Upscaling Aleph-J SRPP tp 100W

VladimirK · 25th February 2010, 11:50 AM

By using high-current j-FETs at Aleph-J input one can upscale this schematic to 100W. Is it reasonable at a price of using p-channel output MOSFETs ?

VladimirK · 25th February 2010, 01:08 PM

This version is more correct. Are there futher corrections?

Nelson Pass · 25th February 2010, 06:48 PM

Ordinarily I let the availability of good cheap N channel output
Mosfets dictate the use of P ch Jfets on the input. It is not
difficult to parallel several of them to get the current up.

Also, on your schematic you definitely want some Gate stopper
resistors on your cascode transistors, say 100-221 ohms.

Bigun · 11:17 PM

I'm a novice when it comes to JFETs but I wonder if some source degeneration on the LTP would help alleviate the device matching requirements.

Also I wonder what the PSRR looks like, there are several resistors from the +35V rail that could be split and turned into RC filters ?

VladimirK · 26th February 2010, 03:31 AM

Quote:

Originally Posted by Nelson Pass

Ordinarily I let the availability of good cheap N channel output Mosfets dictate the use of P ch Jfets on the input. It is not
difficult to parallel several of them to get the current up.

Also, on your schematic you definitely want some Gate stopper
resistors on your cascode transistors, say 100-221 ohms.

Dear Nelson, thanks for your input, the gate stopper must be added. I know opinions that in schematics with NFB, p-chann devices affect sound being used in either output or input stages. Also, in two-stage schematics the input stage theoretically needs more current, more current ... It would be interesting to check which combination is better, under 2 x 50mA input stage conditions, p-ch - n-ch or reverse, IMHO. As for prices, I get both 2SK1529 or 2SJ200 equally for 4 USD/piece. Questions of capacitors are more sensitive. But in USA, you are right, easier to parallel several 2SJ74 than to pay 18 USD for CP650 per piece. Since i already have a lot of j-fets similar to CP650, I am looking for possibilities of using them.

VladimirK · 26th February 2010, 03:46 AM

Quote:

Originally Posted by Bigun

I'm a novice when it comes to JFETs but I wonder if some source degeneration on the LTP would help alleviate the device matching requirements.

Yes, one might want to consider some source degeneration. This schematics is very draft. But in my case I have matched pairs of j-fets.

VladimirK · 26th February 2010, 08:23 AM

One also might want to compare data for CP650 and 2SJ74.
CP650: gm = 0,150 S, Ciss = 20 pF, Idss = 500 mA
2SJ74: gm = 0,022 S, Ciss = 105 pF, Idss = 10 mA
For total current of 50mA, a 5-piece set of 2SJ74 results in Ciss=525pF,
to be compared with 20pF of CP650. So, it seems that CP650 might open a new quality in respective schematics. I know the one and only guru capable of schematics fine tuning and simplification. Hope he could get some interest to such mid power j-fets.

25th February 2010, 11:13 PM	#4
Bigun diyAudio Member Join Date: Jan 2009 Blog Entries: 2	I'm a novice when it comes to JFETs but I wonder if some source degeneration on the LTP would help alleviate the device matching requirements. Also I wonder what the PSRR looks like, there are several resistors from the +35V rail that could be split and turned into RC filters ? __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig. Last edited by Bigun; 25th February 2010 at 11:17 PM.

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25th February 2010, 06:48 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	Ordinarily I let the availability of good cheap N channel output Mosfets dictate the use of P ch Jfets on the input. It is not difficult to parallel several of them to get the current up. Also, on your schematic you definitely want some Gate stopper resistors on your cascode transistors, say 100-221 ohms.

26th February 2010, 08:23 AM	#7
VladimirK diyAudio Member Join Date: Feb 2007 Location: Tashkent	One also might want to compare data for CP650 and 2SJ74. CP650: gm = 0,150 S, Ciss = 20 pF, Idss = 500 mA 2SJ74: gm = 0,022 S, Ciss = 105 pF, Idss = 10 mA For total current of 50mA, a 5-piece set of 2SJ74 results in Ciss=525pF, to be compared with 20pF of CP650. So, it seems that CP650 might open a new quality in respective schematics. I know the one and only guru capable of schematics fine tuning and simplification. Hope he could get some interest to such mid power j-fets.