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Old 30th June 2009, 11:18 PM   #1
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Default j.fet front aleph 5

hi all
i,ve heard you can mod an aleph 5 with a jfet front end and would like to try

can any one here offer any advice on how its done and what parts to use

also what is the limit for the supply voltage on a aleph 5 assuming the heatsinks are up to it ,im running a 25-0-25 at the moment

regards sheafer
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Old 1st July 2009, 03:46 AM   #2
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this may help, you'll want to limit the rails to +/-24v
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Old 1st July 2009, 09:19 AM   #3
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thank you kind sir i will check it out

regards sheafer
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Old 1st July 2009, 01:56 PM   #4
Dansk is offline Dansk  Denmark
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Zen Mod has made a jfet frontend for (me - Thank you, Zen Mod - and) an Aleph 60 here:
jfet input aleph 60
It can be driven using a little higher supply voltage.
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Old 1st July 2009, 07:23 PM   #5
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The 2Sj74's and their duals the 2Sj109's seem to be able to withstand
35 volts in actual amplifiers, which puts them in striking distance for
the Aleph 5.

To keep the dissipation low on the JFETs, you want to reduce the
bias current to about 5 mA each, which is accomplished by raising
R11 from 221 to 475 ohms. This keeps the Vds at about 30V
and the dissipation at 150 mW each.

R12 will then want to be about 820 ohms, and it would be a good
idea to place a resistor in series with the Drain of Q2 at 820 ohms
also.

This works, but it does operate the JFETs at the limit, so don't fool
around and do try to keep the Jfets cool. Paralleling two for Q1 and Q2
is a good idea also.

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Old 1st July 2009, 10:10 PM   #6
EUVL is offline EUVL  Europe
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> do try to keep the Jfets cool.



> Paralleling two for Q1 and Q2 is a good idea also.

Does it not change the open loop frequency characteristics ?


Patrick
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Old 2nd July 2009, 09:03 AM   #7
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thanks guys

you are very helpfull

regards sheafer
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Old 2nd July 2009, 03:10 PM   #8
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Quote:
Originally posted by EUVL
Paralleling two for Q1 and Q2 is a good idea also.

Does it not change the open loop frequency characteristics ?

Not enough to create issues.

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