OT: LM386 Not Loud Enough

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rs2845

Senior Member
Hello all,

I've got all the hardware ready for the alarm system I'm building, apart from the amplifier for my Tenda MP3 module which is to be used as a voice enhanced sounder. If any of you use the SD variant of Tenda's MP3 module then you may know how quiet the unit is with a piezo connected. Well mine is anyway

This is why I added an LM386 configured to have a 200 gain. Still, it isn't loud enough (I can talk over the audio). I need something that outputs 'double' of this LM386, perhaps around 90db.

I was wondering if anyone knows of any amplifier IC's that I can swap with my LM386 to get my 90db?

Tried doing a google search but got nothing helpful.
 
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g6ejd

Senior Member
Which piezo unit are you using?

LM386 maximum power out is 1.25W, a radio at that level would be above ambient noise levels, but would depend on transducer (speaker). Check the data sheet for the transducer to see what sound output level is produced with a given audio power input.

For very loud outputs, my guess is at least 10-20W of power.
 
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rs2845

Senior Member
Hello, thanks for your fast replies.

The piezo is this:

http://www.ario.com.tw/pdf_datasheet.php?products_id=161&osCsid=6jdsp65ohonoh2ud7rkem4k7j4

If I can even get 85db from this piezo I'm happy. They are used in an existing alarm sounder and that is very loud.

Well, due to my budgeting issues I haven't been able to purchase the exact components. The only thing that isn't correct is the 250uF capacitor in the 200gain diagram. I was donated the components by a very friendly JimPerry (thanks once again!)

Can one cap cause such an increase in audibility? Below is a diagram of my circuit- sorry its not high quality or very technical!

**I was unsure about the resistor that connects to the 0.05uF capacitor (pin5). It just says '10'? 10 what? uF? nF? Monkeys? May as well be... Tried attaching an image as well but wouldn't work.

I do have a feeling I've done something wrong. If I can get 85DB from my current hardware then I'd love to!
 

Attachments

Pongo

Pongo

Senior Member
Haku said:
I wired up a VMA2016 2x10watt class D amplifier to a Tenda board last month, astonishingly loud for such a small device - http://dx.com/p/vma2016-2-10w-audio-amplifier-module-44122
Its smaller cousin, the VMA2012 2x3watt is pretty good too - http://dx.com/p/vma2012-2-3w-class-d-audio-amplifier-module-44126

Just a shame there's no UK stockists of those as they take weeks to get here from China.
Click to expand...
Yeah, those VMA chips are amazing, especially to someone who remembers the original Sinclair class D's.
 
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inglewoodpete

Senior Member
ravisispal94 said:
... Tenda's MP3 module then you may know how quiet the unit is with a piezo connected.
Click to expand...
This issue with piezo speakers came up recently. A impedance of piezo "speaker" is too high to get much sound without using a higher voltage output from the amplifier. Have a look at the circuit you have supplied. The HF bypass network (0.5uF and 10 kohm) has a lower impedance than the piezo speaker. It is not intended to be connected directly to a piezo.

Either use a "proper" (electromagnetic) speaker, a reversed audio transformer (Eg 8 ohm to 1000 ohm or more) or a more suitable amplifier (+&-15 volt or higher rails).
 
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Paix

Senior Member
With all due respect Pete, the resistor is 10 ohm, as witnessed on the datasheet page 6 Amplifier with bass boost, where they actually printed the omega symbol which they seemed to be so reluctant to do on most of the other diagrams. Definitely not monkeys Ravi :)

I don't remember the Sinclair class D amp, but before he moved away from extremely budget audio amplifiers, he used to get an awful lot of stuff sent back from what I heard. I guess that he was playing the percentages somewhat.

= = =
I stand corrected Pete. Ravi did indeed interpret the resistor required as 10k and as you were citing that and not the data sheet, I'll wind my neck in a tad. Ravi, you read the resistor value wrongly I believe.
 
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bluejets

Senior Member
Your circuit shown with 1K0 in series with 10uF cap. pins 1 and 8.

Data sheet explains "Gain Control" page 4 ...200 is just the cap. Looks like you might be working about 40..??

Also agrre with JimPerry, resistor is 10R (10 ohm)
 
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Goeytex

Senior Member
Not only is the resistor 10 ohms instead of 10K, the capacitor is shown in the datasheet as .05uf not .5uf.

(.047uf should work fine)
 
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rs2845

Senior Member
Hello all, thanks for your replies. Really appreciate everyone's input.


My apologies with the diagram I provided. That 1kohm resistor wasn't actually supposed to be there, it was late when I drew the thing ;) the only thing I don't have is a 10 ohm resistor- will this really make a world of difference even if I used a em speaker? I have a 330R resistor. Other than that I believe my circuitry is correct.. Unless someone can provide a dummies diagram I can check my circuit against.


I guess I cannot use a piezo without requiring more hardware. Only reason why I wanted to use a piezo was due to the thinness of it.. I'm building a detector that has a speech sounder and strobe all in one small device.. an em speaker will end up taking up necessary space and this is why I can't use a ready made module as my pcb and the Tenda module will take up space. I would like to use a piezo if I can, even if only a few more components are needed- I guess I could buy them if I knew it would give me my output.


Although, I can get an 8 ohm 0.6w 65mm Mylar Speaker from eBay for a couple of quid. Think I could get my sound output now?! I'm new to all this amplification stuff, impedance and whatnot. Learning though (slowly!)


Saying this- I did connect my amp up to a large alarm speaker I found in my garage and the sound was very loud so I can't be far off..
 
