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Old 19th September 2012, 12:07 AM   #301
tinitus is offline tinitus  Europe
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jfet boz
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Old 19th September 2012, 01:13 AM   #302
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by vdi_nenna View Post
I think the 10 ohm at the Source is 22 ohms if voltage is dropped to 16v.
just found there is a jfet BOZ thread with this
curcuit

several mods are suggested to try a lower distortion

one suggests 1k drain, and 5ohm source
maybe I will try something in between

but I better take it there if I have further questions
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Old 18th October 2012, 10:18 PM   #303
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Hello,
I am building a Juma's pre (post 53 schematics).
Question about DC offset .... which is the acceptable value ??? I am able to adjust ch right to 0,0mV, but not able to go under 87,2 mV on Ch left. RGDS
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Old 18th October 2012, 10:45 PM   #304
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then there is something thats not correct. it is way too high for a preamp.
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Old 18th October 2012, 11:31 PM   #305
juma is offline juma  Germany
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Quote:
Originally Posted by gcolnaghi View Post
Hello,
I am building a Juma's pre (post 53 schematics).
Question about DC offset .... which is the acceptable value ??? I am able to adjust ch right to 0,0mV, but not able to go under 87,2 mV on Ch left. RGDS
0mV is what we aim for, but couple of mV up or down due to thermal drift is OK.

87mv is not acceptable - you have wrong value resistor soldered or, more probably, input JFETs are badly mismatched...
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Old 19th October 2012, 07:42 PM   #306
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Resistor value is fine.
Yesterday I forgot to say that BC560 is B (not C). Is the BC560B/BC550C couple an issue ?
After several JFET/transistors swap (tried many combination i am actually at:
- CH left drifting from -8mV to +6mV (pot P1 is in the middle of the run)
- CH right drifting from +1mV to +18mV (pot P1 is at the end of the run)
............... better but not good enough i think
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Old 19th October 2012, 07:51 PM   #307
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by gcolnaghi View Post
- CH right drifting from +1mV to +18mV (pot P1 is at the end of the run)
I guess you should be able to change that with a resistor on the adjust pot

wouldn't +/-10mV be ok ?
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Old 19th October 2012, 08:07 PM   #308
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2-3mV is max offset accepted by me in a preamp
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Old 19th October 2012, 11:20 PM   #309
juma is offline juma  Germany
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Ideally, BJTs should be matched as well as JFETs. P1 can adjust for some level of differences but it's not all-mighty. Also, to minimize offset drift, thermal coupling should be implemented (posts #54 and #82)
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Old 20th October 2012, 01:26 PM   #310
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if you mount a cap on output, would that be ok ?

or maybe your power amp have a cap on input
then it wouldnt matter much, would it ?
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