Faulty 36V switching power supply?

[AlexQ]

I have three 36V DC switching power supplies (2A, 5A and 5A+). I checked them with a multimeter and they all give 36V DC quite precisely but if I switch the multimeter to AC I can measure a fast oscillating voltage between 0 and 38V. Is this normal for a 36V power supply? I measured many switching PSU's in this manner (5V, 9V and 12V DC PSU's) and they never gave more than 0.5V AC and never oscillating. Is there any solution to filter out this variation of voltage? The DC is not oscillating. Should I replace capacitors with higher capacity ones? (the capacitors at the output seem to be rather small)

Two of these PSU's I bought from new and one is used. I am trying to pick up the best one out of these to power up my audio amplifier. I had a careful look inside all of them. One of them is really complex (many electronic parts) and the output is adjustable. I tried all three of them with the amplifier and I get a reasonable amount of buzzing in the speakers when there is no music playing. The buzzing is louder with one of the PSU's. They are made by LiteOn, XPpower and no_name.

Thank you very much for reading this!

 

[davenn]

tell us more about these PSU's ...
show pics
links to www info about them etc

 

[AlexQ]

Thank you for your answer!
The one made by XP power has some info here:
http://www.alldatasheet.com/datasheet-pdf/pdf/276262/XPPOWER/SML150PS36.html
I'll come back with more info soon.

 

[Arouse1973]

Your multimeters AC setting is only suitable for measuring mains frequency really, unless its a true rms type. You cant get any meaningful results otherwise. I would be supprised if XP PSUs were actually doing this. I have used many of them over the years without a problem. But I suppose there is always a first.
Adam

 

[AlexQ]

I thought my cheap multimeter might not be intended to measure highly fluctuating voltages but I couldn't figure a better way of testing PSU's with limited equipment (I don't have an oscilloscope).

A friend of mine mentioned tonight that this might actually be the issue: a 5V SMPS for example works (chops AC from the wall) at a much higher frequency than a 36V one (36V/5V=7.2 so around 7 times higher frequency) so my multimeter is more likely to show constant values both in DC and AC at the output of the 5V PSU than at the 36V PSU.

Tonight I also tested a 31V PSU and got similar results - fluctuations of voltage in AC mode.

My question now is this qvasi AC fluctuations of the output voltage could translate in the noise (buzzing/hum) that I can hear in my speakers (mostly when no music is playing)?
Also, is there a way to improve the quality of the output of such a PSU? (huge capacitors maybe?) All three of my 36V PSU's seem to have LC or RLC filters on the output with small capacitors.
Thank you for reading this thread!