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-   -   Zen9-like Preamp for Power Followers (http://www.diyaudio.com/forums/pass-labs/165982-zen9-like-preamp-power-followers.html)

VladimirK 29th April 2010 06:12 AM

Zen9-like Preamp for Power Followers
 
2 Attachment(s)
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 29th April 2010 06:55 AM

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

VladimirK 29th April 2010 07:14 AM

Quote:

Originally Posted by woody (http://www.diyaudio.com/forums/showthread.php?p=2169732#post2169732)
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 29th April 2010 04:41 PM

Great preamp :up:
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 29th April 2010 06:22 PM

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.

:cool:

VladimirK 30th April 2010 03:44 AM

Quote:

Originally Posted by Nelson Pass (http://www.diyaudio.com/forums/showthread.php?p=2170273#post2170273)
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 30th April 2010 03:52 AM

Quote:

Originally Posted by juma (http://www.diyaudio.com/forums/showthread.php?p=2170194#post2170194)
Great preamp :up:
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 30th April 2010 08:04 AM

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 30th April 2010 08:09 AM

Quote:

Originally Posted by juma (http://www.diyaudio.com/forums/showthread.php?p=2170854#post2170854)
........and SuSy ......



:rofl:

juma 30th April 2010 08:15 AM

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) :rolleyes:


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