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Old 30th May 2012, 06:16 AM   #1
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Question 5W amplification at low voltage

Dear DiYAudio,

I'm completely new in here and in audio circuits as well. Thus, I thought this community might be helpful.

I'm designing a circuit running on battery from (3 to 9 volts) which must have a 5W audio amplification stage. I've been browsing the internet quite a lot for such amps but no one was running under 12volts.

Do you have any ideas ?

I wish you a nice day and thank you for all the knowledge available on that website.
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Old 30th May 2012, 06:50 AM   #2
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Albert what is your load, an 8 ohms speaker?

For 5W into 8 ohms you need about 6.3V RMS signal voltage, which means a peak-to-peak value of about 18.5V.
So you would need at least a total supply voltage of 20V or more.

You can't do that with a 3-6V battery, unless you use specialty amps with output transformers.

jan didden
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Old 30th May 2012, 07:16 AM   #3
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Thank you janneman for your answer !

My speaker is 6Ohm impedance and I want to amplify inputs in the order of 1.5Volts in a range [1.35;1.65]Volts. So maybe 5W is not needed.

The idea is that I have different customized sensors i should amplify the outputs to apply some treatments. I need low voltage because i want all my devices to be portable and run on small batteries.

Thanks a lot.
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Old 30th May 2012, 07:18 AM   #4
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Oh and I forgot !
What do you mean by:
" unless you use specialty amps with output transformers" ?

Thank you for your informations, it helps me to learn audio circuits !
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Old 30th May 2012, 07:35 AM   #5
godfrey is offline godfrey  South Africa
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The LM386 IC is ideal for low power audio amps running off batteries.
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Old 30th May 2012, 07:43 AM   #6
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The Texas Instruments TPA3111 will get you fairly close. It's a class-D mono amp spec'd for 7-W into an 4-Ω Load at 10% THD+N from a 8-V supply. As Jan suggested, you'll need to wring every millivolt possible from the supply and amp, so a bridged load is virtually a must. There's other chips with similar specs to the TI chip, but I can't recall them offhand.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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Old 30th May 2012, 07:59 AM   #7
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Thanks a lot for those clues ! I'll browse the TI website and tell what i find if you are interested.
Thanks again !
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Old 30th May 2012, 11:48 AM   #8
AndrewT is offline AndrewT  Scotland
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a single supply voltage of 9V limits the output voltage from a "normal" amplifier to ~3Vac.
3Vac into a 6ohm speaker is roughly 1.5W. One cannot get more than that from a 9V battery and a normal amplifier. The distortion at that maximum power is likely to be unacceptable and a more reasonable maximum with tolerable distortion is probably around 700mW to 1000mW of output power.

Now you can use bridged amplifiers to get more power output from a fixed 9V supply.

Take a pair of those "normal" amplifiers and design each of them to drive a 3ohms speaker load. 3Vac into 3r0 is ~3W.
If you bridge these two 3ohm capable amplifiers, you can get ~6W into your 6ohm speaker. The same limitations on tolerable distortion apply as before. Expect a usable/tolerable maximum output of around 3W to 4W into your 4W amplifier.

A 386, and the others, designed for low voltage supplies are excellent, but they are severely limited in maximum output power. That is the consequence of designing for high efficiency to suit battery use.
You must use a high efficiency speaker to get higher SPLs, That's why we see headphones used on portable equipment that has limited battery power.
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Last edited by AndrewT; 30th May 2012 at 11:50 AM.
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Old 30th May 2012, 01:20 PM   #9
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Thank you Andrew for your answer !
I'll try to bridge the amplifiers i already have tonight ( only one lm386 at 9v wasn't indeed sufficient last time).

Thanks to your remarks and my readings, i'm seeing clearer but some points are still not clear at all.

I'm considering the fact that i may not need a 5W amplification (I choosed 5W, because in a research paper dealing with such kind of speakers, they said they used that value with no justifications)
Considering my speaker specifications:
Bandwidth: 50-500Hz
Impedance: 6 Ohm
Maximum Input Voltage: 3V
Maximum Input current: 0.5A

Power = voltage^2/impedance, thus 3*3/6 =1.5W, should be sufficient ? I'm right ?

I was able to perfectly use those speaker on my cellphone (3.7V) headphone output without any amplification. Do think they use those texas instrument amplifier you suggested ?

Thanks to all of you, this hep me a lot! i only have this forum and google to ask questions on that topic !
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Old 30th May 2012, 02:23 PM   #10
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The cellphone no doubt uses a lower power amplifier than the TI chip I linked to. Just guessing, the phone probably can't output even 1.5W. It sounds like you could get by without a bridged amplifier, and if so that would allow a greater choice of chip amps and simplify getting the circuit put together. It appears that you don't need much voltage gain.
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
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