From: Larry V East (firstname.lastname@example.org)
Paul -- I just read "The LM386 Cookbook" by you in the latest "Low Down".
Lots of good information there, but I was a bit amazed that you failed to
mention the use of a bypass cap on pin 7. A small electrolytic or tantalum
cap of a few uF from pin 7 to ground will isolate the high gain input stage
of the LM386 from power supply noise, hum, transients, etc. Such a bypass
cap is much more effective in this regard than the "brute force" Vcc filter
that you recommend in your note #2.
You aren't the only one to ignore this; most commercial rigs (and kits)
using the LM386 either fail to bypass pin 7 at all or use too small a value
(0.01uF or 0.1uF) to be totally effective. For example, Index labs uses a
value of 0.1uF in their QRP PLUS and increasing the value to 4.7uF
completely eliminates the "thumps" in the audio output produced by Vcc
transients when the transmitter is keyed.
I don't mean to be critical, but I just don't understand why the proper use
of the LM386 "bypass pin" seems to be overlooked by almost everyone (in
"hamdom", at least) using this device! It's even label "bypass" in the data
books (and your Fig. 2 in your "cookbook" as well).
72, Larry W1HUE/7
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