||25th January 2010 05:59 AM
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.