventually oscillate.

how to stabilize? i think maybe resistor add source?

thank you!
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Old 22nd January 2016, 08:03 AM   #2
juma is offline juma  Germany
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Vssa is essentialy an F5 variant with input and VAS changed to BJTs and MOSFET source follower added at the output. So if you want input stage reverted to JFETs you have to take out parts that were introduced in VSSA to make it BJT input amp.
More detailed answer will be possible after you attach precise/exact schematic of your build.
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Old 23rd January 2016, 06:58 AM   #3
donovas is offline donovas  South Korea
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hello, the circuit is this.

Click the image to open in full size.
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Old 23rd January 2016, 10:12 AM   #4
juma is offline juma  Germany
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Ok. 2sk246 instead of BC550 and 2sj103 instead of 2sj103 (at the input).
D -> C
G -> B
S -> E
Take care about pinout and check your JFETs out of the circuit - make sure that they are OK.

Both 15k and OFF1/OFF2 resistors must be taken out. Install wire jumpers instead of 1m feedback capacitors.

Put 500R multi-turn pots instead of 470R JFET Drain resistors and use them to set 10mA bias through VAS BJTs (about 100mV across the 10R VAS collector resistors) the same way as described in F5 bias setting procedures (that way you'll set the DC offset at the amp's output too). Start with the zero resistance in this pots - otherwise the VAS BJTs will die.

------------------------------------------------------

For future reference, exact schematic means: exact power supply voltage, exact part designations (are your JFETs GR, BL or V grade), what are your VAS and output transistor, and so on...
If you expect someone to invest his time in solving your problem, be prepared to do at least the same.
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Old 23rd January 2016, 02:55 PM   #5
juma is offline juma  Germany
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I made a typing error in the previous post - first sentence. It should say: 2sj103 instead of bc560
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Old 29th January 2016, 06:04 PM   #6
donovas is offline donovas  South Korea
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thank you juma. i will try this immediately
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