Why are current ratings on some dual PSUs lower for the negative rail?

[Andre Gulbis]

First post in EEVBlog Forum.
Looked for an answer to this first but couldn't find it discussed yet - I hope!

TLDR: Why do some +/- rail PSUs have a lower current rating on the negative rail, and can I use switch-mode supplies for audio amplification.


I have come across this a few times before, and now that I am needing to buy an embedded switch-mode PSU it seems to have cropped up again.

When I am looking at the current ratings for the off-the-shelf triple-output PSUs I see something like :  +5V/2A  +15V/1.5A   -15V/0.5A
So the negative rail current rating is much lower.

Is this related to the bias/supply for op-amps, or the fact that certain circuits need a bit of voltage in the negative?  But that makes no sense, because it's not the voltage that is lower, it's the current..
I remember when I was doing a simple voltage inverter with an op-amp I think I needed at least some voltage on the negative rail for the amp to behave itself.
But I am wanting to amplify audio with this supply, so I want the same current capacity on both the rails don't I? In a Class A amp it takes equal amounts of current from both rails doesn't it?
Am I just looking at PSUs that have ratings for different applications than what I want, and I should just keep looking for a symmetrical-current supply?

Just so you know - the 5V is for digital stuff, and of course as I have said the +/- 15 for audio - op-amps and bi-polar transistors.
I could build my own linear supply (which I would prefer...), and have done so many times before, but space is getting to be a problem, so if one of these off-the-shelf switch-modes can give me the 3 voltages and currents I need then I will save space.

But this lower-current negative rail is kinda got me scratching my head and second-guessing myself.
And also I bit worried about using a switch-mode for audio - both the noise in rails and having a switching device in the project itself.
Has anyone had any experience with using a switch-mode for audio amplification, and is it OK or are there potential problems?

Any information about this would be of help - thanks!

[edpalmer42]

+15V or +12V is often needed for things like relays, motors, etc. that need a bit of power.  As you stated, negative voltages are typically used for op-amps or maybe bias voltages for ADCs, etc. that barely need any current, but still need the voltage.  So, the power supply manufacturers just provide the current that's required.  There's no point in providing symmetrical current ratings if it's not required.

However, for your application, you do need symmetrical current ratings so you'll just have to look a little further to find a supply that meets your needs.  You might end up with one supply providing +15V and -15V while a second one provides +5.

Ed

[Andre Gulbis]

Thanks Ed!  I think that's what I needed - someone tell tell me/confirm for me that I wasn't going insane and that I do need a symmetrical supply.
And I would much rather build my own linear one - and would be much cheaper than buying a $70 switch-mode.
As I said, also scared about having the switching in my audio device... my problem is that I'm running out of space!
Ill have to see how small I can get  +/- 15V(1.5A) and 5V(1A) supplies... I guess if it was all straight-forward it would be boring.
Cheers mate.
Andre