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Old 30th April 2004, 02:42 PM   #1
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Lightbulb Please i need some help choosing an amp.

Hi guys, i'm quite new to this place, but i'm in need of some help.
I'm making a backpack with integrated speakers, powered by a 7.5 Ah 12v battery. I've bought a 180w(peak)4ohm 8" bass speaker for the frontal panel, but i don't really have an amp to power it. Also i have 2 eliptical 80w wide-range speakers for the sides and i have a TDA 1555Q based 2x44w amp for those. But the main problem is that i can't find an amp for the subwoofer. It seems that there aren't any ~50-60w single channel amps out there, and i'm pretty new to design my own schematic, so please i need some help. The main problem seems to be the 12v power supply though, because most of the amp chips work @ about 25v and above.

Thanks in advance,
MinD|_eSs
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Old 30th April 2004, 07:42 PM   #2
GregGC is offline GregGC  Canada
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Default Re: Please i need some help choosing an amp.

Quote:
Originally posted by mindless_bg
Hi guys, i'm quite new to this place, but i'm in need of some help.
I'm making a backpack with integrated speakers, powered by a 7.5 Ah 12v battery. I've bought a 180w(peak)4ohm 8" bass speaker for the frontal panel, but i don't really have an amp to power it. Also i have 2 eliptical 80w wide-range speakers for the sides and i have a TDA 1555Q based 2x44w amp for those. But the main problem is that i can't find an amp for the subwoofer. It seems that there aren't any ~50-60w single channel amps out there, and i'm pretty new to design my own schematic, so please i need some help. The main problem seems to be the 12v power supply though, because most of the amp chips work @ about 25v and above.

Thanks in advance,
MinD|_eSs
Try some of the IC designed for car aplications (usualy 12 V bridge configuration for more power).

Try this link:

http://www.st.com/stonline/bin/sftab...able=a_207_203

Uspeh!

/Greg
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Old 1st May 2004, 03:53 AM   #3
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Default power output vs supply voltage

There's a reason why higher power amps usually use higher supply voltages. Briefly, in a transformerless amp, the maximum peak-to-peak output voltage is equal to the power supply voltage. Actually it's two or three volts less, due to voltage drops in the output transistors.

You can now convert this peak-to-peak AC voltage to its RMS value, square that and divide by the loudspeaker resistance to get the theoretical maximum RMS output power. If you work this out, this is the formula you come up with:

Max output power = (supply voltage)^2 /( 8 x speaker resistance).

Using your 12 power supply and a 4-Ohm speaker, this works out to:

Pmax = 144/32 = 4.5 Watts!

In reality, you will be lucky to get more than 3 W RMS, due to unavoidable voltage drops in the amplifier output transistors.

Bridge output amplifiers manage to double the maximum peak output voltage, by suspending the loudspeaker between two amplifier output stages that are driven in antiphase. Doubling the voltage results in four times the power, so the formula for a BRIDGED amp is:

Pmax = (supply voltage)^2/(4 x speaker resistance).

Which, of course, works out to about 9 W RMS in your case (4 Ohm speaker and 12 V power supply). Again, in reality the maximum available will be a little less, probably more like 6 or 7 Watts.

There are only three ways I know of to get more power than this (i.e, more power than you can get from a bridged amp as discussed above) from a given supply voltage, and two of them are impractical to varying degrees:

One, use an output transformer to step up the output voltage (needs a big, heavy, expensive transformer!).

Two, use a custom low-resistance loudspeaker (used by Bose in some car speakers some years ago), or parallel several ordinary speakers to lower the net resistance.

Three, use a high-powered DC-to-DC converter to step up the 12V from the battery, and run the amp off of that increased voltage. This third solution is quite complex, but it is the one used by all those high-powered car audio amps out there that manage to put out hundreds of watts of power while running from the 14 V supply available in todays automotive electrical systems.

If you're happy with power outputs in the 6W range, you can fairly easily build yourself an amp using one of those dual power amp IC's designed for automotive use. If you decide you want more than that, the technical challenges start to mushroom rapidly - not many diy'ers have the knowledge needed to build a reliable, high-power DC-DC converter (I certainly don't!).

Good luck with your project!

-Flieslikeabeagle
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Old 1st May 2004, 08:30 AM   #4
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Default Re: power output vs supply voltage

Quote:
Originally posted by flieslikeabeagl
There's a reason why higher power amps usually use higher supply voltages. Briefly, in a transformerless amp, the maximum peak-to-peak output voltage is equal to the power supply voltage. Actually it's two or three volts less, due to voltage drops in the output transistors.

You can now convert this peak-to-peak AC voltage to its RMS value, square that and divide by the loudspeaker resistance to get the theoretical maximum RMS output power. If you work this out, this is the formula you come up with:

Max output power = (supply voltage)^2 /( 8 x speaker resistance).

