Semisouth Aleph J?

I have been reading here, there, and everywhere about the different Pass amps that have been made. I have many questions which is evidenced by my random scatter-brained post. MY question for this thread concerns the possibilty of substituting the Semisouth power J-Fet's in the Aleph J layout. THis uninformed question is based on the firstwatt articles including specifically the Sweet Spot. I am a newb, but have plenty of time to ask questions :devily: while simultaneously trying to figure out what the hell i am talking about. If this is doable, i will probably go ahead and buy the pcb's from Peter if they are still available. MY thinking is that this amp would be similar to the J2. I would just do the F5, which is a much easier build according to most, but i have limited funds and think, at least for now, this is what i want.
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
It's a pretty similar topology, but the details of biasing the
dual input pairs and the top side output current source are
different.

Specifically, I use a mu follower circuit to due what used to
be the "Aleph" portion, as the lower threshold voltages of the
power Jfets do not allow high enough Vce voltage for the
bipolar control transistors to have as good performance.

:cool:
 
I would think..:scratch2: that i stand to gain very little if anything by replacing one set of fets with another. I have found a simplified version of the mu follower typology. I will see if i can work with that. THe output seems familiar so i will have to go back over the various articles and schematics and see what i can find. This is like treasure hunting and I am a pirate with two eye patches.:Pirate:
 
Forgive me, if in my ignorance I have crossed a line. I can assure everyone that I don't know my mosfet head from my jfet *** (as if that's not already evident). If I am pushing too far into something it is out of ignorance. I believe Bill recognizes this and is trying to steer me in the proper direction. Papa is just too damn kind (thankfully so) and as always offers a small morsel to try to assist others in doing what he loves. For me, the thing the makes Mr. Nelson unique is not his mastery of electronic circuits, but rather his willingness to generously share something that he could keep all to himself. Even IF i had the ability to do him harm, I would not. Now that we have cleared that up, will somebody please tell me why this amp is smoking :scratch:
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
I think you can see significant improvement by simply
dropping the Jfet in on the negative side - the IRF240
on the top (aleph) side can be left as is.

To bias it properly you will need to drop the value of the
resistor from the Drain of the input transistor (and Gate
of the Jfet) to the negative rail to a lower value - I
recommend a 500 ohm pot.

:cool:
 
I think you can see significant improvement by simply
dropping the Jfet in on the negative side - the IRF240
on the top (aleph) side can be left as is.

To bias it properly you will need to drop the value of the
resistor from the Drain of the input transistor (and Gate
of the Jfet) to the negative rail to a lower value - I
recommend a 500 ohm pot.

:cool:

Outstanding!

That is probably the exact thing that Buzz needed to hear.

Thank you very much for your input.
 
:djinn:N Kelson and Bill,
Thank you both for the advice. I found myself on this latest trail because of the reviews at 6moons. His reviews of the F5, J2 and Aleph J are what made me want to go this route. I think I will biuld the modified Aleph J and F5 first and then maybe I will know enough to stumble through the J2. That is, if I even need to. Hey Nelson, where do you get your heatsinks?
 
The best bang for the buck in heatsinks is probably HeatsinkUSA. Russ is in the vendors section of the forum. I've been using his monster extrusions for a while, and he will cut them to any length and width you desire. You just have to finish them, he has no machining or anodizing facilities. A couple light coats of flat black paint works wonders.
 
THanks Bill. I have seen those heasinks on the forum and on his external site. I was hoping for something pretty like what Pass uses. I have seen extruded HS that look cool but they are usually limited in length to about 4 inches. They cold be doubled up, but then you have fets attached to seperate sinks and may have some thermal differences which may lead to altered performance. At least that is what i have read,...I think!
 
I am looking for opinions/experience on the wattage/temperature limits of IRFP 240. I am wanting to build an ALeph J with bias set at around 2.4 or slighlty higher, using two fets with 28V rails. Nelson, if your willing to comment, how hard have you pushed these things and how much is too much in terms of reliabilty. Rad somewhere that 50W per fet is really pushing it. Looking at using Heatsink USA 10" x 2.85" x 6". claims C/W/3" rating of .70, but unable to verify.