Here is an image of an amplifier circuit.
How can I modify this for a 10-50 times more gain. I put a wire from pin 1 to 8, but I didn't get enough gain.
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1\$\begingroup\$ Start with a data sheet -- ti.com/lit/ds/symlink/lm386.pdf. If you've place a capacitor without a resistor between pins 1 and 8, that's maximum gain of 200. \$\endgroup\$– Scott SeidmanCommented Jun 4, 2013 at 22:48
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\$\begingroup\$ The LM386 gain can be varied from 20 to 200, a factor of 10. What value of gain do you want? \$\endgroup\$– Alfred CentauriCommented Jun 4, 2013 at 22:49
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\$\begingroup\$ I dont know exactly, but more than a wire across pin 8 and 1 \$\endgroup\$– skylerCommented Jun 4, 2013 at 23:54
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\$\begingroup\$ Wow, that's the first time I've seen LM386 described as tube-like. \$\endgroup\$– markragesCommented Jun 5, 2013 at 1:21
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1\$\begingroup\$ @markrages They are about the same age, aren't they? ;o) \$\endgroup\$– jippieCommented Jun 5, 2013 at 6:19
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2 Answers
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Leave pin 1 to 8 open. Add another gain stage ahead of the LM386.
simulate this circuit – Schematic created using CircuitLab
This non inverting amplifier has a gain of \$A=1+\dfrac{R_f}{R_i}\$. Gain can be increased by increasing the value of the feedback resistor. Add the LM386 gain stage and total gain becomes \$A=20(1+\dfrac{R_f}{R_i})\$.
The voltage divider and capacitor add a DC bias to the the non-inverting pin. This will allow it run from a single supply, and acts as a 50 Hz high pass filter. Do not forget to add C2 as well. Without it, the amplifier will just slam the positive rail.
However, if this is for that induction loop you keep talking about, I doubt you will ever be able to get enough power out of an LM386.
answered Jun 5, 2013 at 0:42
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\$\begingroup\$ The TL082 is just a 2 gate version of the TL081. \$\endgroup\$ Commented Jun 5, 2013 at 22:59
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\$\begingroup\$ I don't have a good neg. voltage. Can I just split the 12v into 2 6 volt suplies somehow? \$\endgroup\$– skylerCommented Jun 6, 2013 at 14:03
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\$\begingroup\$ @skyler Make the changes I indicated on the schematic, and connect the power pins to 12V and ground. \$\endgroup\$ Commented Jun 6, 2013 at 21:47
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\$\begingroup\$ Ok. So did you add some sort of high-pass filter? \$\endgroup\$– skylerCommented Jun 6, 2013 at 22:23
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There is another way to set the gain, which the datasheet briefly mentions but it doesn't go into any detail.
"Gain control can also be done by capacitively coupling a resistor (or FET) from pin 1 to ground."
See:
http://www.wentztech.com/radio/resources/Technical-Info/_LM386/Unleash-lm386.pdf
which discusses how to use that method to get a gain of over 4500 (73dB). It requires careful layout and grounding.
