136 kHz preselector and amplifier
By Claudio Pozzi, IK2PII - August 2001

download pdf schematic

1.  Introduction

This project was developed to reduce the image frequency response of my direct conversion receiver. It can also e useful as front end for HAM communications receivers and also for selective level meters used as LF receiver. The project focus was selectivity, not gain.

2.  Circuit description

The input filter coils are slug tunable 70 microhenry surplus pot core coils. The capacitor C2 define the filter bandwidth, was chosen by trials. Tuning is sharp, centered at 137.650 kHz for the QRSS portion of the band.
The gain provided by Q1 is enough to compensate the filter loss.
The output transformer T1 is winded on a Amidon FT37/43 toroidal core; primary is 20 turns, secondary is 5 turns.

136 kHz preamplifier schematic

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