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bfgstew

bfgstew

Senior Member
Just use the wire off the end of a resistor, other schematics for the LM386 i have seen use either a 10 ohm resistor or a jumper wire!
 
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boriz

Senior Member
The 85db you refer to is only achievable under specific conditions. Firstly, a sufficiently large voltage must be applied. Secondly the signal must be of a specific frequency (the resonant frequency of the disk). Thirdly the Piezo disk needs to be mounted correctly in a resonance chamber.

All of this is feasible, right up to the point at which you say "speech sounder".

If you want to amplify speech, a Piezo is going to be a very poor substitute for a moving coil speaker. Both in quality and volume.

The 386 is designed to drive a moving coil speaker. If you insist on sticking with a Piezo, as IWP tried to point out, you will definitely need a higher voltage. Usually you find simple flyback inverter type circuits in the loud alarm stuff, like the personal attack alarms. They create very brief, very large voltage spikes at the appropriate frequency.
 
JimPerry

JimPerry

Senior Member
You would get more noise out of a set of cheap earphones than the piezo - (probably blow them up!)

The Poundland speaker would do the job :rolleyes:
 
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geoff07

Senior Member
boriz is right. Forget the speech if you want an alarm, just make sure that the piezo is resonant (try varying the freq around 3-4kHz). The resonance does the work and if you have the right unit it will be LOUD for hardly any power and no components except the input voltage source (no Rs and no Cs needed on a normal hi-z piezo). If speech is also needed, do that separately.
 
erco

erco

Senior Member
Although every self-respecting alarm sytem needs to say "Self Destruct Sequence Initiated" and give a countdown, just like in the movies.
 
fernando_g

fernando_g

Senior Member
erco said:
Although every self-respecting alarm sytem needs to say "Self Destruct Sequence Initiated" and give a countdown, just like in the movies.
Click to expand...
Of course, with 1" bright red displays.

Now on a serious note.....you don't need an amplifier that is optimized to reproduce quality audio (although quality is a subjective word) to drive a single tone piezo.
Additionally piezos are highly capacitive.

In the past I've used VERY succesfully PWM modulators, originally intended to drive power Mosfets in power supplies, to drive piezos at high levels. In particular I like the LM3525A, since it can drive it push pull.

The thing is operated open loop (without any feedback) which will cause it produce maximum output.

See the following:
http://www.alldatasheet.com/datasheet-pdf/pdf/116867/NSC/LM3525A.html
 
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rs2845

Senior Member
Okay.. I have ordered an 8 ohm speaker which is 0.6w.. I should then be able to see how effective my amp circuit really is and until I can find a trusty solution to use my piezo.

I know how loud my piezo can go as I gutted it from a once-working voice sounder and the speech was superb quality.

@Fernando_g, I am liking the idea of these pwm modulators for the piezo. Could you please provide some more specifics? Would I connect my pre-amplified Lm386 circuit to this modular or the low volume audio direct from my mp3 module? Any additional components needed? Will take a read of the datasheet you provided.

Of course, the alarm must hold some self-destruct announcement- that's plan B if the main announcement doesn't work ;)
 
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Paix

Senior Member
On a really bad day, it might make it's own self-destruct announcement without any prompting from yourself :)

How alarming would that be?
 
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rs2845

Senior Member
I've done a little search on Rapid...

Look at this IC:
http://www.rapidonline.com/Electronic-Components/Tda2050v-32w-Audio-Amplifier-82-0632

There's quite a few:
http://www.rapidonline.com/Electronic-Components/Audio-power-ICs-67347

Yay or nay? How well do you think the Tda2050v would drive a speaker or a piezo?

Runs at 4.5v but obviously need higher for better results, which is great as I have 24v.

@Paix- I think that's the time you really do listen to the alarm... There'll be no ignoring that!
 
fernando_g

fernando_g

Senior Member
ravisispal94 said:
@Fernando_g, I am liking the idea of these pwm modulators for the piezo. Could you please provide some more specifics? Would I connect my pre-amplified Lm386 circuit to this modular or the low volume audio direct from my mp3 module? Any additional components needed? Will take a read of the datasheet you provided.

Of course, the alarm must hold some self-destruct announcement- that's plan B if the main announcement doesn't work ;)
Click to expand...
No, it doesn't require any audio source at all. The IC is self oscillating, think of it as a 555 on steroids. It only requires an enable signal. Let me see if I can make a quick sketch.
 
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SteveT

Senior Member
If you want to use the TDA2050v then check out http://diyaudioprojects.com/Chip/DIY-TDA2050-Hi-Fi-Chip-Amplifier/
Have a look at the power supply requirements, forget the 120VA toroidal, you need 18-0-18VAC 160VA toroidal transformer (about £23.50 from Rapid (cheapest I could find)). Very cheap amplifier chip, not so cheap everything else to build a working amplifier. I'm half way through building one myself but have had to put it on hold for a while till my hands stop playing up.

In my opinion - way overkill for what you want.
 
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rs2845

Senior Member
@fernando_g

I managed to get hold of an SG3525 chip. I followed your schematic but am getting no amplified audio.

You say that diagram is dirty, dirty enough to still work or?! I'll strip it down again and rebuild but I checked it a few times now which is odd.

May try a higher voltage too (24v)
 
fernando_g

fernando_g

Senior Member
Do you have a scope?
Is there a triangle wave at the 0.01uF cap? This oscillation is required.

Are ther pulses at pins 11 and 14?

Is the voltage at pin 16 about 5.1 volt?
 
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