John Curl's Blowtorch preamplifier

[joho]

jc-80 input

I hope it's allowed to post a jc-80 question.

When i look at the design i see some servo regulation at the right half of the j-fet pairs(the - input) so i suppose that this preamp needs a unbalanced source.
Can i connect that source with the + input only?
With some balanced preamps needs the - input to ground.

Johan.

 

[john curl]

This design COULD be made with fully balanced inputs. How would that be accomplished?

 

[dimitri]

This will require BT style servo.

 

[john curl]

Not necessarily.

 

[dimitri]

otherwise CMRR will be poor :-(
And please don't insert the servo signal in the audio signal path.

 

[john curl]

Try harder and don't give away any secrets!

 

[john curl]

OK, I'll give a hint. All you have to do is to relocate the servo return that is now connected to the (-) input.

 

[courage]

John,

Thanks for the given hint.

Which MosFets currently available will give the most satisfying result soundwise?

 

[john curl]

The only mosfets that are generally available are the Fairchild equivalent of the IRF510-9510.

 

[Justcallmedad]

Hi All! 🙂

OK, I'll give a hint. All you have to do is to relocate the servo return that is now connected to the (-) input.

It can simply be connected between the two upper/lower fets sources…

 

[joho]

unbalanced use of jc-80

Dear Mr.Curl,
Thank you for the fast response and new hint, but my first question is stil open.
I use unbalanced sources so what to do with that (-)input?
Shorting to ground disturbes the servo i guess.
Resistor to ground? or ... nothing?

B.t.w what are the specs of this legendary preamp?

Thanks again,
Johan.

 

[joho]

Servo relocation

Hi Richard,

Hi All! 🙂



It can simply be connected between the two upper/lower fets sources…

When i look to the schematic in post 1882 there is already a servo at that location.
I missed something?

Johan.

 

[jacco vermeulen]

The difference between left/right and top/bottom.

 

[Justcallmedad]

Modified JC 80

… Just a global attempt to update the JC80 for fun, with today’s components.
Resistors values are changed to fit the Irf devices requirements and to reach a 20kHz bandwith in open loop.
The 2SK389 and 2SJ109 can be replaced by the 2SK170/2SJ74 of course, but also by other ones as suggested by JC, in this case R2/R3/R14/R15 must be adjusted. In my case the SK 389/SJ109 Idss value is 10 mA. Mosfets Idle Id is 50 mA. I changed the original input stage arrangement for this preferred one. It just a starting point, probably the MASTER will see some possible improvements. I didn’t build it, some measurements with real devices could lead to various modifications .

Gain 12dB.
Zout 50R per side.

An externally hosted image should be here but it was not working when we last tested it.

The DC servo could be something like the one I post about the Blowtorch in this post

 

[PMA]

Richard, another "modification":

 

[Justcallmedad]

Pavel, I will be tempted to change some resistors connections, and delete some other ones...

 

[PMA]

It works pretty good, 2SK389/2SJ109 must be V - type.

 

[john curl]

Pavel is upgrading to the Blowtorch, but is this really practical?

 

[Justcallmedad]

John, what is exactly the purpose of the 10R and 20R trim pot between the mosfet sources? (JC80 schematic)

 

[john curl]

AC balance.