PRE4

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

PCBs are not available since 02/2012.

Technical parameters

• Output voltage max. 26 Vp-p / 600 ohm (9.19 V ef)
• Load impedance - can drive even 24 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
• CCIF IMD 19+20kHz at 1Vrms/600 ohm - less than 0.001%
• Gain - adjustable from 1x (0 dB) to 5x (14 dB) by resistors in a voltage divider
• Output impedance - less than 5 ohm
• Supply/consumption - 2 x 15V stabilized, 2 x 100 mA
• Power supply requirements - 2 x 15V stabilized, at least 2 x 200 mA

Measurements

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:

Price:

PCBs are not available now, please see the pricelist.

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Pavel Macura, May 2011, modified 2/2012