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LM386 Mini Amp

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by drewlane, Mar 31, 2003.

  1. drewlane

    drewlane New Member

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    I'm trying to build a tiny mono amplifier with built in speaker.
    I want this thing to be super small so I can put it in my pocket and use it with my walkman or PDA.

    I have very little experience with this, and I'm struggling just to read a schematic diagram. :shock:

    Anyway, I found this little diagram and decided to give it a try:

    [​IMG]

    I'm not sure if this schematic is even what I need, but I went to Rad Shack and got the parts and built it. I used the smallest speaker I could find for this: 8 Ohm, 0.1 W (273-092)

    Well, it sorta works. I've been testing it with my walkman on an FM radio station, and it works for a couple of minutes but after that it starts making a whining, static noise or even starts osciallating. :?

    Can anyone tell me what I need to get this working properly?

    Do I need a different speaker or am I using the wrong circuit?

    I really want this project small, so I want to use the absolute minimum amount of components.

    Thank you!

    Drew
     
  2. Pilot

    Pilot New Member

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  3. Sebi

    Sebi New Member

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    Without Boucherot R-C components (on output) all amplifier IC can make self-oscillation.
     
  4. dave

    Dave New Member

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  5. drewlane

    drewlane New Member

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    I'm using a 9 volt battery. I assume you mean don't use a 9V wall wart.
    BTW, I really want to use a smaller battery. Any suggestions?

    Got it (although I dont' understand half of it), I can see there
    are some good examples on p. 5. Thanks!

    Since my speaker is so tiny. Perhaps I don't need the additional gain provided by jumping pin 1 and 8?


    OK. This is good, too. I must be missing a few components. :)

    If you have time, could you explain the functions of the capacitors and resistors in this diagram? For example, what do C3, C4, and R1 do?

    Thank you.

    Drew
     
  6. drewlane

    drewlane New Member

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    I don't really understand what that is.

    Are those components shown in this diagram:

    http://www.warplink.com/user/kencan/lm386.htm

    Regards,

    Drew[/url]
     
  7. Sebi

    Sebi New Member

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    On this sematic C4-R1 serial components called "Boucherot" .
     
  8. drewlane

    drewlane New Member

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    I finally got this little circuit working, although it took much longer than I expected. It was the lack of "Boucherot" that was causing the problem.

    I ended up using a 6 volt lithium L544 battery, as it was the smallest I could find with sufficent energy. I would still like to go smaller, however.

    I didn't put a on/off switch on the circuit to save space.

    I'm wondering what the drain on the battery is if the device is not plugged into an audio source?

    Will the battery drain at the same rate regardless?

    If this is a problem, what could I use to disconnect the battery after a few minutes of no audio signal? Is there a tiny chip that does this or something else that's really small?

    Thanks again,

    Drew
     
  9. zambo

    zambo New Member

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    Hi im olny 15, but you would be better using a LM308N 14 pin i have a data sheet on it and it has plans for a mini amplifier if you want it i could possible help you

    Zambo ()
     
  10. bogdanfirst

    bogdanfirst New Member

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    look, i have a pcb and schematic for a LM386 amp that i have build and the pcb has a center hole so the speaker fits in perfect....
    i can send it to you
     
  11. Mosfet

    Mosfet New Member

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    Isn't "Boucherot" more commonly called a "Zobel" circuit?

    [Sorry Drew, not nubie stuff.]
     
  12. leejongfan

    leejongfan New Member

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    I have been trying to use this LM386. Actually, when I used breadboard to build my 1st prototype, it worked fine (yeah still had noise but acceptable, at least i can understand the sound)

    But now I am doing the prototype using PCB and its performance is very bad. I cant even hear anything except noise and buzz.

    [​IMG]

    I also used decoupling cap for the power supply and an 8Ohm speaker.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Leejongfan,
    You have a capacitor between pin 1 and pin 8 which increases the voltage gain 10 times.
    If you don't use shielded audio cable at the input then your input wires are an antenna picking up mains hum and buzzing.
     
  14. leejongfan

    leejongfan New Member

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    Hi audioguru,

    Really appreciate your help!
    I searched on Internet to know what "shielded cable" means. But not really understand though.
    Do you know anyways I can build a self cable myself (no need to be so perfect)
     
  15. Hero999

    Hero999 Banned

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    Shielded cable has an outer braid which acts as a faraday cage and prevents it from picking up electromagnetic radaition

    If you don't have any shielded cable then try a twisted pair which might be good enough.
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Drew,
    The datasheet for the LM386 has all the details for it.
    Its quiescent supply current changes a little when the supply voltage is changed but is typically 4mA with a max of 8mA when its supply is 6V and there is no signal.
     
  17. leejongfan

    leejongfan New Member

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    Thank you so much for your reply.
    But could you please elaborate more ? Right now I only have single-core wire
    Twisted pairt: did you mean I have to "twist" or "wound" together 2 wires ? Like this link Twisted pair - Wikipedia, the free encyclopedia
    These 2 wires are for the power supply ( GND and Vcc) or for the input audio signal?
     
    Last edited: Nov 5, 2008
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    All audio equipment is connected with shielded audio cable. Get some.
     
  19. saturn1bguy

    saturn1bguy New Member

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    Perhaps you've heard of coax, or coaxial wire? That's a form of shielded cable. The antenna cable to your television, or from the cable company, that's coax, or shielded cable. Shielded audio cable sometimes looks like really thin coax.

    And sometimes shielded audio cable looks externally like zip cord, like your average 18 gauge, two-conductor power cord, but indeed internally it has a braided-wire shield around the inner conductor, two each for stereo.
     
  20. Hero999

    Hero999 Banned

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  21. tidos76

    tidos76 New Member

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    i am also building an amp with the lm386. i have successfully used it on 3.6v. running a 8ohm speaker. i get alot of distortion at high volumes and i'm starting to get a lot of interference. the guys that previosly posed that they had designs using this amp, would you mind sending them to me? i'm also looking to the smallest design possible.

    leejongfan,
    when using the ocb board keep in mind everything is mirrored if you are using through hole components on the top layer, i ran into the problem.
     

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