Gang,
Get out the 'Data Book for Homebrewers and QRPers'
by our fellow member Paul Harden, NA5N. Turn in
your hymnal to page .... Oh drats, no page numbers.
OK, go look at the LM386N data sheet about 0.5218th
of the way through the book. :-)
Down at the bottom of the page is a 'typical application'
schematic for a 46dB audio amplifier. In playing with
the Pixie 1, 2, and W1FB version and the Super Pixie
I prototyped this thing up on a breadboard.
You don't need anything connected to pin 3. What I was
doing was writing up the assembly instructions for the
Super Pixie and was thinking about the individual(s) that
don't have a scope, audio generator, etc. and figuring
out how you would check to see if it working.
By clipping a test lead to pin 1 or pin 8 I was able to
hear very loudly KRLD radio here in the Dallas/Ft Worth
area. I learned a lot about some legal issues that were
being discussed on a talk show run by two lawyers.
The age of news stations and talk radio.
This little circuit had enough drive to hear the earphones
clearly across the room, so make sure that you put in
the R1 variable on the input for a lot of applications.
Couple of things that I found and would like others to
confirm (or disagree if you find out differently) is:
1. Pin 5 is the output pin number (that is a given).
2. Change R2 from 100 to 200 ohms. I found no
difference in the output, but current to LM386
went from 44 mA down to 33mA (on the average)
with 200 ohm resistor. W1FB shows a 1K, but I
didn't like that value at all.
3. Pin 2 is the + input and pin 3 is the -. Pin-out
diagram in the NA5N book shows it correctly, but
the functional circuit diagram and the typical
application diagram are showing minor differences.
Paul will fix this in the next revision of the databook.
4. C4 at 10uF is a good value for CW and SSB work as
noted by Paul
5. C4 and C3, both electrolytics may be used as
shown or reversed. Polarity doesn't come into play
for this application.
6. R3 and C5 combo made the output less distorted at high
output levels.
So, if you have a group of kids or beginners that want
an instant radio, here is your answer. Less than $2 or
so worth of parts, a $30 prototyping board, and Sony
earphones and you are up and running. :-) And about
$10 for the Gel-cells. It's the little things that add
up, don't they? (LM386 is $0.95 from Digi-Key and
$0.59 from BG Micro and the 1,000 PN2222 NPN PC lead
transistors for $20 from BG Micro are house numbered but
they are good transistors)
Ooops, Phyllis just came in to see what the noise
was. "Honey, that's right. It's coming from that mess
of wires that I have on this board here on the top of
the desk, which is a mess, but it'll be gone by the
end of the week. I'm designing and building my own
radios so that I can save a lot of money by not buying
them off the shelf like everyone else. Look a complete
transceiver for less than $25!!" -- "What? No, you're right.
I didn't count the price of the scope, RF generator, the
gel-cells, the charger, the protoboard, the life time
supply of parts, the books, the club dues, Dayton trips,
the keyer, the paddles, the cables, the antenna(s), but
I am impressed that you know what all this stuff is now."
That's not what happened but we can all see what
reality is with this hobby.
The point here is, change R2 to 200 ohms. W1FM shows
1K for R2, but I found that caused considerable distortion
in the output and a reduced output power to the headphones.
Circuit runs on 9V or 12V.
For those having problems with the little rigs, like the
Pixie, with local AM BC stations being received I'll bet
it may be the LM386 circuit. I'll also write up later
on the web page a thing about mixers and non-sine wave
LO effects. This is taking some time so hold on. This
is going to be a long article, but hopefully this will
help fill in the why and how issues for many listeners.
So if some xcvr's are not in a case you will have a good
chance of receiving additional signals not wanted or
desired.
FYI
Chuck Adams K5FO CP-60 adams@sgi.com
http://reality.sgi.com/employees/adams_dallas/
WIMPS: Qs=000 30m=0 17m=0 12m=0 States=00/00/00
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