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Lm386 amplifier

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alec_t

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would like to do two inputs on it.
Do you mean add a mixer stage? What signal levels/sources will the input devices be? Volume controls needed?
 

JLNY

Active Member
It should be possible using a summing amplifier configuration. An internet search for "op amp summing amplifier" should yield plenty of examples, but you will need to change some things around to work with the LM386 (which is not an op amp per se) and depending on how you want to do your volume controls. Building a separate mixer stage before the input to the to LM386 may be a better way of doing it. What is the intended application here?
 
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Do you mean add a mixer stage? What signal levels/sources will the input devices be? Volume controls needed?
Just another input mixer type.

It should be possible using a summing amplifier configuration. An internet search for "op amp summing amplifier" should yield plenty of examples, but you will need to change some things around to work with the LM386 (which is not an op amp per se) and depending on how you want to do your volume controls. Building a separate mixer stage before the input to the to LM386 may be a better way of doing it. What is the intended application here?
Do I have to add an op amp for just another input and volume knob??
 

dr pepper

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Most Helpful Member
For such a simple amp like a lm386 you could just use the lm386 as a summign amp.
30 secs of googling and I found this, you could do without the pots if you wanted:

https://www.google.co.in/search?q=dual+input+lm386+amp&biw=1280&bih=913&tbm=isch&imgil=fw2aAUSRzMNsXM%3A%3BHIkoI8pRx404dM%3Bhttp%253A%252F%252Fwww.m0716.com%252Flm386%252Flm386-amplifier-circuit-diagram&source=iu&pf=m&fir=fw2aAUSRzMNsXM%3A%2CHIkoI8pRx404dM%2C_&usg=__etS1Vfii07gvc3aqCNZ0VXsOP6c=&ved=0ahUKEwjXieD-_IzPAhWKiCwKHQZcAh0QyjcILg&ei=2UXYV5e5NoqRsgGGuInoAQ#imgrc=fw2aAUSRzMNsXM:

To make the inputs equal replace R6 for 27k and R7 for 10K, if you dont want the pots connect the audio direct to the end of R6/R2 that went to the pots to the audio source.
The values of the input resistors affect the gain of the amp.
 

alec_t

Well-Known Member
Most Helpful Member
Whether or not an opamp (preamp) is needed depends on what the input sources are. You didn't answer all the questions in post #2.
 

JLNY

Active Member
For such a simple amp like a lm386 you could just use the lm386 as a summign amp.
30 secs of googling and I found this, you could do without the pots if you wanted:

https://www.google.co.in/search?q=dual+input+lm386+amp&biw=1280&bih=913&tbm=isch&imgil=fw2aAUSRzMNsXM%3A%3BHIkoI8pRx404dM%3Bhttp%253A%252F%252Fwww.m0716.com%252Flm386%252Flm386-amplifier-circuit-diagram&source=iu&pf=m&fir=fw2aAUSRzMNsXM%3A%2CHIkoI8pRx404dM%2C_&usg=__etS1Vfii07gvc3aqCNZ0VXsOP6c=&ved=0ahUKEwjXieD-_IzPAhWKiCwKHQZcAh0QyjcILg&ei=2UXYV5e5NoqRsgGGuInoAQ#imgrc=fw2aAUSRzMNsXM:

To make the inputs equal replace R6 for 27k and R7 for 10K, if you dont want the pots connect the audio direct to the end of R6/R2 that went to the pots to the audio source.
The values of the input resistors affect the gain of the amp.
Yup, that's basically what I had in mind, but I wasn't sure whether to spell it out quite so bluntly or not. That is probably the minimum configuration for doing it. Here is a quick sketch of what I guess I imagined (I haven't checked all these values to see if they work or not):

386summingamp.png

Adding a cap across pins 1 and 8 and a bypass cap will increase the gain as per the OP's original reference material. The values of the potentiometers R1 and R2 may need to be changed if the signal source being used can't drive a 1k load (OP has not stated the application). Something like a computer headphone jack or sound card meant for driving 32-Ohm headphones should have no problem driving a 1k load, but other signal sources may depend. My coupling capacitor values are selected for a roughly 20-30 Hz cutoff frequency, which may be a bit overkill here.

