Loudspeaker Amplifire

[Fahime]

Hi.
I have a bell project. It has a melody generator, its output is amplified by a transistor and goes through a loudspeaker. Which one should I amplify to reach a louder sound of speaker, voltage or current or both?

 

[audioguru]

Ohm's Law says that if you increase the voltage across a load then the current in it will also increase.

A loudspeaker operates from an AC signal. An amplifier is made to make an AC output signal. A transistor has a fluctuating DC output level that wastes power heating the transistor and heating the loudspeaker and forces the voice coil of the speaker away from its magnet so it works poorly.

Why not use an IC audio amplifier instead of a transistor?
What is the circuit's supply voltage?
What is the supply's max current rating?
What is the impedance of the speaker?
How much power do you want for the speaker?
What is the output level from the melody generator?

 

[Fahime]

Dear,
The following parameters seem to be important for answering to my broblem:
1. The voltage supply is about 3 volts
2.Its max current in the collector is 40mA.
3.The speaker is a 8 OHMs and 0.5 W.
4. the output level of the melody generator is equal with the supply voltage.
Thanks for your care in my question.

 

[audioguru]

Hi Fahime,
Your supply voltage is very low. If the transistor works perfectly then its max current is 3V/8 ohms= 375mA. The average current is 188mA and the power output at clipping is 141mW.

Since your transistor's max current is only 40mA then the speaker has a max voltage of only 0.32V across it so the transistor isn't working properly. The power output is only 1.6mW. Little earphones operate at much higher power levels.

An MC34119 (Motorola/ON Semi) is a bridged audio amplifier IC in an 8-pins case and works with a supply voltage down to 2V. With a 3V supply it gives 100mW at clipping into 8 ohms. Because it is bridged, its ouput doesn't need a big coupling capacitor.
Philips has a similar amplifier IC.

 

[Fahime]

What should I do if I want to use a buzzer instead of a loudspeaker?
I replaced it but it doesn't have any voice. What's the reason?

 

[Hero999]

Obviously it won't work with a buzzer because a buzzer only gives a single tone.

 

[audioguru]

Dis you try using a low voltage power amplifier IC driving a speaker?

 

[Fahime]

Dear Audioguru,
I'm trying to find the Ic. I haven't found it yet. Whereas the cost is really important inmy project, I should replace a buzzer. I have seen similar projects that work successfully with a buzzer but I don't know the trick. Actually I would find the IC and experience it because it seems to be more practical with a loudspeaker. Any way, do you know how can I use a buzzer instead?

Thanks for your kindly attention.

 

[Dr.EM]

Are you thinking of piezo transducers? They require a different kind of amplification (low current but higher voltage). Only really effective at a tight frequency range though.

 

[audioguru]

Digikey and Farnell don't have the MC34119 amplifier IC in a dual-in-line package anymore, just tiny surface-mount ones. Philips and ST Micro used to have similar ones but I never used them, don't know their part numbers and don't know if they are still available. Most new low voltage amplifier ICs are extremly tiny for use in cell phones.

 

[fingaz]

The NJM2073D is a 8 pin dil 2 channel amplifier and can be run on as little as 1.8V upto 15V. I'm not sure about availability or price. Also can be bridged to provide one channel. Obviously the output power is dependant on the operating voltage so lower voltage=less amplification.

datasheet is here, with exapmle circuits (only one chip, a variable resistor and a couple of capacitors required)https://www.ortodoxism.ro/datasheets/newjapanradio/de05009.pdf

Theyr'e quite often used in small amplified speakers (like pc speakers, or personal cd player speakers) so you may be able to salvage one from a set of unwanted amplified speakers.

 

[Fahime]

piezo transducer???

Are you thinking of piezo transducers? They require a different kind of amplification (low current but higher voltage). Only really effective at a tight frequency range though.

I don't know its name is piezo transucer or something else. I attached it to see. It has a round golden color metal inside that a smaller circle from another kind of metal is stuck on it.
Anyway I don't know how to boost its output voice.

 

[Fahime]

piezo transducer???

Are you thinking of piezo transducers? They require a different kind of amplification (low current but higher voltage). Only really effective at a tight frequency range though.

I don't know its name is piezo transucer or something else. I attached it to see. It has a round golden color metal inside that a smaller circle from another kind of metal is stuck on it. please see the picture in this link: https://www.electro-tech-online.com/attachments/19112006-jpg.10240/
Anyway I don't know how to boost its output voice.

 

[Thunderchild]

stick to the speker it is eaier. connect the amplifier in place of the transistor. depending on the output power you want you will find it easier with at least 6 volts as it is easier to find and amplifier for this voltage and it will have more power the TBA820M is a nice one chanel amp giving up to 2 W at 12 volts I think it will work on 3 but not sure but I have used it with no problem at 6 V

 

[Hero999]

Don't double post.

Connect it to 9V battery, if it beeps then it's a buzzer but if it makes a click sound then it's a transducer.

 

[audioguru]

The piezo beeper or transducer is packaged in a tuned resonant cavity (its black plastic case) so it will be loud only at one frequency of from 3.5kHz to 5kHz. If it is not in the tuned resonant cavity then it won't be loud at any frequency.

I played with a piezo transducer when I looked at a Keys Finder Project and its frequency needed to be carefully adjusted to its resonant peak for it to be loud. I used only 3V for a supply and a bridged amplifier driving it. Two Cmos inverters didn't provide enough current to drive it (not even when paralleled) so I used complimentary emitter-followers.

 

[Hero999]

What about those piezo transducers used in musical greetings cards and other novelty items?

They're loud enough to be annoying and they can't be that resonant or they wouldn't be able to play irritating music so effectively.

 

[audioguru]

You know that even a cheap and small speaker has a reasonably flat frequency response over a wide frequency range.
The noise-makers in greeting cards are just tiny whistles.

I have two frequency response graphs of piezo transducers. One is absolutely horrible. This one is also horrible but it is much better than the really bad one:

 

[Fahime]

I tried the piezo transducer. It's loud but I need a bit louder. I think I should use a H Bridge. What do u think?
Meanwhile, is there any 3volt voltage regulator? If yes, what's its name?

 

[audioguru]

A piezo transducer needs lots of volts, even 6Vp-p isn't very much.

An H-bridge is on or off for motors. It would sound horrible with sound, but maybe your melody chip makes a single level square-wave so it doesn't matter. Try it.

There are 3.3V regulators but I haven't seen a 3V one.
An LM317 adjustable voltage regulator can easily be set to 3.0V.