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 current.

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

PCBs are not available now.

Recommended preamp layout and wiring

Technical parameters (updated 06/2022)

• Output voltage max. 26 Vp-p / 600 ohm (9.19 V ef)
• Load impedance - can drive 32 ohm headphones
• Freq range (-3dB) 0.1 Hz – 220 kHz for potentiometer set at 100%
• Freq range (-3dB) 0.1 Hz – 150 kHz for potentiometer set at 50%
• THD at 1Vrms/600 ohm - less than 0.001% through audio range, 0.0001% at 1kHz
• CCIF IMD 19+20kHz at 1Vrms/600 ohm - less than 0.0004%
• Gain - adjustable from 1x (0 dB) to 5x (14 dB) by resistors in a voltage divider
• Output impedance - 10 ohm
• Supply/consumption - 2 x 15V stabilized, 2 x 120 mA
• Power supply requirements - 2 x 15V stabilized, at least 2 x 200 mA

Measurements

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.

New measurements taken in June 2022

Home

Pavel Macura, May 2010, updated June 2022