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Old 21st November 2013, 01:55 AM   #1
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Default LM386, not what I expected.

http://m.youtube.com/watch?v=GYV95Fs...%3DGYV95Fs6UGE

Pins 1 & 8 are left open, so a Gain of 20. Most of these "headphone amplifiers" have a gain of 50, or 200. I can't even toss a 10uf betweens 1 & 8 without nasty distortion.

I posted this on another forum. Just was curious to see how people were implenting their single chip lm386 audio amplifiers

*I'm using a 1k & cap in series from pins 1 to 5. This is actually supposed to be a bass boost, which really just cuts the midrange down & lets the low-er frequencies play. I don't care for the bass boost, but this is a GREAT way to cut down on the nasty hiss I'm getting.

It just sounds poor. I'm getting distortion at like 1/8 of a turn.

Thoughts?

This is a pretty common circuit.

Thanks
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Old 21st November 2013, 12:14 PM   #2
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For some reason I can't get the datasheet at TI.com. You'll have to give us more details. What Voltage are you using? You do know it only produces 1.6W? Hiss is very odd though. Tere's no reason for anything modern to hiss badly. Also we need to know what your load is. That will affect where it clips. Min load is 4 ohms.

Have just seen your video and you should replace that 10uF back with a resistor. It probably won't hurt to much but the amp will probably be oscillating since it's driving a 100nF load. Put in 4.7 or somewhere near. 2200 or 4700 in place of the 470u will do better still.
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Old 21st November 2013, 01:28 PM   #3
macboy is offline macboy  Canada
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"Modern"? The LM386 is at least 25 years old, probably older. I have some in my parts box that are easily 25 years old. The Datasheet is available at Speaker Amplifier and Modulator - Mid/High-Power Amplifier - LM386 - TI.com

You might try a capacitor in series with the input pin. The input is 50 kΩ so you could use say 1 F or higher. The output capacitor should be big, 470 F is OK, but you can go higher. Voltage of this cap should be more than 1/2 the supply.

I don't see a bypass capacitor mounted directly at the chip. This is absolutely necessary. Put a 47 to 100 F cap from pin 6 to 4 directly over top the chip, keeping the capacitor leads trimmed as short as possible. Your capacitor on the breadboard power supply rails can stay there, but you also need the "local" bypass capacitor right at the chip. Normal (aluminum) electrolytic would work, but a tantalum may be better. Keep the output zobel as designed: 0.05 (0.047) F and 10 ohms. Don't mess with that, it is often necessary for stability. Certainly, taking out the resistor and replacing it with a capacitor creates a pure capacitive load which is a bad thing in general.

Your 4 ohm speaker may ultimately be the cause of all your problems. Try a 8 or 16 ohm speaker. This chip is not designed for difficult 4 ohm loads, and the power output is very low with 4 ohm (only 200 mW max, vs. over 1 W at 16 ohm). Since the little chip will distort at over ~ 200 mW into 4Ω, that's less than 1 V. At a gain of 20, that is less than 0.05 VRMS at the input. So you will never be able to turn the volume pot very high.
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Old 21st November 2013, 01:32 PM   #4
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Quote:
Originally Posted by TylerTraulich View Post

*I'm using a 1k & cap in series from pins 1 to 5. This is actually supposed to be a bass boost, which really just cuts the midrange down & lets the low-er frequencies play. I don't care for the bass boost, but this is a GREAT way to cut down on the nasty hiss I'm getting.
Put 5.6K in series with 10u from pin 8 to pin 1. This will give you 6dB of bass boost, but gain will remain 20 1.2kHz on and up.

Do put a 1uF from bypass to ground.

You may be over-driving the chip -- back off the input voltage.

Last edited by jackinnj; 21st November 2013 at 01:36 PM.
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Old 21st November 2013, 01:57 PM   #5
macboy is offline macboy  Canada
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Quote:
Originally Posted by TylerTraulich View Post
...
*I'm using a 1k & cap in series from pins 1 to 5. This is actually supposed to be a bass boost, which really just cuts the midrange down & lets the low-er frequencies play. I don't care for the bass boost, but this is a GREAT way to cut down on the nasty hiss I'm getting.
In the datasheet (under application hints), it states that when you do not bypass pin 1 to 8 for gain boost, then the minimum value of R between pin 1 and 5 is 10 kΩ, otherwise the amp will become unstable. You are exceeding this minimum.
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Old 21st November 2013, 02:02 PM   #6
Mooly is online now Mooly  United Kingdom
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I remember as a youngster having no real success with various power amp chips of the day. They always seemed to be oscillators first and amplifiers second and easily provoked into failure. I've learnt a lot since then but discrete designs were always more successful in my early experiments
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Old 21st November 2013, 02:32 PM   #7
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The LM386 was a favorite for ham radio low power operation since it's so simple to implement. It's generally worry free.
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Old 21st November 2013, 03:05 PM   #8
johnr66 is offline johnr66  United States
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From your video, I see some issues. Big problem is your grounds. If that is the grounds on the right, you have signal, power and output all together and that will cause oscillations and crackle with music. You must isolate the signal grounds from the output. No wonder it won't work at higher gains! The cap on the supply, and the output RC filter must be close to the IC. You should not spread the components out so far like you have. Don't use the LM386 with 4ohm loads. Use 8 ohm minimum with 9v supply.
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Old 21st November 2013, 04:02 PM   #9
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Quote:
Originally Posted by Mooly View Post
I remember as a youngster having no real success with various power amp chips of the day. They always seemed to be oscillators first and amplifiers second and easily provoked into failure. I've learnt a lot since then but discrete designs were always more successful in my early experiments


that sounds like ... dunno. hard to belive.
i never managed a discrete amp on the first attempt, in fact i had waaay too many problems with them.
chips on the otherhand are very easy. if done with least minimum care...
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Old 21st November 2013, 07:33 PM   #10
kuroguy is offline kuroguy  United States
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I remember using the LM386 on a project in elementary school, something like 4th grade. it came in an 8 pin DIP package back then. That was 36 years ago. This chip has been around forever and has always been a piece of cake to implement.

Dude, I was 9 and I was able to make it work well. What's up with that?
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