lm386 pin usage changes from schematic to schematic, please clarify why

[musicman70]

Hello, I am a new to electronics but have built the lil gem amp and i love it. in my quest to understand some of the lm386 schematics i see that different pins are used, for example in the lil gem and the noisy cricket mkII, the input goes to 2, 3 and 4 are joined to ground and 5 signal out, 6 +v, 1 and 8 gain.

In the big daddy for example the input goes to pin 3, 2 and 4 are joined to ground, this seems to be opposite? to me
also i have seen schematics where the +v goes to 7 instead of 6, also i have seen 7 on 5 joined ,usually by a cap, or pot

this confuses me as i try to conceive new circuits, or attempt to integrate with current circuits 
any explanation or clarification is greatly appreciated

Thank you

[artifus]

http://www.ti.com/lit/ds/symlink/lm386.pdf

[Jdansti]

Opamps can be used many different ways, often as inverting or noninverting amplifiers.  While Wikipedia isn't always a good source of information, I think that this article might help.

http://en.m.wikipedia.org/wiki/Operational_amplifier_applications#section_2


Please see my corrected post below.

[NazzTazz]

LM386 is NOT an opamp

[musicman70]

Thanks
i thought i had a handle on it, the spec sheet makes sense in context to the lil gem, all seemed clear.
but once i see it used differently, even the spec sheet does not bring it clear.

can i assume{

"in the big daddy the reason the signal goes to pin 3 is because the fet buffer inverts it"
or maybe " the lm386 has two inputs out to mono, so use either and ground the other"
}

these concepts are something that i can understand in context with the circuit.
this would also help me reverse engineer the spec sheet and all the wiki's
its all Chinese till i see it on the board, then reverse the docs
again i am new to this, these 386 builds are the gateway drug

Thanks again

[Jdansti]

LM386 is NOT an opamp

Thanks for the correction. I'll edit my post. 

[NazzTazz]

Thanks
i thought i had a handle on it, the spec sheet makes sense in context to the lil gem, all seemed clear.
but once i see it used differently, even the spec sheet does not bring it clear.

can i assume{

"in the big daddy the reason the signal goes to pin 3 is because the fet buffer inverts it"
or maybe " the lm386 has two inputs out to mono, so use either and ground the other"
}

these concepts are something that i can understand in context with the circuit.
this would also help me reverse engineer the spec sheet and all the wiki's
its all Chinese till i see it on the board, then reverse the docs
again i am new to this, these 386 builds are the gateway drug

Thanks again

LM386's inputs are similar to opamp's ones. This means, you'll get an inverting output if you connect your input signal to pin 2, and a non-inverting out when input goes to pin 3.

About unused input pin, check the datasheet:

INPUT BIASING
The schematic shows that both inputs are biased to ground
with a 50 kW resistor. The base current of the input transistors
is about 250 nA, so the inputs are at about 12.5 mV
when left open. If the dc source resistance driving the LM386
is higher than 250 kW it will contribute very little additional
offset (about 2.5 mV at the input, 50 mV at the output). If the
dc source resistance is less than 10 kW, then shorting the
unused input to ground will keep the offset low (about 2.5 mV
at the input, 50 mV at the output). For dc source resistances
between these values we can eliminate excess offset by putting
a resistor from the unused input to ground, equal in
value to the dc source resistance. Of course all offset problems
are eliminated if the input is capacitively coupled.
When using the LM386 with higher gains (bypassing the
1.35 kW resistor between pins 1 and it is necessary to bypass
the unused input, preventing degradation of gain and
possible instabilities. This is done with a 0.1 μF capacitor or
a short to ground depending on the dc source resistance on
the driven input.

AM Radio PowerAmp (page 7) shows an example of inverting output.

LM386's are nice chips. I used a couple of them to build a quick'n'dirty poweramp for my comp speakers. Bass boost feature (page 6) (actually, if you check internal schematics, you'll understand it's kinda feedback loop) helps a lot with my crappy 3" speakers.

Some nice boxes are LM386-based (Lovepedal Plexi 800 for instance), gotta check their schematics to discover new ways to use that chip.

[Jdansti]

This power amp can be used as an inverting or noninverting amplifier depending on whether the input is at pin 2 or 3.  Maybe NazzTazz or someone else could explain when we should use one configuration vs another.

It can also be used as an oscillator.

Edit: Sorry NazzTazz-posted right after you did.

[deadastronaut]

i love a 386 for its pure simplicity..you can get a nice quick n dirty distortion out of those...with the 386n-1 to 386n-3/4.....great for breadboarding...

heres one i did a while back...nice easy build..with a great sound...i'll have to add a tone to it one day..

http://www.aronnelson.com/gallery/main.php/v/chickpea/BIGBASTARD14SEPT2010.gif.html

and demo  (testing out decay) some 386 designs have a fizzle on the dying notes....cured!

as you can see i used input 2, and tied 3/4 to ground, and added a 10uf to pin 7 for stability (that i read somewhere)....input 3 was just too noisey imo.

http://www.youtube.com/watch?v=W_5SZVyUP-Y&feature=plcp

just my 2p... rock on...

[PRR]

> in the lil gem and the noisy cricket mkII, the input goes to 2
> In the big daddy for example the input goes to pin 3,

The input section of the LM386 allows two inputs, pin 2 or pin 3. Like a store with two entrances.

They work 90% the same and in many cases you can use either one. Ground the unused input or it will pick-up crap from the air.

While learning, working from tested designs, it may be best to just copy which input pin was used in the plan you are copying. There are a few oddball tricks where it matters which input you use. Particularly non-standard "Gain" pin connections (gain pins set resistor-leverage against the input pins, and then it can matter which input pin you use.)

For growing your skills: understand how the LM386 gives two groundable inputs yet sets its output to half-supply and sets AC/Audio gain. It is a clever circuit and not THAT hard to analyze, though you better build some chops with 1 and 2 transistor circuits first.