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Old 13th August 2005, 06:07 PM   #1
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Default 15vdc amp

I have a power supply rated at 15volts DC @ 2500mA (no load tested 18.97v). I want to use this to power a couple of small amplifiers that need to provide atleast:

5w @4 ohm x 2
10w @ 6 ohm x 1

I was thinking a TDA2009a, however the supply voltage at rated power is 24volts, and I'd only be providing 15v. This power supply is (almost) set in stone, as I do not wish to mess with mains power on this project, and this power supply is already available and working.
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Old 13th August 2005, 11:08 PM   #2
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the 10W amp needs to be comprised of 2 amplifiers in a bridged (differential output) configuration. you would need more then 7.5V of output swing.
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Old 15th August 2005, 12:00 AM   #3
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> power supply rated at 15volts DC
need to provide at least:
5w @4 ohm x 2
10w @ 6 ohm x 1


Do math.

15VDC into a perfect amplifier is 15V peak-peak, or 15/2.8= 5.3V RMS.

5.3V RMS in 4Ω is (5.3^2)/4= 7 Watts
5.3V RMS is 6Ω is (5.3^2)/6= 4.6 Watts

Working in Bridge will quadruple these powers.

Real amplifiers will give only 70%-80% of these powers.

So you are cool for the 4Ω loads directly.

The 6Ω 10-Watt won't work without a bridge amp. That gives around 4.6W*4*75% or say 14 Watts, not far from your goal. (Note that the bridge amp chips each see a 3Ω load.)

Total DC power demand on sine-test will be 1.5 to 2 times the sine output power, which is 5+5+14= 24 watts times fudge-factor is 36 to 48 watts DC. You have 15V+2.5A= 37 watts DC. You may not quite make these numbers on sine-test, but for speech/music use the power supply is ample.
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Old 15th August 2005, 12:12 AM   #4
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> TDA2009a

Or just see the TDA2009a datasheet. Page 5: at 15V, 1% THD, you can get 5 watts in 4Ω (OK), about 2.6W in 8Ω, so probably 3.7W in 6Ω. You demand 10W in 6Ω, so you gotta do something else. You cast your 15V in stone, so the first trial-hack is the bridge. In this mode, with 6Ω load, each side sees 3Ω. The 2009 does not carry a rating for 3Ω operation. It will drive 4Ω, and page 8 shows 18W into 8Ω bridge, so you are not far off. Peak current in Bridge will be 15V/6Ω = 2.5A, comfortably below the 3.5A rating. It will probably work, though THD and losses will be higher than for 4Ω operation.
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Old 15th August 2005, 02:01 AM   #5
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I think I am going to go with a +/- 18 volt supply that I also have laying around (but was hoping to use for a better project, but what can you do?), and using four TDA2050s. One for each channel of the stereo mid/tweet, and two in a bridged configuration for the woofer.

Any thoughts?
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Old 16th August 2005, 09:01 PM   #6
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> a +/- 18 volt supply

36V peak-peak gives around 10V RMS. 10V RMS in 4 ohms is 25 watts, somewhat above your 5W goal. With 2X4 plus 6 on a bridge, you are going to need over 6A DC, over 200 Watts DC power, which may be more than you have.

What was wrong with the 15V plan? Sweet size for a shelf-system.
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Old 18th August 2005, 12:52 AM   #7
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I'm useing the TDA7297 amp right now, It runs on 16 volts . I have one running on 18 volts, but I still cant get the distortion down.. I have no Idea what I did wrong. But anyways you can look into that. Its a dual bridge and puts out 15 watts per channel, of cource at 10% THD.
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