speaker size for LM386

[Alistair Ballantyne]

Hi,
Simple LM386 amp with 8 ohm speaker. What would be your max wattage recommendation for the speaker?
Thank you.
Alistair

 

[FvM]

You ever looked into datasheet? It specifies maximum output for different supply voltages, e.g. 0.2 - 0.25 W for 6V.

 

[danadakk]

This might help -

Regards, Dana.

 

[Alistair Ballantyne]

Hi Dana,

Thanks. Would not have thought of that so very interesting and useful.

Still not totally clear though. Is that the data sheet for the LM386?

So at 9V it has a max output of 0.5W?

What would be the effect of using a, say, 5W 8ohm speaker?

Regards,

Alistair

 

[KlausST]

Thanks. Would not have thought of that so very interesting and useful.

Still not totally clear though. Is that the data sheet for the LM386?

So at 9V it has a max output of 0.5W?

At least after post#2 you should have downloaded and read it......
If then still questions exist, then please refer to the datasheet chapter/page.

What would be the effect of using a, say, 5W 8ohm speaker?

If you don't hear very loud, nothing happens.
Otherwise you might overload the speaker, maybe damage it.

Klaus

 

[danadakk]

Hi Dana,

Thanks. Would not have thought of that so very interesting and useful.

Still not totally clear though. Is that the data sheet for the LM386?

So at 9V it has a max output of 0.5W?

What would be the effect of using a, say, 5W 8ohm speaker?

Regards,

Alistair

Unless I am mistaken a 5W speaker rating refers to voice coil power handling
capability, so anything below this is fine. The 8 ohms is going to be fine for
the LM386. If you find volume a tad low you can always do a bridge configuration.

LM386 stereo amplifier in bridge 2 watts | ElecCircuit.com

See LM386 stereo audio amplifier circuit, 2 watts. Using 3 ICs in a bridge BCLmodel. Cheap and easy build. Stereo small LM386 amplifier x3.

www.eleccircuit.com

You can drop the bottom LM386 if you are only doing mono.

You might want a clip on heatsink depending on how you are driving them.


Regards, Dana.

 

[Audioguru]

The Stereo Bridge circuit has a couple of serious problems:
1) If there is the same signal (mono) input to each channel then the center LM386 has the same signal on both of its inputs which cancel and produce zero output.
Then there is no bridging. The stereo speakers produce an out-of-phase sound.

2) The LM386 is overloaded when bridged. Each of the three amplifiers has a 4 ohm current. The LM386 works poorly with a 4 ohm load.
If there is a signal on only one channel then the middle LM386 and the playing channel are trying to produce double the current in one 8 ohm speaker which reduces the output level due to overloading.
Then there is a small bridging power boost and lots more heating.

With a 9V supply, the output power of an LM386 amplifier with a 9V supply and an 8 ohm speaker is 0.45W with low distortion. The amplifier heats with 0.5W.
With a 9V supply, the output power of a bridged LM386 amplifier with a 9V supply and an 8 ohm speaker is 0.6W with low distortion. Each amplifier heats with 0.8W.
0.6W is far from the 2W that is claimed.

 

[danadakk]

The Stereo Bridge circuit has a couple of serious problems:
1) If there is the same signal (mono) input to each channel then the center LM386 has the same signal on both of its inputs which cancel and produce zero output.
Then there is no bridging. The stereo speakers produce an out-of-phase sound.

2) The LM386 is overloaded when bridged. Each of the three amplifiers has a 4 ohm current. The LM386 works poorly with a 4 ohm load.
If there is a signal on only one channel then the middle LM386 and the playing channel are trying to produce double the current in one 8 ohm speaker which reduces the output level due to overloading.
Then there is a small bridging power boost and lots more heating.

With a 9V supply, the output power of an LM386 amplifier with a 9V supply and an 8 ohm speaker is 0.45W with low distortion. The amplifier heats with 0.5W.
With a 9V supply, the output power of a bridged LM386 amplifier with a 9V supply and an 8 ohm speaker is 0.6W with low distortion. Each amplifier heats with 0.8W.
0.6W is far from the 2W that is claimed.

1)Would not the middle amp have its NI input grounded, after dropping out the bottom
amp for mono operation. So the signal would be inverted, as needed....?

2) The bottom amp is not present.

Heating is of concern, reason for prior comments about heatsink.

I think forum looked at this problem before. I had tried to do a sim
to see if I could get legit power levels. The web has a disparity of models,
and a suitable speaker model I could not find, so did not proceed.

Regards, Dana.

