SMPS thoughts/questions

[slomatt]

Greetings. I'm in the initial phases of designing a LM3886 based amp and have become very interested in using switched mode power supplies (SMPS). My company has a good number of power supplies for 1U rack-mount cabinets sitting around and I've been evaluating their ratings. As expected many have a minimum required current for the 5v output, I'm assuming that this is to ensure stability. The min current seems to range from 1-2A, and can be provided by a resistor across that output.

I've read the article on Decibel Dungeon that describes using 4 modified SMPSs to create a +/- 24v supply, the obvious downside being space considerations and expense.
http://myweb.tiscali.co.uk/nuukspot/...clonesmps.html

In looking at the power supplies we have here I found one that has 3 different 12v outputs each rated at a max of 15A and for a total 12v max of 35A.

This got me thinking, would it be possible to use two dual 12v SMPS supplies to get +/- 24v? An example of a possible supply is:
http://www.rackmountmart.com/ps/ps2007.htm

This supply is designed to support dual processors and has 2 12v supplies rated at 18A max each and 25A total combined.

The big question is if the output grounds can be isolated from each other and from the chassis ground so that the two supplies could be run in series to create a single 24v @ 25A supply from a single SMPS.

Has anybody looked into using one of these dual supplies? Any thoughts or suggestions?

- Matt

[Nuuk]

Matt, as you say the $64,000 question is can you isolate the grounds of the SMPS you are asking about. I would suggest that only the supplier of those modules can tell you for sure!

[Paswa]

move it to power supply section

[slomatt]

Paswa,

Since this post covers power supplies specifically for the 3886 chipamps I figured this was the best forum for it, I hope that is a correct assumption.



I've found a supplier of 24v and 27v SMPS power supplies of differing wattages. They come with and without enclosures and some are medical grade. The 120w (5A) supplies start around $45. The best part is that the technical support department has confirmed that the chassis and DC grounds are floating so the supplies can safely be run in series. The 24v supplies can be adjusted between ~22-27v and the 27v supplies go a little higher.

My question is, how much current does a air of 3886s require? According to the datasheets at the National site the output is 68w/4ohm and 38w/8ohm for Vcc of +/- 28v, which is fairly close to the max output of the SMPS. According to equation (6) this equates to peak currents of 5.8A and 3.1A respectively. Based on this it would require a supply that puts out +/- 24v @ 12A to be stable at 68w output.

It seems that most people run two 3886s on a 160-240VA toroid. Based on the above it doesn't seem that this would be enough power to run the chips to the max output for the supply voltage. Will +/- 24v at 5A (240w) be enough to drive a pair of 3886s to an output level of say 30w? Any thoughts?

- Matt

[Nuuk]

I think AndrewT will be along shortly to point out that we are talking about music amplifiers here, and they don't need full power 100% of the time.

I'll just say that I have run LM3875s with a single Skynet 8080 module - with the -12v rail rated at only 0.5A!

[Paswa]

i have a small question here

My SMPS can deliver +12V @ 8Amp and -12V @ 0.5Amp.
the negative rail is just too low.

I've a LM1875 amp.

I think the amp can't be connected to the smps +-12V supplies as the negative rail is too low..

but If if if I connect it, what would be the result,

if me amp will be blown?
or the smpps will be blown?
or any other......??

[Nuuk]

No - I am not psychic!

[Nuuk]

But I should add that you should be careful when using an SMPS like that. It would be better to use two of them in series, using only the +12v outputs that are rated at 3.5A (see this)!

And the speakers I used were very efficient and didn't require the amp to draw too much power from the SMPS.

[Paswa]

n how about my question??

[Nuuk]

If you connect it? Depends on your speaker load and how loud you want to listen but it should be OK.

If you are asking what could go wrong, I can't say as I haven't experienced any problem. It could be that the SMPS can't supply enough power and if it the amp clips badly, it could harm your speakers. I really don't know if it could harm the SMPS itself (it is protected by a fuse). Where is AndrewT when you need him?

If you are using an LM1875, I assume that you don't require much power anyway and it should be fine. But I hope you will understand that I can't guarantee anything, only report my own experience!