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LM386 Mini Amp

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drewlane

New Member
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:



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
 

Pilot

New Member

Sebi

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

drewlane

New Member
Pilot said:
First thing is... Don't use +9/-9 as a power supply.
These you can use with a single ended 9V supply.
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?

Pilot said:
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?


Pilot said:
This is a common ic used in amateur radio low power home built transceivers and a good example of this ic in use can be found at....
http://www.warplink.com/user/kencan/lm386.htm
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
 

drewlane

New Member
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
 

zambo

New Member
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 ()
 

bogdanfirst

New Member
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
 

leejongfan

New Member
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.



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

audioguru

Well-Known Member
Most Helpful Member
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.
 

leejongfan

New Member
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.
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)
 

Hero999

Banned
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.
 

audioguru

Well-Known Member
Most Helpful Member
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.
 

leejongfan

New Member
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.
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?
 
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audioguru

Well-Known Member
Most Helpful Member
All audio equipment is connected with shielded audio cable. Get some.
 

saturn1bguy

New Member
But could you please elaborate more?
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.
 

Hero999

Banned

tidos76

New Member
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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