EDIT: inverting and non-inverting inputs were switched. Fixed.
 
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dr pepper

Well-Known Member
Most Helpful Member
Looks good, 680n caps nice touch, even less 470's might be an idea, max audible volume would probably increase as I doubt this circuit will be used with a speaker that can reproduce less than 80hz or so anyway, so theres no point wasting power trying.
 
For such a simple amp like a lm386 you could just use the lm386 as a summign amp.
30 secs of googling and I found this, you could do without the pots if you wanted:

https://www.google.co.in/search?q=dual+input+lm386+amp&biw=1280&bih=913&tbm=isch&imgil=fw2aAUSRzMNsXM%3A%3BHIkoI8pRx404dM%3Bhttp%253A%252F%252Fwww.m0716.com%252Flm386%252Flm386-amplifier-circuit-diagram&source=iu&pf=m&fir=fw2aAUSRzMNsXM%3A%2CHIkoI8pRx404dM%2C_&usg=__etS1Vfii07gvc3aqCNZ0VXsOP6c=&ved=0ahUKEwjXieD-_IzPAhWKiCwKHQZcAh0QyjcILg&ei=2UXYV5e5NoqRsgGGuInoAQ#imgrc=fw2aAUSRzMNsXM:

To make the inputs equal replace R6 for 27k and R7 for 10K, if you dont want the pots connect the audio direct to the end of R6/R2 that went to the pots to the audio source.
The values of the input resistors affect the gain of the amp.
Hi Dr Pepper ok thanks for leading me to this.
Whether or not an opamp (preamp) is needed depends on what the input sources are. You didn't answer all the questions in post #2.
Alec I am just a novice in electronics but I built this amp from youtube cause it was simple. So certain questions would be out of my knowledge.
Yup, that's basically what I had in mind, but I wasn't sure whether to spell it out quite so bluntly or not. That is probably the minimum configuration for doing it. Here is a quick sketch of what I guess I imagined (I haven't checked all these values to see if they work or not):

View attachment 101192

Adding a cap across pins 1 and 8 and a bypass cap will increase the gain as per the OP's original reference material. The values of the potentiometers R1 and R2 may need to be changed if the signal source being used can't drive a 1k load (OP has not stated the application). Something like a computer headphone jack or sound card meant for driving 32-Ohm headphones should have no problem driving a 1k load, but other signal sources may depend. My coupling capacitor values are selected for a roughly 20-30 Hz cutoff frequency, which may be a bit overkill here.

EDIT: inverting and non-inverting inputs were switched. Fixed.
Hi JLNy thanks for that schematic I will try this.
 

JLNY

Active Member
Alec I am just a novice in electronics but I built this amp from youtube cause it was simple. So certain questions would be out of my knowledge.

Hi JLNy thanks for that schematic I will try this.
Glad I could help. Fair point about the caps, Dr. Pepper. 470nF should be fine as well, or even lower if those aren't available, but I would try not to go lower than about 180nF for C3 and C4.

Also, I think you may be over-thinking Alec's question a bit. :D I think his main question (and my question as well) is simple: what is your input device? is it a computer? a phone? a guitar? an Arduino? A bit of context as to what you are using and what you are trying to do with your circuit would be helpful. If something we said is too complex, just ask us to clarify. It's not a bad thing to ask questions, even if they seem basic. :)

The reason that knowing the input device is important is because different devices will have different output impedance and output signal levels. Something like a guitar pickup has a high output impedance and might not be able to drive a low impedance load like in my schematic. A source with too low a signal level might need more amplifier stages to drive the LM386, and too high a signal level might overdrive the chip at all but low volume.

Let us know how it works if you build it!
 
