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Old 16th September 2010, 04:07 AM   #11
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the 150W per chip is total power 30v * 5A = 150w only if chip is shorted

into a 6 Ohm load 5A the chip dissipates a few watts, the load has the rest.

30v rail 18v output each chip outputs 3A into 6 ohm load
the load sees 18 *3 = 54 watts
The chip dissipates (30-18) * 3A = 36 Watts

therefore the 3 chips should be able to handle a 2 Ohm load.
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Old 16th September 2010, 06:21 PM   #12
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The 3,17 A of your SMPS is the DC current. The effective value of a sine wave is 0,707 times the peak -> 3,17 A(crest) * 0,707 = 2,24 A(eff). That means for a 2 Ohm load, you will reach the current limit at 4,48 V(eff) which leads to ~10 W of output power.

For 150 W into 2 Ohm you need 17,32 V(eff) and 8,66 A(eff) at the amplifier output. That corresponds to 24,4 V(crest) and 12,2 A(crest). You need a few volts more to make up for the losses in the output stage, so a 30 V power supply is okay, but 3,17 A is so far from the required current that you cannot even hope an SMPS may deliver that much more for even short amounts of time.

You are right, each chip will deliver a third of the total, i. e. 50 W into 6 Ohm. Redshift did not account for the effective voltages and currents. His 450 W(crest) turn out to be 225 W(eff) which is what your power supply should be able to supply, if you need 30 V(eff) and 5 A(eff) continuously.

The worst case dissipation is UČ/(2*PIČ*Rload). For a ±30 V power supply and 2 Ohm that is ~91 W total (plus a little for the chips own consumption) or around 30 W per chip. Real life works in your favour, because you will not always operate exactly at the point of highest dissipation. Real life works against you in that speaker impedances dip below the nominal impedance which means even higher losses.
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Last edited by pacificblue; 16th September 2010 at 06:25 PM.
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Old 16th September 2010, 07:12 PM   #13
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I understand now.

Thanks to all of you for your replies!... It looks like the 36VCT transformer is much better suited for the job after all

I love this forum!
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