What To Do With Those 2SJ28's

[PSz.]

Very nice! How does it sound?

 

[Michael Rothacher]

Just a bench proto right now. Trying to decide if it merits further effort.

 

[wdecho]

And this is with the 2SJ28? If so nice.

 

[Michael Rothacher]

It's an iteration of #5.

 

[Nelson Pass]

That's really great. I see that your thd figures are comparable to some
of the prototypes I made with the transformers.

 

[Soundhappy]

Ah, the joys of clip leads and duct tape

Congratulations sir Rothacher
First lab bench photo from new house

 

[Michael Rothacher]

I see that your thd figures are comparable to some of the prototypes I made with the transformers.

That's welcome news.

 

[vanofmonks]

Are the variable resistors set so there is an equal drop on the 47K resistors? How much current at idle?

 

[Michael Rothacher]

Currently, I have them set for 1.65A with the midpoint between the drains set at about 22V. The circuit is pretty easy to trim, but it is fussy in that it requires decently matched complements of low Vgs. Probably too much fuss for general DIY.

 

[Soundhappy]

Probably too much fuss for general DIY.

New amp design with vfet is never too much.
Sadly matched pairs is very rare actualy.
Acronman source....the old souvenirs.

 

[Michael Rothacher]

I only had about 20 pieces left and I was able to find adequate matches, but it's possible I was just lucky.
I also tried some other things in the N-channel position, but that's for another time...

 

[Soundhappy]

Maybe two versions : one for matched pairs lucky guys and the next
for unmatched vfet so circuits with voltages adjustements ,trimmers etc.
make sings lonely vfet's anyway

 

[PSz.]

Hi Mike,
any thoughts on initial resistance values in "The Soundhappy" amp? How about a CCS on it?

 

[Michael Rothacher]

You could do a CCS, but it changes the rest, so you'd have to first decide if you want that and what type, etc.

 

[PSz.]

I'm torn between "The Soundhappy" and this Hungarian amp.

He says he runs it at 40-45V and 1.2-1.5A. I've got the 2SK79s, 2SK60s and 2SJ49s sitting here...

 

[Michael Rothacher]

Might as well try that one first, pretty much the same deal.

 

[PSz.]

Any idea what potentiometer value to use?

 

[Michael Rothacher]

I'm not sure I get the schematic as drawn. Doesn't seem to leave much voltage for the 2SK79. What sort of performance is this giving?

 

[PSz.]

The circuit is apparently a thought exercise to build an amp with no fixed resistors. It was implied that this same circuit could be used at 120V 40mA or 40-45V 1.2-1.5A. Perhaps I misunderstood. As a preamp he says:

- THD 1v eff. Output voltage: 0.07%
- THD 10v eff. Output voltage: 0.5%
- THD 20v eff. Output voltage: 1.5%

and provides these scope traces

The Hungarian thread is here:
http://www.avx.hu/forum/index.php?/topic/20255-v-fet-single-ended-amplifier/

 

[Michael Rothacher]

As a preamp.