portable speaker amplifier (7.4W 4Ω)

Hello there !

I have a speaker that I don't use along with its rest set.
**broken link removed**

I'd like to make an amplifier for it, so I can connect my mini-jack devices (for example my mobile phone, or an mp3 player) and make the speaker be heard louder than the devices can drive it.

Of course I want an output regulator to control the sound. Also I think I should power it with AA batteries of 2450mAh (I have lots of these, rechargable also, so I won't have to buy new ones).

I've read some things about the Class A, B, and AB amplifiers but couldn't decide which I should choose for my build.


Can you suggest any drawings for my situation?
I will build the enclosure.

The easiest solution is to use an amplifier IC, such as LM386.

godfreyl said:

The easiest solution is to use an amplifier IC, such as LM386.

Click to expand...

I am not looking to build something easy. More like to build an amp that will produce sound as clear as it can.(With no distortions etc, I'd like the best audio quality, I can get on a fair price for the parts)
Is LM386 good for this?

kgeorgeg7 said:

More like to build an amp that will produce sound as clear as it can.(With no distortions etc, I'd like the best audio quality...

Click to expand...

LM386 is not good for this aim. It's quality is better suited to portable radios.

Other IC's, such as LM3886, give very good quality. It is usually used for higher power like 40W to 50W, but you could use it with a lower voltage supply e.g. +-12V to +-15V for lower output power.

You could also build an amplifier with discrete transistors, but this is likely to cost more.

Note: The name is similar, but LM3886 is very different to LM386.

godfreyl said:

LM386 is not good for this aim. It's quality is better suited to portable radios.

Other IC's, such as LM3886, give very good quality. It is usually used for higher power like 40W to 50W, but you could use it with a lower voltage supply e.g. +-12V to +-15V for lower output power.

Note: The name is similar, but LM3886 is very different to LM386.

Click to expand...

LM3886 seems to be very nice choice. But, how can I power it with so many Volts?

To be operational it needs 20V =< |V+| + |V-| =< 84V

I don't think I can find batteries of that range

godfreyl said:

You could also build an amplifier with discrete transistors, but this is likely to cost more.

Click to expand...

Actually, I have a lot of transistors that I am not going to use. I have many npn and pnp like 2n3906 2n2222 2n3055 etc. (many of these) (I think the 2n3055 is the most powerful I have)

So could I build something with transistors only? w/o amps

kgeorgeg7 said:

LM3886 seems to be very nice choice. But, how can I power it with so many Volts?

Click to expand...

Sorry, I forgot it had to be portable, thus battery powered. Even two 9V batteries wouldn't be enough.

I had a look through this list of ICs that could work with a single 9V battery, but none of them look very good. Seems like you can have low voltage or high quality, not both.

Here's a list of ICs that would work with two 9V batteries. A few of those look good e.g. LM1875, but I'm not sure if you want to use two batteries.

Actually, I have a lot of transistors...
So could I build something with transistors only?

Click to expand...

Yes. That may be your best option. Unfortunately I don't have any circuits for battery-powered amps. Hopefully someone else can help.

I searched a bit and found these:
TDA2003
NTE1232

Will they be ok for my case? They can be powered with a low voltage of 8+ Volts (8 to 18), and they seem to be ok.

But I need your opinion too

Hello kgeorgeg7,

TDA2003 is good, but only take the PCB layout of the datasheet. This IC will make very strong oscillation if the layout of the PCB is not good.

Regards

Rainer

Hello again guys,

Finally I had some time to focus on this project. But,

I couldn't manage to make an amplifier with LM386 or a Class AB circuit with transistors. With both ways I had huge distortions. I could barely hear the signal that should have been on the output...

Please I need some help and a nice circuit



Edit: I'd like to build something only with LM386 741 411 or with transistors only.

