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Old 7th February 2012, 05:42 PM   #1
camus is offline camus  Spain
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Default Using LM338 and LM337 for 250VA supply

I build this power supply to feed 3 TDA7264 amps (25W+25W each). It is powered by a 250VA toroidal 230 to 17.5-0-17.5 with the ultimate goal to provide 18V. As the LM7818 and LM7918 regulators didn't cover the power I needed (Aprox. 465mA), I went for the LM338 and LM337 regulators and have been having more problems than normally expected. After many rectifications to the design, some burnt components and a couple of minor explosions, I think I finally got it. (see attached PDF's).

I would really appreciate any feedback on the design, flaws or errors I might have missed, ideas, etc. Thanks!!
Attached Files
File Type: pdf powerAmp250VA_diagram.pdf (13.2 KB, 142 views)
File Type: pdf powerAmp250VA_PCB.pdf (42.2 KB, 67 views)
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Old 9th February 2012, 05:25 PM   #2
CBS240 is offline CBS240  United States
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Hi

First off, 17.5VAC X SQRT2 = 24.7VDC or ~23.7V after rectifier. Loosing 6V in a regulator for a power amplifier, with the product of the current creates significant heating, wasted heat. Awfully inefficient. 18V + 1V(rectifier)--> 19VDC/SQRT2 = 13.42VAC. If you wanted to have voltage regulation, I would not look to drop more than a couple volts. 20VDC supply would need 21VDC/SQRT2 = 14.8VAC.

Will each amplifier chip have it's own set of regulators? If not, the LM337 may come up dry on current. A single LM337 supplying 3 amplifiers may need a helper pass transistor.

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Old 10th February 2012, 04:26 PM   #3
camus is offline camus  Spain
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Default totally correct

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Originally Posted by CBS240 View Post
Hi

First off, 17.5VAC X SQRT2 = 24.7VDC or ~23.7V after rectifier. Loosing 6V in a regulator for a power amplifier, with the product of the current creates significant heating, wasted heat. Awfully inefficient. 18V + 1V(rectifier)--> 19VDC/SQRT2 = 13.42VAC. If you wanted to have voltage regulation, I would not look to drop more than a couple volts. 20VDC supply would need 21VDC/SQRT2 = 14.8VAC.

Will each amplifier chip have it's own set of regulators? If not, the LM337 may come up dry on current. A single LM337 supplying 3 amplifiers may need a helper pass transistor.

Thanks! I somehow got the 18V value stuck in my brain, while 20V is much efficient and in fact even the spec sheet of the power amplifier sets everything at 20V (talk about seeing the tree and not the forest).

You are also right on the regulators, a single LM337 won't do. Due to my very limited knowledge placing a set of regulators on each amplifier seems easier than setting a helper pass transistor (had to look it up and just learning about it).

Again, thanks a lot for you knowledgeable response!!
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