CW RECEIVER FOR 80 AND 40 METERS
WITH I.C. AUDIO STAGE
The easy 80 and 40 meter Simplest Ham Receiver described on this site
has proven to be (after 2 decades!) versatile, stable, durable and effective. However,
interest and curiosity has been expressed for a version with an audio amp section using integrated
circuits instead of transistors.
The circuit shown above using the 8-pin LM741 op-amp with an 8-pin LM386 audio amp was chosen because
it had been used successfully as an amplifier driving a speaker for an electric guitar, an MP3 player,
a small Theremin, and an infrared-pulse detector. It has an
absolute minimum of support components. The source is
Getting Started in Electronics by Forrest M. Mims III
and also appears in his Engineers' Mini-Notebooks: Communication Circuits
where it is shown as an amplifier for a crystal radio. The only modifications are the
substitution of the 100K resistor for a 100K potentiometer and omission of a switch for the
10uF capacitor; both changes were made for simplicity and to maintain high gain. Performance is about the same as the Simplest Ham Receiver,
but with much more volume, no Audio Q-Multiplier
is needed to enhance performance. All parts are very
inexpensive and available virtually everywhere.
Shown above left is the audio power supply: a +9v/-9v dual-polarity using two
9-volt batteries for the 741/386 board. At right is shown a separate 9-volt supply providing
regulated +6.8 volts for the oscillator(s). There is no connection between the two sections,
not even the ground. Having two separate common grounds seems to prevent unstable interaction between
the audio section and oscillators and appears to reduce hand-proximity capacitance detuning
of the VFO (if a VFO is used). The OSC PWR switch chosen is the type that attaches
to the 10k VOLUME pot [or, use a 10k pot that includes an on/off switch]: even with power constantly applied
the crystal oscillator will not function
unless a crystal is inserted in the circuit; this plus the VFO I/O switch permits
either, neither, or both the crystal oscillator and
the VFO to function at any time while the OSC PWR switch is "on".
Instead of just substituting these modifications on my existing Simplest Ham Receiver, an entirely new version was built using a cigar box.
The cigar-box version has all parts very close
together and no metal panel for the VFO section. When using headphones with my station's setup, and
when transmitting more than about a watt, the antenna lead-in line to the receiver requires an external
cutoff switch to prevent gross overloading; back-to-back diodes do not suffice (diagram at right). However, when used
with transmitters running about a watt or less, the signal can be monitored comfortably without
use of a cutoff switch as long as the volume is turned all the way down. When using a
simple external audio amplifier and speaker no cutoff switch is needed.
PHOTOS OF THE CIGAR BOX VERSION
In photo at left, a cutoff switch is housed in a plastic box.
The cables that connect the box from the T/R device to the receiver are common ones left over
from a DVD player. The cutoff switch is not needed at this station when
a simple audio amp and speaker is used instead of headphones. This simple addition makes the operation
of the receiver much easier and more pleasant. Oscillator crystals also are shown, including
one inserted in a crystal holder: the crystal oscillator is located under the chassis.
Less than $50 was spent on this project, but lot of parts were on-hand.
If everything was purchased the total might come to over $75, which isn't bad for a 2-band
receiver capable of receiving more bands if used with converters.