Driving a Speaker: Op Amp or LM386?

**bugmenot** · 19th November 2008 10:36 PM

I need to drive a small (8Ω 250mW) speaker. The way I know how to do this is to use an Op Amp and a NPN/PNP push pull (the Op Amp alone can't provide enough current).

Is there a better way to do this? Perhaps an Op Amp which is designed to put enough current out?

I know the LM386 is supposed to be for audio output. *But*, it multiplies voltage x20. I don't need to do that - just provide enough current. And it has a relatively low input impendance. And, it requires a bunch of external components (as shown in the datasheet). Is there a better way to drive a speaker, using an LM386?

**audioguru** · 19th November 2008 11:24 PM

The minimum voltage gain of an LM386 is about 9. It oscillates if it has less gain.
Simply set it with its default gain of 20 and feed its input with two resistors as a 1/20th attenuator. Then the voltage gain is 1.

The attenuator will raise its input resistance.
It does not need a bunch of external components (only 4).

**bugmenot** · 20th November 2008 01:02 AM

Originally Posted by audioguru

The attenuator will raise its input resistance.
It does not need a bunch of external components (only 4).

Thanks. Do you have a diagram - the datasheet shows many more for its simplest?

Still, what is the advantage of it over a high current op amp?

**audioguru** · 20th November 2008 01:27 AM

Originally Posted by bugmenot

Thanks. Do you have a diagram - the datasheet shows many more for its simplest?

Still, what is the advantage of it over a high current op amp?

The datasheet shows a very simple circuit. I would add a supply bypass capacitor.

The LM386 amplifier has internal parts that allow its inputs to work at 0V and it has negative feedback resistors built in. Its supply current is low and it works from a supply voltage as low as 4V.

A power opamp might have a high supply current, needs negative feedback resistors, input biasing resistors and an input capacitor. its minimum supply voltage is high.

Here is the minimum parts schematic in the datasheet of the LM386 with the input volume control repaced by an attenuator resistor:

**transistor495** · 20th November 2008 07:43 AM

I won't use LM386 more because,
It will produce noise at output.
More chance of getting oscillated.

It is not suitable for your 250mW speaker. You will get 'purprup'.

250mW speakers are usually used with 3v driven amplifiers. For eg, a pocket radio.

LA4510 is the amplifier IC suitable for you(3V). It needs little more external components. But works great!

**audioguru** · 20th November 2008 03:43 PM

Originally Posted by transistor495

I won't use LM386 more because,
It will produce noise at output.
More chance of getting oscillated.

The LM386 has a small amount of noise when its gain is 200 but no noise when its gain is 20.
Its datasheet recommends an RC network at its output to prevent oscillation.

It is not suitable for your 250mW speaker. You will get 'purprup'.

Of course it is suitable for 250mW. Its output into 8 ohms at clipping is 250mW when its supply is 7V.

250mW speakers are usually used with 3v driven amplifiers. For eg, a pocket radio.

LA4510 is the amplifier IC suitable for you(3V). It needs little more external components. But works great!

No.
The Sanyo LA4510 amplifier produces a very distorted square-wave of 250mW into a 4 ohm speaker with a 3V supply. Its power into 8 ohms at clipping with a 3V supply is only 120mW. It produces 250mW at clipping into 8 ohms when its supply is about 5V. It uses more parts than an LM386.

A TDA2822M amplifier produces 250mW at clipping into 8 ohms when its supply is about 3.5V (it is a bridged amplifier). It uses very few parts.

**transistor495** · 21st November 2008 04:08 AM

The Sanyo LA4510 amplifier produces a very distorted square-wave of 250mW into a 4 ohm speaker with a 3V supply. Its power into 8 ohms at clipping with a 3V supply is only 120mW. It produces 250mW at clipping into 8 ohms when its supply is about 5V. It uses more parts than an LM386.

