Planning LM386-4 amplifer and speaker circuit build and I have a few questions (speakers, bridge/parallel or push-pull options?)

Thread Starter

bignobody

Joined Jan 21, 2020
68
Hi there!

Near the beginning of this year I made my first LM386-4 based amplifier and speaker circuit (some details in this thread here: https://forum.allaboutcircuits.com/threads/first-time-using-a-voltage-regulator-sanity-check-plus-decoupling-capacitors-questions.166722/post-1473500 ). It's been "good enough" for all my audio experiments so far.

I'd like to add a LM386-4 based amplifier circuit and speaker to my current project, so I came back to my previous one here for reference. But I think it's natural to want to do better than we did last time so I have some more questions.

My circuit is running at 9V DC.

I should start with talking about my speaker options. Both are rated for 8 ohms, but the first is rated for 0.5 watts and has a larger heavier magnet (aside: is there any reason to prefer a heavy magnet? This doesn't seem like a "feature" to me in a half watt speaker), and the second option is 3 watts. My first build is using the 8 ohm 3 watt speaker.

From my reading on the LM386-4, I think it can power in the 0.25 to 1 watt range. Given my first speaker option maxes out at half a watt, I think I should continue to use the 8 ohm 3 watt speaker.

But if the LM386-4 maxes out at 1 watt and my speaker has "room" for 2 more... Can I get there?

I stumbled across this video today:

"LM386 Amplifier. Parallel. Double the power and performance."

I should start by saying I am taking this video with a huge grain of salt - comments are disabled and they don't offer any real "proof" of what the circuit has achieved beyond being able to make some music audible.

But it did start me down the path of looking at parallel, bridge and bridge-parallel circuits, because it's not really a problem for me to add a second LM386-4.

I found what claims to be a Bridge Parallel amplifer circuit searching the web, but beyond the diagram it's pretty light on details and it could be riddled with errors for all I know.
http://www.seekic.com/circuit_diagram/Amplifier_Circuit/Using_LM386_as_BTL_amplifier_circuit_diagram.html

Still, I am going to try to breadboard this and see what kind of results I get.

I have another question related to this, though. Assuming a bridge-parallel would work for me, is it still possible to add things like a gain control potentiometer (normally done as a RC circuit between pins 1 and 8 when using a single IC)? I know both ICs are supposed to be working as identically as possible (except the polarity reversal).


Alternatively, reading some other threads on these forums regarding lm386 and adding a Push-Pull Output stage was also suggested:
http://www.bristolwatch.com/radio/lm386_power_amp.htm

This is also intriguing idea, and I may even have all the necessary components laying around, but I do note that the author states:
"The output is several watts as opposed to 400 mW."... I'm not sure if they consider "several" as more than 3.


Assuming I do have all the suitable components I will also try to breadboard this and see.


I will update this thread with my results, but I'd appreciate any input or suggestions anyone has, even if it's "don't bother, because..."
 

sghioto

Joined Dec 31, 2017
1,453
A bridged output will generally quadruple the output power for a given supply voltage and load but you have to consider the operating limitations of the chip which is mainly the power dissipation. With a recommended max 18 volt supply and 8 ohm load you can probably get 3 watts but you may need to heatsink the chips especially if the output is continuous.
SG
 

Audioguru again

Joined Oct 21, 2019
1,914
The first video is completely wrong. The ICs are not in parallel, instead they are bridged. The author did not read the datasheet and says he has no formal electronics training. In a bridged amplifier, the 470uF output coupling capacitor is not needed.

The audio output power is not doubled, instead the heating in each IC is almost doubled.
The maximum allowed heating in each LM386 is 1.25W when it is almost melting.

1) With a 9V supply, the output of one LM386 is 6Vp-p into 8 ohms when it is distorting fairly badly, its output power is 0.56W and its heating is 0.52W. The output power with low distortion is 0.45W like a cheap clock radio.
2) With a 12V supply, the output of one LM386 is 6.5Vp-p when it is distorting fairly badly, its output is 0.66W and its heating is 0.8W.
3) Bridged with a 9V supply, the output is 7Vp-p when it is distorting VERY badly, the output power is 0.77W and the heating in each IC is 0.9W.
4) The second bridged amplifier is also wrong and uses a 12V supply. The output is 7Vp-p again and the output when it is distorting VERY badly, the output power is 0.77W again and the heating in each IC is the maximum allowed heating of 1.25W.

It ain't designed to be a bridged amplifier. That is why a bridged amplifier is not shown in the datasheet.

It is silly to add a push-pull output stage, instead buy a more powerful amplifier.
 

Thread Starter

bignobody

Joined Jan 21, 2020
68
Thank you both for your answers!

I did try breadboarding the bridge-parallel circuit I found (with the exception that I had to use a 500K pot as voltage divider as I didn't have a 470K) and sent in a signal from an audio oscillator circuit I made. It may have been a little louder than my other amp and seemed to be pushing more current, but as neither of you will be surprised to hear the ICs got very hot very fast.

1) With a 9V supply, the output of one LM386 is 6Vp-p into 8 ohms when it is distorting fairly badly, its output power is 0.56W and its heating is 0.52W. The output power with low distortion is 0.45W like a cheap clock radio.
Thanks again for your detailed response. Based on this point, it sounds like I might as well stick with the 0.5 watt 8ohm speaker instead of the 3 watt if I'm still going to use the LM386-4.

It is silly to add a push-pull output stage, instead buy a more powerful amplifier.
I will probably still experiment with it, because I'm still a beginner and every experiment will help me grow. I'm not really clear what's silly about it. But if I was going to buy a more powerful IC (say something "one step up" from a LM386-4) do you have a recommendation?
 

Audioguru again

Joined Oct 21, 2019
1,914
Modern amplifiers are class-D (high frequency switching with very low amount of heating).
They are also stereo and are bridged. An example is the PAM8403. With a 5V supply, its output with low distortion into 8 ohm speakers is 1.8W per channel and into 4 ohm speakers is 2.5W per channel. Its recommended maximum supply is 5.5V.

There are many PAM amplifiers, some are mono and some have a higher output power:
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiP_pqK6LfrAhUIWN8KHYfIAVsQFjAAegQIAhAB&url=https://www.diodes.com/products/analog/audio/&usg=AOvVaw1IXe5nzVxASuTurglOld_p
 
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