Electronics > Beginners

LM386n datasheet inquiry

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bdunham7:

--- Quote from: bitman on May 26, 2019, 06:25:14 pm ---
This is the first time I work with that low voltage for real, but it doesn't fit with my observation. Note - I'm talking AC coupling here, not DC - not sure if that makes a difference. Even if I set the vertical resolution to 50mV so the sine is easy to see, it doesn't freeze (trigger) at that low voltages. I see two or three sines overlayed and moving side to side - clearly the sync isn't working. Even when I hit stop are there overlayed sine curves.  The moment I get a bit more voltage than 150mV I see a steady curve.  I've tried to adjust the trigger level but regardless of where I put it on the sine I see no correct output. I


--- End quote ---

I think Audioguru has explained the clipping and max voltage well enough.  There are amps that work differently (push-pull and h-bridge) that do not have ground-referenced output and can have peak-to-peak outputs nearly double the supply voltage.   These are commonly used in car audio.  The LM386 is not one of those.

As far as your scope trigger issue, something is wrong.  Just to check for sure, I took my DS1054Z and connected it to my signal generator which was set to 1KHz, 50mV p-p.  I then simply pressed the "AUTO" button and a few seconds later I had a clean, stable sine wave on my screen showing 52mV Vp-p and 17mV RMS.  I then dialed down the amplitude until the scope triggering failed, which was at 7mV.  8mV gave a stable signal.  Using more advanced trigger options, I was able to trigger reliably on a 1mV signal, although it was pretty noisy--not a pretty picture.  Post your setup exactly, with a photo if needed, showing scope settings, probe setting, etc and I'll try to see where you are going wrong.  Perhaps your input is very noisy??

Zero999:

--- Quote from: bitman on May 26, 2019, 07:07:25 pm ---
--- Quote from: Audioguru on May 26, 2019, 06:31:45 pm ---The graph is on Figure 3 of your datasheet. With a 12V supply and an 8 ohm load the LM386 gets too hot when playing loudly. With a 12V supply and an 8 ohm load the graph shows a maximum peak-to-peak output of 6.5V which is +3.25V to -3.25V which is 0.66W.

You did not say the frequency. The 250uF output capacitor feeding an 8 ohm speaker reduces the level of frequencies below 400Hz.

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Where are you finding the information?? I'm obviously not reading the datasheet right so how are you determining those values?
I've done frequencies between 600Hz to 1.2KHz - typically the test frequency was set at 1KHz.

Note - because my ears would object, I did not have a speaker on for most of the time - I put the probe straight on pin 5 (before the two caps). I did not see any significant change putting the probe after the cap (but still no speaker).  I did notice that values would change once I had a load (the speaker) on. So when I just want to test the circuit in silence, should I just add a properly sized resistor?

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In theory yes, it should be possible to replace the speaker with the a resistor of the same value, but in practice, the impedance of the speaker varies with the frequency and at resonance will be higher than what's written on the back of the speaker or when measured using a multimeter.

Here's an example of an 8 Ohm speaker I found, which has an impedance of over 40 Ohms, at 200Hz.

https://impactaudio.co.uk/collections/visaton-miniature-speakers/products/visaton-frs-5-xts-8-ohm

bitman:

--- Quote from: bdunham7 on May 27, 2019, 01:37:22 am ---I think Audioguru has explained the clipping and max voltage well enough.  There are amps that work differently (push-pull and h-bridge) that do not have ground-referenced output and can have peak-to-peak outputs nearly double the supply voltage.   These are commonly used in car audio.  The LM386 is not one of those.

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Audioguru absolutely did. I've learned the mistake of skipping the graph sections when I try to find key value points in a data sheet. Quite frankly I didn't even read the datasheet before I started with this very simple circuit - it took several failed attempts before I downloaded it (and then realizing there are more than one kind of LM386n). Lots of beginner confusion made bigger by using too low of a Vdd.  Well, and using the wrong type of caps.  My only frustration here is simple - I just needed something that would illustrate sound in basic electronics in a talk about how codecs works (I work in IT and only dabble in electronics as a hobby). I thought I could quickly put something together that would allow a simple live demonstration. I wasted so much time on screwing around with this, that it ended up with no live demonstration *grrr*. Thinking you know how to do something when obviously you don't is frustrating.


--- Quote from: bdunham7 ---As far as your scope trigger issue, something is wrong.  Just to check for sure, I took my DS1054Z and connected it to my signal generator which was set to 1KHz, 50mV p-p.  I then simply pressed the "AUTO" button and a few seconds later I had a clean, stable sine wave on my screen showing 52mV Vp-p and 17mV RMS.  I then dialed down the amplitude until the scope triggering failed, which was at 7mV.  8mV gave a stable signal.  Using more advanced trigger options, I was able to trigger reliably on a 1mV signal, although it was pretty noisy--not a pretty picture.  Post your setup exactly, with a photo if needed, showing scope settings, probe setting, etc and I'll try to see where you are going wrong.  Perhaps your input is very noisy??

--- End quote ---

Wow - ok, interesting. It was (I took it all apart now - sorry) very spot on around 150mV things would no longer be steady for me. Maybe I'll just repeat your setup and see.  Noise - yeah, there's plenty here in my "electronics lair".  From watching Dave's videos I know he would get a heart-attack if he saw how I connected the scope to the bread-board. Definitely not isolated - typically it's not an issue. I do mostly simple CMOS/TTL stuff, or basic micro-controller setup talking I2C or similar to components - nothing is very low voltage or prune to not work if there's noise. So I've never invested in the right cables and connectors. I have a ton of cheap crocodile cables or the hookup wires for bread-boards that I use to connect PSU, Scope, Meter etc. to the right points. I know it's far far FAR from efficient.  So maybe that's what is really happening here - but that's the equipment I have so my options are a bit limited.

I've got a good supply of perfboards now and I'll start doing soldered circuitry more than just bread-boards. But as you can probably tell, I make a lot of mistakes so using bread-boards allows me to figure those out, correct them before I move on. That probably won't change.

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