2yr before I have made a reflex receiver which used LA4510 on its output. I hve feeded the output to 4ohm 750mW 4" speaker inside a 4" speakerbox. I wondered such a nice big sound produced from this little 3V amplifier. There was no any distortion and one of my best assembly, still I'm using that.

LA4510 will produce distortion and oscillation if recommended circuit is not followed thoroughly.

My 1995 model Philips pocket radio(4 transistor superheterodyne) has an LA4510 on its output. Great sound

**audioguru** · 21st November 2008 01:47 PM

Originally Posted by transistor495

LA4510 will produce distortion and oscillation if recommended circuit is not followed thoroughly.

Didn't you see its datasheet? With a 3V supply and a 4 ohm load it clips at only 140mW which is a very low amount of power. The power will be much less when the battery voltage runs down.
The output power is about the same or is less into 8 ohms.

**transistor495** · 22nd November 2008 11:51 AM

Originally Posted by audioguru

The power will be much less when the battery voltage runs down.

I'm running my receiver(1*BF494,2*BC549, LA4510 reflex AM receiver) on 2*AA rechargeable batteries. It works for months even after 1hour normal usage daily and produces BIG SOUND from a 4" speakerbox. I think it is a free energy generator

Also a 250mW of power don't give to a 250W speaker. An 750mW speaker can produce more nice sound from it.

So 140mW can be handled more nicely by a 250mW speaker

**audioguru** · 22nd November 2008 07:36 PM

A little speaker with a low power rating is garbage. A bigger speaker is more sensitive and produces better bass frequencies.

Your two Ni-Cad or Ni-MH batteries produce only 2.5V so the output power of your LA4510 is only about 100mW at clipping into 4 ohms which is almost nothing.

**cab4word67** · 28th February 2009 12:24 AM

I am new here and love what I found so far. My problem I am useing 5/8" diamiter speakers 8 ohm very small in model railroading and need more volume. I have tryed the basic LM386 set up. And it works. my quesiton is will 12 volts hurt this in the long run? We have 15-18 volts at all times and I have thought of installing a voltage regulator .And is there something better to use, Space is a preimum

Chris

**audioguru** · 28th February 2009 01:15 AM

Originally Posted by cab4word67

I am new here and love what I found so far. My problem I am useing 5/8" diamiter speakers 8 ohm very small in model railroading and need more volume. I have tryed the basic LM386 set up. And it works. my quesiton is will 12 volts hurt this in the long run? We have 15-18 volts at all times and I have thought of installing a voltage regulator .And is there something better to use, Space is a preimum

Chris

A little LM386 works well with a 6V to 9V supply. When it has a 12V supply the max output power is slightly more than with a 9V supply but the extra voltage is wasted by making the LM386 get hot.

Can your tiny speaker survive 0.7W of power?

**cab4word67** · 28th February 2009 02:10 AM

The speakers are in fact 1 wt rated. should i drop the supply voltatage with an ajustable regulator so to stay down around 9v and if so is my regulater going to get hot. Again this is inside a small plastic train and space is hard to find. Thanks for the fast replys.
Chris

**audioguru** · 28th February 2009 03:43 AM

If the LM386 is in free air for cooling, its max allowed dissipation is 1.1W
With a 12V supply, its output at 3% distortion is 0.7W and its dissipation is 0.82W.
Since there is not any space for cool air then it might over-heat and fail.

With a 9V supply, its output at 3% distortion is 0.6W and its dissipation is 0.5W so it will not over-heat.

A voltage regulator would share the heat so both the LM386 and the voltage regulator will get hot but not too hot.

**cab4word67** · 28th February 2009 05:20 PM

Thanks for all the help we will be testing this out today with a couple new speakers. Question after finding the set volume how would I go about replacing the 100K pot with just a resistor of set value? does #3 then go to ground with the 0.1 cap or to #4? Just trying to make smaller and we wont need the adjust ability because the input has some adjust to it also.
Thanks we can now here with little speakers Chris

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