PRE4 is based on a succesful Dispre 2-JFET preamplifier. The main difference is that the PRE4 uses only single differential input biased by constant current source, rather than complementary differential topology used in the Dispre 2-JFET. The main reason to do so is a global lack of 2SJ74 Toshiba JFET devices, that are getting very hard to buy and genuine parts are almost unobtainable. The input stage of PRE4 uses 2SK170BL devices, that are still in production and quite easily obtainable.
It is possible to use PRE4 as a headphone
amplifier, without any components changes needed.
Input JFETs are selected and matched for Idss tolerance +/-0.5mA
Output stage idle current is set at 25 mA.
PCBs are not available now.
Speed of the circuit is demonstrated by measurement of step response to square wave. It is measured with the input RC filter, and shows real speed of the assembled preamplifier.
THD is at the limit of my measurement method:
Pavel Macura, May 2011, modified 2/2012