Mini-A to Mini-J

grataku · 10th August 2005, 05:22 PM

A simple enough permutation of the Mini-A it would seem, not very original either.
The PS voltage for the mini-A seems to be perfect for a clean swap.

I have the mini-A circuit breadboarded, I have the j-fets and a higher value resistor to get the feedback down from the original Mini-A which uses higher gain mosfet for the differential.

Anyone with me?

winslow · 10th August 2005, 08:11 PM

I'm curious.

rtirion · 10th August 2005, 09:56 PM

Wouldn't the drive of the upstream MOSFET require a healthy bias
through the tiny JFET?

I think you would use a 2SJ109 JFET. According to the toshiba
datasheet these have a max Pd of 200mW. Sadly no Tja is
specified, so let's asume 150 to 200K/W.

At Ids of 8mA and 15V across a JFET half means 0,12W.
That's 0,24W for the 2SJ109 pair, and +36 to +50 degrees celsius
above ambient for this very small unit.

Too rich for me, so I think you will need to cascode.

btw I am building one of these jfet-cascode A.... right now.
Will keep you posted.

Regards

grataku · 11th August 2005, 01:34 PM

Quote:

Originally posted by rtirion
Wouldn't the drive of the upstream MOSFET require a healthy bias
through the tiny JFET?

What is the 'upstream' mosfet? I am confused.
if you mean the ccs that's going to change to something more appropriate

rtirion · 11th August 2005, 02:07 PM

I am sorry. I should have specified Q7, original schematic by
Grey. You can find the schematic here.

At higher frequencys (>1k) Q7 needs more and more current.

That's why you need the relative high bias for the differential pair.

Hope this helps

10th August 2005, 05:22 PM	#1
grataku diyAudio Member Join Date: Dec 2000 Location: -	Mini-A to Mini-J A simple enough permutation of the Mini-A it would seem, not very original either. The PS voltage for the mini-A seems to be perfect for a clean swap. I have the mini-A circuit breadboarded, I have the j-fets and a higher value resistor to get the feedback down from the original Mini-A which uses higher gain mosfet for the differential. Anyone with me?

10th August 2005, 09:56 PM	#3
rtirion diyAudio Member Join Date: Nov 2000	Jee Wouldn't the drive of the upstream MOSFET require a healthy bias through the tiny JFET? I think you would use a 2SJ109 JFET. According to the toshiba datasheet these have a max Pd of 200mW. Sadly no Tja is specified, so let's asume 150 to 200K/W. At Ids of 8mA and 15V across a JFET half means 0,12W. That's 0,24W for the 2SJ109 pair, and +36 to +50 degrees celsius above ambient for this very small unit. Too rich for me, so I think you will need to cascode. btw I am building one of these jfet-cascode A.... right now. Will keep you posted. Regards

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10th August 2005, 08:11 PM	#2
winslow diyAudio Member Join Date: Aug 2005	I'm curious.

11th August 2005, 02:07 PM	#5
rtirion diyAudio Member Join Date: Nov 2000	I am sorry. I should have specified Q7, original schematic by Grey. You can find the schematic here. At higher frequencys (>1k) Q7 needs more and more current. That's why you need the relative high bias for the differential pair. Hope this helps

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