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Old 25th January 2010, 05:59 AM   #1
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Default SIT-Preamp

Behind this schematics stands an idea to try using SIT transistors with triode-like output characteristics for simple preamp design. The common source stage based on KP926A Russian SIT transistor is used for amplification (nearly 20 dB) at 100V power supply voltage and 175mA quiescent current, class A operation. Unfortunately, KP926A has 1600pF of Ciss (instead of this transistor much better would be SONY 2SK60 which have Ciss = 190pF). To handle the 1600pF of input capacitance of KP926A one needs a buffer stage, which I decided to make like well known j-fet follower with 25mA quiescent current and plus-minus 15V supply voltages. One needs a decent j-fets for such a buffer, like for instance CP650. In my case I used Russian KP601A, which is close to CP650. The Ciss for KP601A is 6pF. Before this buffer a simple volume control based on 10kOhm ALPS resistor stands more than comfortably. In fact, I used a voltage divider constituted by 7,8kOhm low-noise constant resistor and the variable part of ALPS below it. Capacitors used include Elna Cerafine, Mundorf polypropylene etc. This schematics has no NFB, its output resistance (measured by immitance meter) is 200 Ohms. I compared it sonically with the good Parasound JC2, which has only 60 Ohms output resistance. Power amp was 50W version of Pass Zen9, Shanling CD T-1500 player, speakers are Paradigm Studio 100 v4, cables Stereovox (speaker), Nordost (interconnects), power is provided by PS Audio Power Plant Premier. I can say that I love my Parasound JC2 very much, but in this comparison SIT-preamp sounded more musical. It gave more detailed and pleasant low-mids and the bottom end. At the beginning I thought that SIT-preamp has lower output resistance than JC2, but it is not. So, I do not know what a magic stands behind it. Speaking about transparency of highs the two preamps are very close, in some cases maybe JC2 is a bit better (difference on the level of tiny cabling effects), but JC2 is connected to XLR outs of Shanling (known of being better) on contrary to RCA outs for SIT-preamp. I would be grateful for help in understanding what a “devil” is behind this SIT-sound.
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Old 25th January 2010, 06:39 AM   #2
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Very nice. What kind of resistor did you use for R7? How about an active load or a choke? Output impedance will go up but still be reasonable.
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Old 25th January 2010, 06:56 AM   #3
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Originally Posted by analog_sa View Post
Very nice. What kind of resistor did you use for R7? How about an active load or a choke? Output impedance will go up but still be reasonable.
I did try CCS load and local NFB, it gives no plus to sound compared to simple resistor load. For R7 I used 11 pcs of 3K3 2W resistors in parallel, but even better would be using of 3K6 resistors in order to make them less hot. A choke I believe would be a best solution, but I did not try it.
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Old 25th January 2010, 07:59 AM   #4
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Hey Vladimir,
Those SITs look promising. Do you have a spec-sheet? If searching for a low Ciss, triodelike FET the one below looks great. Hard to find though and expensive on eBay though. A pity they dont make tham any longer

Another way to get a FET like 2SK170 to get triode-characteristics is to use localfeedback.
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Old 25th January 2010, 09:04 AM   #5
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Originally Posted by revintage View Post
Hey Vladimir,
Those SITs look promising. Do you have a spec-sheet? If searching for a low Ciss, triodelike FET the one below looks great. Hard to find though and expensive on eBay though. A pity they dont make tham any longer

Another way to get a FET like 2SK170 to get triode-characteristics is to use localfeedback.
Lars, thanks a lot for the interesting datasheet. As of KP926A, I did some measurements by myself, enclose it. Enclose also datasheet in russian just for some minor information.
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File Type: jpg SIT-preamp Measurements of KP926A.JPG (172.2 KB, 1471 views)
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File Type: pdf SIT-preamp KP926 russian datasheet.pdf (42.4 KB, 178 views)
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Old 25th January 2010, 09:07 AM   #6
juma is offline juma  Germany
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Hi Vladimir,
I really like your preamp and I would like to build one myself but I have a problem finding a source for KP926A (google wasn't my friend this time). Can you recommend the on-line store or some other way to buy these SITs ?

I suppose this is the device you used:
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Old 25th January 2010, 09:22 AM   #7
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Originally Posted by juma View Post
Hi Vladimir,
I really like your preamp and I would like to build one myself but I have a problem finding a source for KP926A (google wasn't my friend this time). Can you recommend the on-line store or some other way to buy these SITs ?
I suppose this is the device you used:
Yes, the foto is correct. I made a search at russian site www.ChipFind.ru and it gave several possible suppliers

I bought them at http://www.anion.ru

Last edited by VladimirK; 25th January 2010 at 09:26 AM.
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Old 25th January 2010, 09:36 AM   #8
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Hey Vladimir,

Not so easy to locate them anyway as my russian is a little rusty. Could you help us out?

But shouldnt those suckers be great as output devices? Couldnt find Pmax in the datasheet though.
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Old 25th January 2010, 09:43 AM   #9
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Originally Posted by revintage View Post
Hey Vladimir,

Not so easy to locate them anyway as my russian is a little rusty. Could you help us out?

But shouldnt those suckers be great as output devices? Couldnt find Pmax in the datasheet though.
Yes, they look like output devices (almost TO-3 case), Id max = 16A, Vds max = 450V, dissipated power = (150-Tambient)/2,5 W
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Old 25th January 2010, 09:46 AM   #10
juma is offline juma  Germany
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Originally Posted by VladimirK View Post
Yes, the foto is correct. I made a search at russian site *лек‚€онн‹е компонен‚‹ и €адиоде‚али | ChipFind.ru and it gave several possible suppliers

I bought them at
Thanks Vladimir,
I found another one (I think it says Pd max = 50W, transconductance = 2 S ):
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Last edited by juma; 25th January 2010 at 09:50 AM.
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