John Curl's Blowtorch preamplifier - Page 388

Justcallmedad · 9th April 2008, 08:30 PM

How do you manage gain (without global negative feedback) and fet input overload in such a configuration

john curl · 9th April 2008, 08:31 PM

Please don't complicate, yet. Richard Later, we can do that.

Justcallmedad · 9th April 2008, 08:55 PM

Ok John

I am trying to find a suitable small floating voltage source (150 mV) maybe a schottky ? 5819 or the like.

john curl · 9th April 2008, 08:57 PM

PMA, if we could have a floating 50mV voltage source in place of the short, we should be near optimum. We might need slightly more voltage, but this would be a good start.

john curl · 9th April 2008, 08:59 PM

Please stay aside, Richard, until we get this resolved.

john curl · 10th April 2008, 05:38 AM

Everyone, PMA's link is now working. It is very interesting to compare the various configurations. Once we get the baseline right, then major changes can be added to see the overall effect. Richard, your input is valuable, but just let us get this baseline nailed down first. The only idealistic limitation that I see at this time is controlling what would normally be controlled by Idss. An added 50-75mV floating voltage source should do that.
Of course, other inputs are welcome.

PMA · 10th April 2008, 05:42 AM

Floating voltage (between upper and lower circuit) - it works, but with higher voltage (1V), and smaller resistor (1 ohm or nothing). It controls Id and keeps low distortion. I will be back in an hour or two.

PMA · 10th April 2008, 05:52 AM

Diode works ...

john curl · 10th April 2008, 06:00 AM

But, what is the current, now? PMA?

PMA · 10th April 2008, 06:56 AM

Now the current is 22mA. It is all a game of JFET threshold voltage, Vdiode (or voltage source).

Also, distortion depends on JFET grade (BL, V, GR). Maybe I could make a comparison and add to a current web page article.

We have pretty nice result with 2 diodes (9.5mA). Resulting dI(outdif)/dV(indif) is 1mA/7mV.

9th April 2008, 08:30 PM	#3871
Justcallmedad diyAudio Member Join Date: Jun 2004 Location: Paris	How do you manage gain (without global negative feedback) and fet input overload in such a configuration __________________ Richard Perez

9th April 2008, 08:55 PM	#3873
Justcallmedad diyAudio Member Join Date: Jun 2004 Location: Paris	Ok John I am trying to find a suitable small floating voltage source (150 mV) maybe a schottky ? 5819 or the like. __________________ Richard Perez

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9th April 2008, 08:31 PM	#3872
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Please don't complicate, yet. Richard Later, we can do that.

9th April 2008, 08:57 PM	#3874
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	PMA, if we could have a floating 50mV voltage source in place of the short, we should be near optimum. We might need slightly more voltage, but this would be a good start.

9th April 2008, 08:59 PM	#3875
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Please stay aside, Richard, until we get this resolved.

10th April 2008, 05:38 AM	#3876
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Everyone, PMA's link is now working. It is very interesting to compare the various configurations. Once we get the baseline right, then major changes can be added to see the overall effect. Richard, your input is valuable, but just let us get this baseline nailed down first. The only idealistic limitation that I see at this time is controlling what would normally be controlled by Idss. An added 50-75mV floating voltage source should do that. Of course, other inputs are welcome.

10th April 2008, 05:42 AM	#3877
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Floating voltage (between upper and lower circuit) - it works, but with higher voltage (1V), and smaller resistor (1 ohm or nothing). It controls Id and keeps low distortion. I will be back in an hour or two.

10th April 2008, 06:00 AM	#3879
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	But, what is the current, now? PMA?

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