L.F. Pre-amplifier.
This circuit was designed by Dave Pick G3YXM. It was first published on Dave's web site and more recently has been reprinted in the September 2000 edition of the RSGB's Rad Com magazine.
Most commercial amateur receivers have a relatively poor performance when tuned
directly to the 136kHz LF band.
It is common for receivers to have an attenuator permanently in circuit below
1.8MHz to prevent overload from
high powered broadcast stations. Unfortunately this and a general 'lack of
performance' below 500kHz severely reduces the ability of the
receiver to pick up weak amateur transmissions. However, a pre-amp will overcome
these losses and restore your radios sensitivity.
This 136kHz Pre-Amp can be assembled
in less than 15 minutes
The pre-amp consists of a top coupled bandpass filter using a pair of Toko
719VXA-AO17AO coils followed by a FET
amplifier and a bipolar transistor buffer. The overall gain is 10dB. This is
enough to overcome the losses of most commercial receivers. The bandpass filter has a
narrow response which prevents the preamp being overloaded by broadcast and other high powered
transmitters operating in the LF / MF spectrum.
Input / Output impedances are suitable for 50 / 75 Ohm systems.
The circuit is mounted on a single sided 44 x 34mm printed circuit board.
136kHz receive pre-amplifier.
The circuit is available as a kit of parts, or ready assembled tuned and
tested.
It requires a nominal 12V supply at about 10mA.