I have a 400W ATX SMPS lying around. I was thinking If I could use it to power a home theatre amplifier board??? The SMPS could be switched on by a simple method but I need to know if it is Ok to power two 12v 5A boards using this supply which gives 20A on its 12v rails??? Will the smps be able to power the sudden rush for current when I play tracks with heavy bass?

 

That power supply is capable of making your amplifier work, but will inject noise to the audio as the switching supply is noisy by nature.
Miguel

 

I thought SMPS could provide the purest supplies as its used to power most digital devices these days including the pC.. No way I can filter them???

 

Yes, it is possible to filter, using chokes and capacitors; how clean they come out depends on how dirty they are and how much filtering is done. Give it a try as-is first, then compare filtered.

 

smps can be good for some amps.

1) smps is very good at supplying a steady current. Class A amps, and most tube amps, mostly draw a steady current. That makes them a great place to use smps.
2) amps tend to have very low PSRR (power supply rejection ratio) abvoe the audio band, especially for tube amps / class A amps that typically don't utilize lots of negative feedback. so if you are going to use a smps to power an amp, make sure a) you have sufficient filtering / choking, and b) your smps can live with that.
3) smps is VERY power in dealing with dynamic loads. if you have an amp that draws nothing and then suddenly lots of current, your smps will have trouble delivering that much current that quickly. all class B amps (90%+ of that amps out there) are such a beast, and class D amps are even worse in that reguard.

so, smps can be good for amps that draw a relative steady current. for other amps, the best power supply is an unregulated power supply.

 

An audio amplifier that uses a 12V supply has an output of only 1.5W at clipping into 8 ohms or 3W into 4 ohms.
If the amplifier is actually two bridged amplifiers then the power into 8 ohms is 5.3W or 10.6W into 4 ohms.

Maybe your home theater has 5 bridged amplifiers.

 

3) smps is VERY poor in dealing with dynamic loads. if you have an amp that draws nothing and then suddenly lots of current, your smps will have trouble delivering that much current that quickly. all class B amps (90%+ of that amps out there) are such a beast, and class D amps are even worse in that reguard.

This is exactly what I needed.

Compared to a transformer which works like a hi-current- ready to serve at any instant the low's is given out,an SMPS is not strong. I also had doubt if an SMPS could handle the amperage for heavy bass. Also Class-B is the one Im having as its power hungry any time.

An audio amplifier that uses a 12V supply has an output of only 1.5W at clipping into 8 ohms or 3W into 4 ohms.
If the amplifier is actually two bridged amplifiers then the power into 8 ohms is 5.3W or 10.6W into 4 ohms.

Maybe your home theater has 5 bridged amplifiers.

I'm using LA4440's which each are stereo amps for 6W per channel and I've bridged them to give 19W . I had only built 2 channels for testing them. I was on a doubt whether to settle for this chip as it needs only single channel 12v supply and If I could power it with the SMPS, it would be more cheaper. Now I think, I should drop this and go for MOSFET amps which gives higher gain and performance despite the odd power supplies.

 

I also had doubt if an SMPS could handle the amperage for heavy bass.

most of the computer atx power supplies run at 20khz or above. they should have no problem dealing with bass (<1khz).

Now I think, I should drop this and go for MOSFET amps which gives higher gain and performance despite the odd power supplies.

most amps are dual-supplied: meaning that they have a positive and negative rail. Most smps is single supplied: just one positive or one negative rail. you can put two smps together, using one of them to power the positive rail, and another the negative rail, but whether that is feasible depends on their topology.