2x J310 Push-Pull Low Noise HF Pre-Amplifier 30 KHz- 30MHz

Fully built and tested HF low-noise preamplifier, based on a push-pull design with two J310 JFETs, optimized for 30kHz to 30MHz.

Bypass relay: The preamp is automatically enabled when DC power is present – either via the Bias-T or through the dedicated DC input connector
Bias-T functionality: DC can be injected via the output SMA, powering the amp through the coax
Gas discharge tube for input overvoltage protection

Performance:
Approximately 15dB gain from 500kHz to 30MHz
8-10 dB gain even above 55MHz
Lower gain observed above 15MHz in the 30 kHz-30MHz version
Noise figure: ~3dB
Output P1dB: +15dBm
Input/Output impedance: 50Ω

Technical Overview:
Two J310 JFETs biased at ~25mA each
Transformer T1 splits the input for balanced amplification
Transformer T2 recombines the signals, cancelling even-order harmonics
R1 and L1 provide negative feedback, shaping gain and bandwidth
Optional modification for 50kHz – 30MHz:
(Still usable down to 30 kHz for LF band users)
Replace T1 and T2 with new windings on BN73-202 binocular cores
T1: 2-turn primary / 12-turn center-tapped secondary
T2: 12-turn center-tapped primary / 2-turn secondary

By sacrificing the upper frequency response (30-55 MHz), you can now achieve a wider band from 30 kHz to 30 MHz without tweaking the feedback network.