Smallest possible simplest possible 7W amp for 2.2 ohm load?

[BAED]

I have a 2.2 ohm planar magnetic speaker which can handle 3.94V RMS (7W).

I don't need to play high fidelity audio, this is just a loud siren / megaphone application.

I have a couple of goals:

• To start with, it just needs to handle input from a basic pulse / tone generator like a 555 timer. This could be the initial simplistic design.
• For a more complex / useful design, I would like to eventually have the ability to feed it audio to use it as a megaphone/loudhailer.

I'm open to suggestions for either the 'tone pulse' amp or the 'audio' amp and I will need to decide whether to go for the full audio version or not!

If I wanted to build a very small basic SMD component amplifier for this, what device / topology would you suggest?

Cost is not a problem, but it needs to be ultra light weight (shaving off individual grams will matter here)

 

[bogdan_borko]

You could use this speaker (siren) in series with 1R8 5W resistor, then you have 4R load that could be driven with some simple 15W chip amp.

 

[OldDIY]

TDA2003 etc.
PAM8403 ($0,30)

 

[joensd]

A lot of class-d ICs will happily drive 2ohm-loads.
TPA3251, TPA3255.
They need about 18V min. supply voltage though and will have 20W output power then.

There are lots of boards around but they might be too expensive!?

Another alternative, much cheaper and probably your best option is a TPA3118 in PBTL-mode that is 2Ohm stable:
DC8 24V TPA3118 PBTL 60W Mono Digitale Audio Verstarker Board AMP Modul Chip 1X60W 4 8 Ohm|power amp|digital amplifier boardpower 24v - AliExpress
or similar. Just search for "TPA3118 PBTL".

They should even make an OK hifi amplifier.

 

[NareshBrd]

You can build a simple transistor based tone generator to make the siren sound if that is all you want to do.
No need for chip amps.
1875, 2030A, 2050, all will work at +/- 12 volts.
4440 and others in 12 volt class...

 

[BAED]

Thank you for your responses!

The series resistor idea could work in a pinch, I don't need it to be super efficient. This is to be battery powered and portable (7.2V lithium) but the total weight of the amplifier board is the most important thing. Cost is not a problem, just needs to be really lightweight.

Using a TPA3118 in PBTL-mode might be ideal, I could design a board with that but with an integrated boost converter to generate the voltage required (is a full 18V really needed to drive the speaker at <4V @ 7W?)

 

[Mooly]

Why not try the speaker fed direct from the 555 first (use the old high current output type). Add series resistor to limit the current to 200ma (about 39 ohm give or take).

See how loud it is to you. It might suffice as it is.

If you want more then just add a single transistor stage with the speaker as the load. That would dump the rail voltage across the speaker. Add series resistors to suit.

 

[OldDIY]

One Mosfet

 

[Mooly]

That would do nicely and is more efficient. You could then use the CMOS versions of 555.

 

[xrk971]

TDA8932 in PBTL is tiny and cheap at $3. To get 7w for 2.2ohm load is maybe 6v supply since the BTL doubles the output swing. But these chips have minimum voltage to work. I think 8v is lowest it can go and the TDA8932 and it is 4ohm stable. Add 2ohm resistor in series and good to go. It’s actually a nice sounding “HiFi” amp. TPA3116 works too.

1pcs Digital power amplifier board module 35w mono amplifier module TDA8932 high power low power consumption|Integrated Circuits| - AliExpress

DC8 24V TPA3118 PBTL 60W Mono Digital Audio Amplifier Board AMP Module Chip 1X60W 4 8 Ohms|power amp|digital amplifier boardpower 24v - AliExpress

 

[OldDIY]

 

[Mooly]

The FET version may be better because it would take a lot of base current to fully saturate the transistor... just thinking out loud now... but the available gate voltage is limited by the battery supply.

I still say yes to the FET but look for a type with a low Vgs threshold.

 

[OldDIY]

Darlington

 

[Elvee]

Why not use a CMOS power MOS driver? They swing to within mV of the power rails, have a high input impedance, are tiny in modern packages and have ridiculous peak current ratings.
In addition, they often come in dual version, meaning you can use them in a BTL configuration and quadruple the already maximal output power.
Another advantage of the PP is that you are not going to fry or offset the moving coil of the speaker with DC.
An example, among many hundreds of other possibilities:
https://ixapps.ixys.com/Datasheet/ixdd415.pdf

TI, IR, ST, MicroChip and others have their own alternatives

 

[NareshBrd]

If you insist on a chip amp, there is an IC which is very common here in FM radios, 6283, about 25 cents for the chip, and 60 cents for the assembled PCB.
It works from 4 Volts to about 15, depending on speaker impedance.
But the above circuits are a simple elegant solution for your needs.

 

[bansuri]

https://datasheets.maximintegrated.com/en/ds/MAX9768.pdf


difficult package tho

 

[Mooly]

Darlington

Erm... you might find the collector can not reach close to ground in practice. A discrete arrangement with two transistors would be better and with driver of the pair having its collector fed from the + supply.

 

[OldDIY]

A reverse diode may be required.

 

[Mooly]

That could be a good idea

 

[Mark Tillotson]

If I wanted to build a very small basic SMD component amplifier for this, what device / topology would you suggest?

Class D chip - find one rated to 2 ohms that can run from 6--12V or so, with a bridge mode class D you only theoretically need 6V to get about 8W max rms sinewave. Class D won't need much heat dissipation which ticks the "very small" box nicely.