 

[Audioguru]

Many amplifiers can drive a 4 ohms load with ease. Then they work well when bridged with an 8 ohms speaker.
But an LM386 heats a lot causing a reduced output power when driving double the current of an 8 ohm speaker.

The DIP 8 pins package or the surface-mount package of the LM386 are not made to have a heatsink and a heatsink is not needed with a non-bridged amplifier and an 8 ohm speaker. The bridged amplifiers will be hot and will survive without a heatsink if they are not enclosed.
Yes, a forum looked at the horrible bridged amplifier before but the datasheet of the LM386 does not show a bridged amplifier so I think it is a bad idea and does not provide much power gain.

 

[betwixt]

I wouldn't recommend that schematic, using a common bridge between two channels will cause horrible crosstalk problems.

However, that wasn't the original question, the answer to which is:
Any loudspeaker with a power rating higher than the amplifier can produce is safe to use.
In general, the higher the power rating of a loudspeaker, the less sensitive it is so try to use one rated a little higher than the amp for best volume. Using one with a much higher power rating will do no harm but will sound quieter.

Brian.

 

[c_mitra]

I wouldn't recommend that schematic, using a common bridge between two channels will cause horrible crosstalk problems.

And, most importantly, you gain very little (if any) and lose a lot (quality sound).

In my younger days (that was very long ago), we were told to focus on performance and then try to optimize the cost.

Keeping the two channels separate as much as possible is a good design sense, even if it costs one IC and a couple of capacitors extra.

Any loudspeaker with a power rating higher than the amplifier can produce is safe to use.
In general, the higher the power rating of a loudspeaker, the less sensitive it is so try to use one rated a little higher than the amp for best volume. Using one with a much higher power rating will do no harm but will sound quieter.

If you drive a speaker at a much lower power (say a 10W speaker being driven at 1W), you may severely compromise the dynamic range. The lows will disappear and the highs will sound strange. Human ear has a somewhat logarithmic sensitivity that is very difficult to match with a speaker. It will be ok for normal speech but the apparent quality of a music will suffer.

 

[Audioguru]

A speaker with a low power rating is usually cheap, small and sounds awful because it is a Shrieker, not a speaker.

 

[Alistair Ballantyne]

I wouldn't recommend that schematic, using a common bridge between two channels will cause horrible crosstalk problems.

However, that wasn't the original question, the answer to which is:
Any loudspeaker with a power rating higher than the amplifier can produce is safe to use.
In general, the higher the power rating of a loudspeaker, the less sensitive it is so try to use one rated a little higher than the amp for best volume. Using one with a much higher power rating will do no harm but will sound quieter.

Brian.

Hi Brian,

Stupidly I see this has answered one of my later questions so thanks!
One other if I may:
A simple LM386 mono amp (schematic below) driving an MP3 player through a basic 8 ohm / 1.5W speaker.
How long would you expect a 9V battery to last powering this?
I made a box to use as external radio but the duration was short lived - c.3 hours.
I didn't think the unit would draw much current but clearly I've got this wrong.
Thanks
Alistair

 

[betwixt]

Its a class AB amplifier so it draws some 'idle and low volume' current that stays fairly steady and an additional higher current when the volume exceeds low level. That makes it hard to predict.

The low current will be about 4.5mA but at volume peaks it will rise to around 100mA (assuming 0.7W into 8 Ohms and about 75% efficiency).
How long it will run on a battery depends on the battery capacity, how much current is drawn and for how long. As you can see, it isn't a straight forward calculation.
You can get an estimate of the time by looking at the Ah (Amp Hour) rating of the battery from the manufacturer, that is the amount of current you can expect to draw for one hour before it reaches a predetermined 'terminal' (=dead!) voltage. For a PP3/1604 size for example the rating is normally about 400mA/H for zinc-carbon and about 500mA/H for alkaline. Rechargeable batteries are usually about half the capacity of 'one time' ones.

Brian.

 

[Audioguru]

The datasheet of the LM386 shows that its output power us 0.45W with low distortion into 8 ohms when the supply is 9V.

The impedance of the speaker, the supply voltage and how loud you play determines the current draw.
If your speaker is 8 ohms, the supply is 9V and you play music or voices so that the loudest parts reach 0.45W
then the average output power is about 0.09W. The datasheet of the LM386 amplifier shows an idle current
4.3mA and heating of 115% of the output power. Therefore the average power is almost 0.194W.
Then the 9V average current is 0.194W/9V= 21.6mA plus the 4.3mA of idle current for a total average current
of 25.9mA.

If you build it with messy wiring on a breadboard then it probably oscillates and uses a lot more current.
A Name-Brand alkaline battery lasts a lot longer than a Super Heavy Duty piece of garbage from "over there".