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Glad I could help. Fair point about the caps, Dr. Pepper. 470nF should be fine as well, or even lower if those aren't available, but I would try not to go lower than about 180nF for C3 and C4.

Also, I think you may be over-thinking Alec's question a bit. :D I think his main question (and my question as well) is simple: what is your input device? is it a computer? a phone? a guitar? an Arduino? A bit of context as to what you are using and what you are trying to do with your circuit would be helpful. If something we said is too complex, just ask us to clarify. It's not a bad thing to ask questions, even if they seem basic. :)

The reason that knowing the input device is important is because different devices will have different output impedance and output signal levels. Something like a guitar pickup has a high output impedance and might not be able to drive a low impedance load like in my schematic. A source with too low a signal level might need more amplifier stages to drive the LM386, and too high a signal level might overdrive the chip at all but low volume.

Let us know how it works if you build it!
Ok I did not understood his question then. Yes I am using it for a guitar and voice.
 

JLNY

Active Member
Ok I did not understood his question then. Yes I am using it for a guitar and voice.
Ah, that could be a problem, then. If you are going to be driving this directly from a guitar pickup with no preamplifier of any kind, it will likely not be able to drive the 1k potentiometer at the input. The microphone for voice will probably also need a preamplifier, but the exact configuration will depend on what kind of microphone it is. I am sure there are many schematics online which you can use for microphone and guitar preamps. If the guitar has some kind of active preamplification, it might be okay, but I am by no means an expert on musician's amplifiers.
 

audioguru

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Most Helpful Member
The volume control resistances are much too low at 1k. Many circuits cannot drive it. Use 10k or 20k.

A guitar magnetic pickup must have a high resistance load. Usually a vacuum tube or a Jfet is used. It does not need much gain because the pickup output level is high. The resistance of its load determines the amplitude of the medium-high audio frequency peak that is a resonance that creates the "twang" sound of an electric geetar. Here is a guitar preamp that has 3M input resistance and a graph showing the peak determined by the load resistance. The gain of this preamp is fairly low.
 

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Ah, that could be a problem, then. If you are going to be driving this directly from a guitar pickup with no preamplifier of any kind, it will likely not be able to drive the 1k potentiometer at the input. The microphone for voice will probably also need a preamplifier, but the exact configuration will depend on what kind of microphone it is. I am sure there are many schematics online which you can use for microphone and guitar preamps. If the guitar has some kind of active preamplification, it might be okay, but I am by no means an expert on musician's amplifiers.
I already built the amp but with one input and sounds ok.
The volume control resistances are much too low at 1k. Many circuits cannot drive it. Use 10k or 20k.

A guitar magnetic pickup must have a high resistance load. Usually a vacuum tube or a Jfet is used. It does not need much gain because the pickup output level is high. The resistance of its load determines the amplitude of the medium-high audio frequency peak that is a resonance that creates the "twang" sound of an electric geetar. Here is a guitar preamp that has 3M input resistance and a graph showing the peak determined by the load resistance. The gain of this preamp is fairly low.
ok the one input that I have has 10k volume. maybe I try the two input scheme with 10k and see what happens.
Thanks guys.
 

audioguru

Well-Known Member
Most Helpful Member
My graph of the frequency response of a magnetic guitar pickup shows that with a load of only 10k or 22k its high frequencies are cut very badly, much worse than an AM radio. Do you like the sound to be muffled like that (or are you deaf to normal high audio frequencies)? The guitar preamp circuit I posted has an input resistance of 3M which is 300 times higher than 10k. Look at any guitar amplifier and see input resistors of at least 1M.
 

JLNY

Active Member
Perhaps instead of using a discrete JFET, it may also be possible to just use a JFET-input op amp like a TL072 as a buffer. TL072 is a dual op amp so you would be able to buffer both inputs with minimum parts count. EDIT: if you are using a single rail supply, you may need some biasing resistors and a capacitor on the input.
 
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