Using your 12 power supply and a 4-Ohm speaker, this works out to:

Pmax = 144/32 = 4.5 Watts!

In reality, you will be lucky to get more than 3 W RMS, due to unavoidable voltage drops in the amplifier output transistors.

Bridge output amplifiers manage to double the maximum peak output voltage, by suspending the loudspeaker between two amplifier output stages that are driven in antiphase. Doubling the voltage results in four times the power, so the formula for a BRIDGED amp is:

Pmax = (supply voltage)^2/(4 x speaker resistance).

Which, of course, works out to about 9 W RMS in your case (4 Ohm speaker and 12 V power supply). Again, in reality the maximum available will be a little less, probably more like 6 or 7 Watts.
---
Three, use a high-powered DC-to-DC converter to step up the 12V from the battery, and run the amp off of that increased voltage. This third solution is quite complex, but it is the one used by all those high-powered car audio amps out there that manage to put out hundreds of watts of power while running from the 14 V supply available in todays automotive electrical systems.

I am getting a bit confused in this matter, since i don't pretty much understand the difference between peak, sinuidal and RMS power .Ayway, if i'm correct i get the idea of the bridged amp like this
=input1=[AMP(+-)]==[SPK]==[(-+)AMP]=input1= if that's so wouldn't the speaker have to be double-coiled? Sorry for the stupid questions, they may seem so, but i'm really in the dark here. By the way i found this one form GregGC's reply, Blagodarja Greg(TDA7256) it may happen to be a good solution. It says it's output power is 30w but i can't quite catch the idea of the RMS power. Because if its 30w RMS i believe it's pretty much for a chip like this. The step-up transformer for the supply voltage might be a good idea, though, i'm not pretty shure how'll i manage to build it...Again,sorry for my lack of knowlege, things will get clearer in time.

Greets from Bulgaria,
MinD|_eSs
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Old 1st May 2004, 09:14 AM   #5
b<ben>n is offline b<ben>n  Australia
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http://www.siliconchip.com.au/cms/A_30614/article.html
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Old 2nd May 2004, 09:35 PM   #6
GregGC is offline GregGC  Canada
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mindless_bg,

Based on your knowledge in the audio amp field I'd suggest you start with the simples possible solution (TDA7256). Build some experience and knowledge in the field and than go to more complicated (and more efficient) solutions.

On the question RMS vs Peak power:
http://www.myhometheater.homestead.com/RMS.html

And general info:
http://www.electronics-tutorials.com...lectronics.htm

Uspeh!
/Greg
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Old 3rd May 2004, 07:43 PM   #7
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Thanks for the help guys i think i've found a fine solution - the TDA1562Q(Thanks b<ben>n, that was quite useful) although'm not shure that 70W wouldn't be too much for my lousy speaker, so i might try the TDA7256 first and then see if i'm satisfied with the results. Thhough i'm gonna have a hard time getting my hands on one of theese, since stocks of this sort are not very diverse in bulgaria, and ordering is expensive and SUCKS ... Anyway thanks a lot guys, i'll post pictures ASAIF, so you'll see what've i came up with.
Thanks, greetz from Bulgaria
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Old 3rd May 2004, 07:46 PM   #8
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Thanks for the help guys i think i've found a fine solution - the TDA1562Q(Thanks b<ben>n, that was quite useful ) although'm not shure if 70W isn't going to be too much for my lousy speaker, so i might try the TDA7256 first and then see if i'm satisfied with the results. Thhough i'm gonna have a hard time getting my hands on one of theese, since stocks of this sort are not very diverse in bulgaria, and ordering is expensive and SUCKS ... Anyway thanks a lot guys, i'll post pictures ASAIF, so you'll see what've i came up with.
Thanks, greets to all you who bothered to write a row, especially to GregGC for giving me those handy guidelines. C Ya Soon!
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Old 4th May 2004, 12:22 AM   #9
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Default power output vs supply voltage (correction)

I just realised I made a mistake writing down the power output formula for a bridged amp. As I said correctly in words, a bridged amp puts out four times the power of a single-ended amp for the same supply voltage if we ignore voltage drops in output devices. That means the correct formula for a bridged output amp is

Pmax = (supply voltage)^2/(2 x loudspeaker resistance)

(not 4 x loudspeaker resistance as I posted earlier. Sorry!).

-------------
earlier mindless_bg wrote:
>I am getting a bit confused in this matter, since i don't pretty much understand the difference between peak, sinuidal and RMS power
-------------

This link might help, it's well written and well illustrated:
http://www.eatel.net/~amptech/elecdisc/voltages.htm

-Flieslikeabeagle
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