LM380 audio boost WORKS!

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From: Paul Harden (pharden@aoc.nrao.edu)
Date: Fri Mar 22 1996 - 20:11:35 EST


Here is the positive feedback modification for the 40-9er to boost the
audio output from the LM380 ... breadboarded and tested only moments ago.
It works, and works great. (Schematic below). Measurements based on my
14-pin LM380, but should be virtually identical to the 8-pin LM380.

LM380 GAIN PROFILE with a 9V battery (at 8.6v loaded)

       NO FEEDBACK CIRCUIT WITH FEEDBACK CIRCUIT
 Vin Vout Av Gain Vout Av Gain
----- ------------------- ---------------------
 10mv 400mv 40 32dB 1200mv 120 42db
 20 850 42 32 3000mv 150 43
 30 1500mv 50 34 5100mv 170 45
 40 1800 45 33 6400mv 160 44
 50mv 2200mv 44 33 Output saturates above
 60 2100 35 31 50mVpp audio input
 70 2000 29 29
 80 2000 25 28
100mv 2000 20 26

Where: Vin=input test sine wave, peak-peak voltage
      Vout=output sine wave, p-p, with 8-ohm load (speaker)
        Av=voltage gain (Vout/Vin)
      Gain=db 20log(Av)

SUMMARY: On both headphones and speaker, kicking in the feedback circuit
makes the audio sound 2-3 times louder. I could hear audio in the
earphones with Vin down to 4mV. If it were CW, I could have copied it.
It is a 3 component modification, which can be added on the circuit side
of the board, takes no extra board space, and pretty easy to do.
 
MODIFICATION CIRCUIT

                             _______ LM380N-8 (8-pin flavor)
Audio in from | |
40-9er RFC3 ---------------3|-IN | + C13
                            | OUT |6------)|--*----->to headphones
              -----*---*---2|+IN | | or speaker
              | | | | | |
            .001 15K | |_______| |
             cap res | |
              | | |--------- 1Meg Res -----|
             gnd gnd

MODIFICATION STEPS FOR THE 40-9er:
1. Remove power (hey, all instructions always start with that one.
   You don't wanna get knocked on your butt from that 9v battery!)
2. Cut trace on LM380 to isolate pin 2 (+IN) from ground.
3. Solder 15K resistor on LM380 from pin 2 to pin 4 or 5 (ground).
4. Solder 1Meg resistor from (-) side of C13 to LM380 pin 2.
5. Solder .001 cap across the 15K resistor, or from pin 2 to ground.
6. Apply power; there should be a very noticeable increase in audio
   gain.

I did not try different values for the feedback resistors. If you roll
your own, make the resistor from pin 2 to ground to be .015 times the
feedback resistor ... or, the same ratio as above. The cap from pin 2
to ground prevents audio oscillation and can be .001 to .1 range.
Frankly, my sloppy breadboard of the above didn't need the cap, but
added it as a precaution anyway.

Try it, have fun, work somebody with it, and let us know how it went.
And, I think us QRPers are the first human beings to actually employ
the positive feedback design from the LM380 application notes AN-69.

Have a good weekend,
Paul NA5N


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