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Old 14th October 2013, 11:54 AM   #11
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I`ve tried that circuit about 4 years ago and it sounds pretty nice.
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Old 14th October 2013, 12:38 PM   #12
flg is offline flg  United States
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Doesn't this design look like 1/2 a Blowtorch? Just sayin... And if you were to go balanced?
Or, even the Vendetta MC Pre-Amp?
Or, is it maybe the BA-4 front-end? Although, then, as already mentioned, it would need a little tweaking to get to the fully useable output voltage swing of the typical BA output stage (Or F4).
But I like it!
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Old 14th October 2013, 01:01 PM   #13
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Quote:
Originally Posted by Nelson Pass View Post
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...

At 31:20 to 31:38 you did say that. Regards
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Old 14th October 2013, 01:32 PM   #14
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Old 14th October 2013, 02:29 PM   #15
Zen Mod is offline Zen Mod  Serbia
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output voltage swing is determined by cascode voltage potential

so , roughly (asymptotic to ) +/-18V ........ meaning 36Vpp

so , not exactly blowing F4 from pants territory , but almost

cascoding da thing everywhere and rising PSU voltage accordingly.... will bring ya (as FE) in PA realm .... (think about including amplified diode in output drain vertical , as inputing part of OS )

and , yes ........ Soroboruo .........
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Last edited by Zen Mod; 14th October 2013 at 02:32 PM.
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Old 14th October 2013, 03:33 PM   #16
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Well, someone asked me so, I'm taking a stab at an easy method to get that + & - 22V for F4 or BA FE out of this design.

Use 10ma Idss JFETs
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Last edited by flg; 14th October 2013 at 03:36 PM.
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Old 14th October 2013, 08:16 PM   #17
juma is offline juma  Germany
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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.
Output swing is happening in realm of voltage difference between the PS rail voltage (45V) and cascode voltage (18V). How much it will be depends on current available to output pair (supplied by 1K/3W resistors to both pairs) and Id of JFETs on one side and the loading resistor (1k5 - the one between output pair drains and the ground) on the other side.
The output swing will be the product of these 2 values, so with the lower value of loading resistor comes the lower gain and the clipping comes sooner (less available swing). For example, with 4k7 loading resistor I got 19V_peak before clipping.
Basically, the thing here is about current swing that gets converted to voltage on the loading resistor (transimpedance amp, I/V converter, that kind of stuff...)

Last edited by juma; 14th October 2013 at 08:44 PM.
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Old 14th October 2013, 08:51 PM   #18
Dzius is offline Dzius  Poland
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Quote:
Originally Posted by juma View Post
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) :
Can't find gate stoppers R21 and R24 on your PCB.
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Old 14th October 2013, 09:54 PM   #19
juma is offline juma  Germany
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Quote:
Originally Posted by Dzius View Post
Can't find gate stoppers R21 and R24 on your PCB.
Looks like I forgot them...
There are few options to get out of predicament:
to interrupt the gate trace and solder them on the bottom side of the PCB,
or to shorten the gate pin of the JFET and solder the resistor directly to it. Third option is to use BJTs (BC550c/BC560c) as the output pair (works great) so you don't need the gate stoppers.
The last option would be to draw the PCB again with R21,R24 on it...
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Old 14th October 2013, 09:56 PM   #20
Mr. dB is offline Mr. dB  United States
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Will this result in a product offered in the DIY Store?
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