John Curl's Blowtorch preamplifier

[plep]

Dear John,

You are right; experience and wisdom are in your words.
Thank you for taking the time to answer to beginners like me.
The beauty of the linearity of the curves was so appealing…

Can you tell us how far your are pushing the power supply voltage?

Best regards.
Philippe.

 

[john curl]

I try to use +/- 30V supplies. This drops in half when actually operating each device, because the 200 ohm drop resistors need so much voltage to get the current necessary.

 

[Justcallmedad]

John, why have you choose Mosfets instead of Fets for the cascodes devices, higher bias current for the output stage?

 

[john curl]

I chose mosfets for higher power dissipation and higher voltage breakdown than possible with jfets.

 

[courage]

John, which brand/type MosFet do you recommend?

 

[Charles Hansen]

If you do a few calculations based on John's previous post you will see the answer.

He said he is dropping 15 volts across a 200 ohm resistor, so the front end and the folded cascode together are drawing 75 mA. If you run the front end with "blue" Toshiba JFETs, they will draw about 5 mA. If you run them with "violet" JFETs, they will draw about 10 mA. So that means he is running at least 65 mA in the output stage. With 15 volts across the output devices, you need a part rated at 30 volts that can dissipate 1 watt and run at 65 mA. There are no complementary audio JFETs that can even come close to meeting these requirements. However, this is easy for the TO-220 sized Hitachi lateral MOSFETs.

 

[Justcallmedad]

I chose mosfets for higher power dissipation and higher voltage breakdown than possible with jfets.

Ok, John but the power/voltage conditions could be identical to those of the input fets, except if we want to choose a higher current for the cascode devices, but why choose a higher current? best linearity?

 

[Justcallmedad]

Vendetta or P5 X ??? 😉

 

[Upupa Epops]

And whatabout to try popular d' Agostino's twist - crowd of small ones in parallel ? 😉 It will look interesting.....

 

[john curl]

It is actually more like 10V across each 200 ohm resistor. I forgot to subtract the input-output voltage differential of the final fet follower. Yes, more current is better than less current in the output devices.

 

[Elso Kwak]

And whatabout to try popular d' Agostino's twist - crowd of small ones in parallel ? 😉 It will look interesting.....

Don D'Agustino long lost his touch....
I heard the latest Krill space heaters!

 

[courage]

I heard the latest Krill space heaters!

Had a good laugh Elso.

Charles & John,

Thanks for shedding some light on voltage/current matters regarding the JFets/MosFets.

 

[EUVL]

MOSFET Source Follower

I wish to refer to the schematic showed in Post #759.

I have been trying to come up with a MOSFET version for some time, but have not been able to come up with something more intellegent (elegant) than this (see PDF attachment).

The problem with this is of course the lack of adjustment for bias. One can perhaps replace the Zeners with resistors or trimmers, but I am still not entirely happy.

Any hints / suggestions ?


Patrick

 

[courage]

John,

Your last post concerning the voltage-drop accross the 200 Ohm resistors, my close observation of the BT Line stage board and additional simulations, gives me reason to conclude that there are, apart from the 2 gain-set resistors, 4 bias source-resistors mounted underneath the PCB to prevent the JFETS from exceeding their maximum dissipation level.

Is this conclusion correct?

Thanks again.

 

[courage]

To illustrate what I meant in my previous post, it concerns the source-resistors R7/R8 and R14/R15.
R10 is used to set the gain of the line stage.

 

[john curl]

Sounds good to me. 🙂

 

[Justcallmedad]

It is actually more like 10V across each 200 ohm resistor. I forgot to subtract the input-output voltage differential of the final fet follower. Yes, more current is better than less current in the output devices.

And more voltage… I guess that it’s better to get more voltage available for the mosfets (essentially the triode like ones) even if it’s rather on the top for the fets.

 

[courage]

John Curl: Sounds good to me.

John, I understand your reason for this cryptic answer, but may I assume that my previous conclusion(s) are kind of confirmed by you? 🙂

 

[jacco vermeulen]

Maybe Mr Hansen confirms in 2 hours from now ?

 

[joho]

Bias adjustment for output fet's

Hi Patrick and other gents,

Following this thread with highest interest, i made "a reel life viro experience board".
For bias adjustment i use a current source and some resistors ...
(thanks to Zinsula)
This works wonderfull !

Johan.