LM386 is not bad for the price (it's your only real choice if you're restricting yourself to only that or the 741 or LF411).
It is a very old IC (maybe 20 years or more?), many people have successfully used it, and if you follow the
datasheet there isn't anything to go wrong since there are only about 5 components.
Use the datasheet circuit, you don't need any other circuit. It's not hi-fi, but having distortions louder than the
signal is clearly not normal.
You probably had too high an audio input. Or, maybe you tested it with a mains power supply. Anyway, it is
hard to troubleshoot with no more information. You'd need to post your circuit diagram, and maybe a photo of your
assembly.
No point trying a discrete transistor audio amp, because it is likely that it won't be any better than the LM386 if you're
trying to run this from batteries. And it would be a lot more complex.
As for LM741, forget it. It is not suitable for this. And the LF411 isn't useful for this purpose either.

All of the circuits I made with LM386 are based on this **broken link removed**

I just assemblied the circuit that is suggested on the site above. And I still got awful distortions...
I also tried all of the suggested circuits of the LM386 datasheet. Same result.

All of my circuits are tested both with a rail from an old psu or batteries.


All of my resistors are rated for 1/4W and my capcitors are electrolytic and ceramic

It's worth just sticking to batteries for now (say 6V or 9V), because the PSU may be causing issues.
Also, definitely remove the capacitor between pins 1 and 8 for now, to reduce the gain.
The LM386 has quite high gain, so that is possible that the input may be too high. Also, keep the cable
short (or screened) at the input.
It's been a long time since I used an LM386, but I can't think of any other reason for audio problems with it.
Is this on a plastic breadboard or stripboard? It might be worth trying to keep component wires short, in case it
is causing any issue.
Finally, it may be worth replacing the speaker with some headphones briefly to check the circuit (the 4ohm impedance
of the speakers may be too low).

sky_123 said:

It's worth just sticking to batteries for now (say 6V or 9V), because the PSU may be causing issues.
Also, definitely remove the capacitor between pins 1 and 8 for now, to reduce the gain.
The LM386 has quite high gain, so that is possible that the input may be too high. Also, keep the cable
short (or screened) at the input.
It's been a long time since I used an LM386, but I can't think of any other reason for audio problems with it.
Is this on a plastic breadboard or stripboard? It might be worth trying to keep component wires short, in case it
is causing any issue.
Finally, it may be worth replacing the speaker with some headphones briefly to check the circuit (the 4ohm impedance
of the speakers may be too low).

Click to expand...

I'll buy a new 9V battery which I will only use for now on for the Vcc.
I also tried without the capacitor between 1 and 8. Same reslut.

The breadboard I use is **broken link removed**
The description says that it is made from a solid and water-resistive ABS (No idea what that is, I guess not the Advanced Breaking System )

As for the speakers, I really didn't ever had any projects with them (I am studying Computer Engineering and we usually focus on digital circuits and not so many analog ones) and I don't know many things about them.

The speaker I am using says it is 7.4W (RMS ?) and 4Ω.
So, that means that it can handle a Voltage of V^2=P*R => 5.45V rms max ?
and a Current of I^2=P/R => 1.37 A rms max ?

Or do other equations apply to speakers?


I have a nice pair of headphones and I wouldn't want to try them on the circuit. I am afraid of burning them :/
Should I try with a speaker that is rated for more Watts? Or less? I think I should try one with more


Ah, and 1 more question, should I connect the negative side of the battery(the ground on the schematics) on the ground of the input?
Either way, I tried them both and still get distortions.

Hello kgeorgeg7,

what is the sound of the distortions? Is it only noise or overmodulation?

In case of noise, I think your circuit will oscillate. Make a direct short from pin 3 to pin4, without volume control. If it is quiet then, your connections to the volume control is too long and must be shield.

If their is any more noise, control your connections to ground. All must be as short as possible to pin 4, also the capacitors 0,1µ and 100µ from Pin 6.

If it is overmodulated, reduce the input signal and look for an other speaker, because the technical data of the IC is specified for 8Ω load impedance.

I hope it will help.

Regards

Rainer

Good suggestions from Rainer. Also, take a look at this page which shows a breadboarded LM386 circuit photo. The wires should be about that length, and not much longer. If you scroll to the bottom of that page, you can see in a photo the small 0.1uF capacitor that has been connected between pins 4 and 6.