"Crazy" power for preamp

[VladimirK]

I have tried this schematic (a combination of components of various ZENs) as a preamp for Zen9 and got an unexpected good and powerful sound. The jFET is a russian vertical SIT transistor KP926A with the triode-like output characteristics. Quiescent current is 150 mA, what seems to be crazy for a preamp. Undistorted sinusuidal output voltage is up to 27V, that is good for a power follower.

[VladimirK]

Enclose measurements for KP926A

[bigwill]

Wow, there are solid state devices with curves like that?

[kenpeter]

Yep, SIT or V-FET: JFET with a gridlike gate buried inside the
channel... Acts like a vacuum triode, it has an intrisic space
charge (or carrier gradient) feedback we know as "Mu".

All the Japanese V-FETs dried up and dissapeared long ago,
is this Russian version an available component, or rare???

Such feedback doesn't have to be intrinsic. O.H.Schade of
RCA proved that long ago, transforming a set of Pentode
curves into a Triode with feedback (not simply strapping
the screen to the plate.) Nelson's Zen a modern example.

But intrisic feedback from the space charge is awful quick
way to stabilize voltage linearity, where you want minimum
phase shift in your local feedback loop. It should be faster
than any feedback path taken through external resistors.

Mu of an input device inherits to the output in a Darlington
or Sziklai arrangement. So you can abuse a small signal SIT,
or Triode, and scale up the output current by the Beta of an
ordinary BJT. Mu linearizes voltage gain for both, if anodes
are wired directly together. But from the BJT's perspective,
this is an external feedback path, not an intrisic one.

[Lumba Ogir]

kenpeter,
I find that report a little bit vague...

[flg]

Yep, my circuit does similar things. I just refer to it as a VAS...

[VladimirK]

Originally posted by flg
Yep, my circuit does similar things. I just refer to it as a VAS...

I guess the powerful and rich sound from the schematics in post #1 arise due to the triod-like characteristics of the single amplifying device.
I also tried to make the schematics more fast by employing casode - between the source of KP926A and the ground I inserted smaller jFET (not SIT) and made adjustments at 100 mA current. Due to decreased input capacitance of modified circuit it became very fast, it started to reproduce square wave up to 1 MHz, and I was close to heaven thinking I got something extraordinary. But during listening I have got ordinary sound, similar to what i have with my good preamp Parasound JC2. Cascode did not allow the triod-like features to operate. My current conclusion: the single-stage amps like some Zen versions benefit strongly from special preamplification stage; amplifier-speaker interaction is more important than good measurements with resistive load.

[VladimirK]

Originally posted by bigwill
Wow, there are solid state devices with curves like that?

These transistors are in TO-3 case and were originally designed for power electronics, still available in Moscow for 7-8 USD. Their drawback is large spread of initial currents at Vgs=0, so matching of pairs is difficult. Another drawback is that input capacitance is not specified by manufacturer, I guess it could be similar to or even larger than that of LU1014.

[VladimirK]

In the schematics from post #1 one can replace the current source by drain resistor. Without negative feedback, the voltage amplification factor was nearly 12 with 2K2 resistive load at the output.

[VladimirK]

Originally posted by kenpeter
Yep, SIT or V-FET: JFET with a gridlike gate buried inside the
channel... Acts like a vacuum triode, it has an intrisic space
charge (or carrier gradient) feedback we know as "Mu".

All the Japanese V-FETs dried up and dissapeared long ago,
is this Russian version an available component, or rare??? ...

These KP926A stiil available, and moscow designer Konstantin Musatoff offers two model of power amps based on KP926A in output stage, with output transformer. These transistors fit well many tube schematics and can be loaded with 300V at 150mA.