The preamplifier board contains a high dynamic range and low noise JFET amplifier and the control circuit for it. It uses a common gate schematic with norton feedback to reach the necessary performance (The design was taken from the single band RA3AO transceived published in the Radio magazine in 1983). The main purpose of this amplifier is to impoove the noise figure on the quiet higher bands (15m,12m,10m).
It has a low gain (8-9 dB), so it only slightly degrades the IMD DR when in use (see specifications page).
Two reed relays are used to switch it on/off and to take it out of the signal path during the transmitting.
The schematics of the Preamplifier board is here.