Zen9-like Preamp for Power Followers

[VladimirK]

For various follower-type power amps, there is a need in preamp with large enough voltage swing and a voltage gain of more than 20dB. This can be solved by a simplified Zen9-like stage, with a Radio Frequency j-FET transistor at its input. I have put together a prototype, as shown in the picture, and made some measurements.
Available voltage swing is nearly 80V p-t-p. Idle current is 220mA, output impedance (no NFB) is [ 270 Ohms, phase shift=-0,53deg at 1kHz] , and [ 270Ohms, phase shift=-3,6deg at 100kHz]. Voltage gain with a connected load of 1,3kOhms is 12,2 (22dB). Square wave response at 70V p-t-p, 100kHz, 1k3 load is shown in the foto. Larger size curve belongs to output signal, smaller – to input. Slew rate = [ U(0,9)-U(0,1) ] / dT = 380V/uS at the front edge, and 212V/uS at the decay. Trying to use the NFB as it realized in Zen9, with 33kOhms and 330kOhms resistors, makes time response of the stage much worse. I guess I should take more difficult way for adjusting the gains of the channels. Also I plan a decent shunt PS for this stage.

[woody]

Nice design sure has enough drive current. Is the PK903 expensive and hard to find?

[VladimirK]

Originally Posted by woody

Nice design sure has enough drive current. Is the PK903 expensive and hard to find?

KP903A and KP926A are russian parts. KP903A is an analog of USA CP650, in a screw-like case, even more convenient. There are a lot of sources for them in Moscow, something 1...2 USD/pcs. In USA CP650 as I googled costs 17 USD. KP926A are power SIT-transistors, something similar to unobtanium japaneese 2SK60, but quality is much worse than 2SK60 (big gate capacitance, big leakage etc.). They could be sourced in Moscow for 6...8 USD/pcs. But I would much prefer 2SK60. As I understand, Nelson Pass even ordered special development of power SITs according to his specification.

[juma]

Great preamp
Pity that those devices are not available outside of Russia. I would like to build with them a differential version as a voltage gain stage for a circlotron power buffer with laterals.

[Nelson Pass]

Nice circuit. I do believe that I would trade away a little of the
gain to get a slightly lower output impedance, lowering the
value of the output resistor to ground.

[VladimirK]

Originally Posted by Nelson Pass

Nice circuit. I do believe that I would trade away a little of the gain to get a slightly lower output impedance, lowering the value of the output resistor to ground.

Thanks a lot Nelson for this hint, since I was going to mess with device matching etc. We can get two profits from this simple step - decreasing out impedance and gain matching.

[VladimirK]

Originally Posted by juma

Great preamp
Pity that those devices are not available outside of Russia. I would like to build with them a differential version as a voltage gain stage for a circlotron power buffer with laterals.

Hi, Juma
I could try to get such parts and then mail to you. Please tell me a quantity. A problem is that they can not be matched easily, espesially KP926A, for differential operation.

[juma]

Thanks Vladimir for your kind offer. Right now we are working here (in Serbia) on establishing the "Russian connection" - people are interested in tubes and power Ge transistors too. If we fail, I'll be free to contact you

When it comes to matching, I suppose some source degeneration and SuSy differential feedback can do the trick.

[Zen Mod]

Originally Posted by juma

........and SuSy ......

[juma]

Don't worry people, everything is OK with SuSy concept - Zen is laughing to me (an internal joke about fashion trends)
He is sooooo witty (as in: full of ... wit)