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Old 28th December 2008, 09:37 PM   #1
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Default Power supply requirement for Class D amp.

Hi all,

I need some guidance to properly size a linear transformer for a very near future power amp. (will be built during the X-mas vacation)

Class D IC would be TDA8920BTH from Philips that runs at about
+- 27Vdc (from a +-20Vac Xtrm), implementation would be to use each stereo IC as BTL therefore power output would be 138W at 8 ohm per channel (based on spec sheet)

I have not done many Class D power amp so I'd like to know how much VA would be required per channel, would a +- 20Vac, 5A (200VA) sufficient..or do I require more power (VA) like a typical AB power amp.

Any help or rule of thumb to size the power xtrm would be welcome.

Regards,
Eric
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Old 29th December 2008, 04:48 PM   #2
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For 138W class D a 200VA transformer is easily strong enough. But take care with your e-caps.
There is no evident reason to reduce their value vs. class AB. The rail sagging is also happening in class D and can lead to undesired early clipping, similar as it can happen in class AB if the caps are to small.
Furtheron most class D amps have less PSRR than class AB.
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Old 29th December 2008, 10:19 PM   #3
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Danke

I will then buy either 2 x 200VA or a 400VA to power both channel.

Eric
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