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Old 13th October 2013, 07:19 PM   #1
juma is offline juma  Serbia
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Default LSK pre - BAF 2013

At the BAF2013 Mr. Pass presented a nice JFET preamp. Today I cobbled up a quick prototype of one channel to test its behaviour and it's very nice. I have a BL rated Toshiba JFETs so some values had to be changed and some parts added (P1 to set the DC offset at the output and harmonic content, caps in cascode voltage divider to improve HF extension a bit). Serious build with buffer added at the output will follow. For the time being here is the schematic and the PCB layout of the preamp (copper side view, 45x50mm) :

EDIT (Oct. 27, 2013): 'final' schematics and pic of built preamp are in the post #66
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Last edited by juma; 27th October 2013 at 08:43 PM.
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Old 13th October 2013, 09:17 PM   #2
CeeVee is offline CeeVee  Portugal
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WOW Juma you were not distracted...fantastic !
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Old 13th October 2013, 09:48 PM   #3
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Does this preamp have enough voltage gain for an f4?
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Old 13th October 2013, 10:24 PM   #4
CeeVee is offline CeeVee  Portugal
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heheheh----ZM would love to answer that question
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Old 13th October 2013, 10:58 PM   #5
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I see the 45v rails, but I don't really know how to calculate output voltage swing.
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Old 13th October 2013, 11:06 PM   #6
CeeVee is offline CeeVee  Portugal
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...i believe the original has LSK and LSJ of 15mA ( C grade ) Idss JFETS....this one has 2SJ and 2SK of BL grade so in principle a little lower...this should help to calculate voltage drop on R19 and R25 resistors and the resulting absolute max voltage swing.

Last edited by CeeVee; 13th October 2013 at 11:19 PM.
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Old 13th October 2013, 11:26 PM   #7
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Quote:
Originally Posted by MASantos View Post
I see the 45v rails, but I don't really know how to calculate output voltage swing.
With 30 volt drop of the 30ma through the 1k resistors the voltage on the fet is about 15 volts . swing from video Nelson said I believe a swing of 10 volts has low distortion see the link above for all of that speech.
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Old 14th October 2013, 12:20 AM   #8
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Of course I neglected to mention that a simple pair of self-biased
complementary followers for this circuit will take the output impedance
down to something like 20 ohms.

Of course we would want to use the same LS parts...

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Old 14th October 2013, 12:22 AM   #9
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you didn't

I mean , neglected

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Old 14th October 2013, 05:29 AM   #10
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Quote:
Originally Posted by MASantos View Post
I see the 45v rails, but I don't really know how to calculate output voltage swing.
Since the Vgds maximum for the P-channel parts is spec'd at 25 V, you night want to consider that to be the limiting factor. That would make the maximum output to be 5 Vrms. (18 VDC on the gate plus 7 V peak signal level). Your choice.
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