Dispre 2 - JFET
Dispre 2 - JFET is a modification of Dispre 2 V4 preamplifier. The main difference is that Dispre
2 JFET has Toshiba 2SK170/2SJ74 devices in the input stage,
not the bipolar transistors that were used in the Dispre 2 V4.
This modification resulted in a design of a new PCB board, as it
was impossible to simply replace BJTs by JFETs, circuit
modification of the input stage had to be made. At the same time,
I added two resistors to enable setting of the output stage idle
Dispre 2 JFET can be also used as a headphone amplifier, without any components changes needed.
Dispre 2 JFET has complementary differential circuit topology with self biased JFET input stage (this topology was introduced by John Curl). This maximizes linearity of the input stage and distortion suppression does not rely only on an overall negative feedback.
Input JFETs are Toshiba parts with Idss
range 4 - 9mA. The JFETs are selected and matched
for Idss tolerance +/-0.2mA
Output stage idle current is set at 40mA, and it works in a class A up to 8V into 100 ohm load. For standard link load 600 ohm and higher, the output stage works in a class A for the whole range of output voltage up to clipping.
Dispre2 - JFET has wide bandwidth and high slew rate again, similarly as Dispre 2 V4.
Some of my customers required an option of overall gain reduction, not to use the potentiometer only at the beginning of its track. So I added two resistors, that enable to make a choice of overall gain from +1 to +5 (0dB to 14dB).
PCBs are not available now.
Recommended preamp layout and wiring
Speed of the circuit is demonstrated by measurement of 100kHz square wave. It is measured without capacitor in the input RC filter, to show speed and stability of the preamplifier.
Pavel Macura, May 2010