[MRCA] Mini Speaker Amplifier LM386 tips

[B. Smith]

        Well having set up "Operational" displays at  dozens of Air 
Shows, military rallies and ham fests as well as  the annual Field Day.  
I've found the audio output  from the LM386 sufficient for boosting the 
audio from most military sets  and it is very clean. It allows visitors 
to listen in - - - and having worn a headset for most of my career it 
gives my ears a break.
        A small LM386 amp can run off a 9 volt battery for hours and 
hours and the best part is with its own power supply it is completely 
floating and isolated from the AC mains and or other power supplies.  
The batteries are cheep at the Dollar Store.
Z

On 3/31/2015 1:31 PM, Ray Fantini wrote:
> The LM-386 is a fairly piss week device with maybe a half watt output at best. Instead of wasting time with that the LM-383 will provide five to six watts of output into a 4 ohm load, requires a minimum of external components and has output protection to take care of shorts or thermal issues. The 383 was designed from the ground up as an audio power amplifier, the 386 works for that but is perhaps more a general use op amp? The only bad thing about the 383 for mill use is its maximum recommended device voltage is 20 VDC but it will withstand up to 40 so it's not designed to be connected directly to a 24 volt bus.
>
> Ray F
>
> -----Original Message-----
> From: MRCA [mailto:mrca-bounces at mailman.qth.net] On Behalf Of B. Smith
> Sent: Tuesday, March 31, 2015 11:48 AM
> To: mrca at mailman.qth.net
> Subject: [MRCA] Mini Speaker Amplifier LM386 tips
>
>            When building your own "mini" audio amplifier  for your military radio using the LM386 chip it is important to not get greedy with the gain of the chip.  The chip has "default" amplification factor of 20 without any external caps or resistors. I suggest that you start off with that default  gain of 20  when using a military handset  set audio to drive the input. Combinations of caps and resistors between pins 1 and 8 can be added to allow selection of various  gain levels from 20 to 200! It is very easy to overdrive the chip with the high audio level from a military radio resulting in distortion.  Consult the data sheet for the chip for examples of different gain settings.
> The Radio Shack amp uses a unique 1 transistor stage prior to the LM386 and varies the gain on that transistor prior to the input of the LM386 -
> - - it is a great circuit and easily copied.  Then following that single transistor stage  on the Radio Shack amp using the  LM386 a combination of 10uF and a 100 ohm resistor is utilized between pins 1 and 8 to allow a fixed gain setting.
> Z
>
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