How do I source an Audibly noiseless (< 3dBA) power supply?

This is not a shopping question; this is a "what is the best way to solve this problem" question.

To power a medium-size project, I'm looking for a 15W/12V board mount power supply, taking mains AC in (auto-switching voltage is nice, although US-only would be acceptable) and providing reasonably-regulated 12VDC out.

I first tried the cheapest thing I could find on DigiKey with parametric search: a 35W Delta PSU, cageless (plain board.) Unfortunately, it had quite significant coil noise, which is right out for this application -- the device will be in a bedroom and ears are very picky about coil whine. As clarified in the comments, I'm looking for something as low as 3 dBA audible noise at 1 meter.

Second, I tried a more expensive model, with a perforated case: a 25W TDK/Lambda. It, too, has coil noise, although less of it. But, this is still too much.

I understand why the coil noise happens: These switching power supplies regulate the voltage by sending a signal all the way to the mains side, using a flyback regulator. But, this is not acceptable to me.

Unfortunately, I have not found any parametric search (digi-key, mouser, farnell, jameco) that rates board mount power supplies on noise. And they don't even advertise the topology used -- a transformer-based down converter with buck topology on the DC side would probably be fine for my needs, for example.

So, other than buying one of each of everything in stock, which I can't really afford to do, how can I find a power supply that doesn't make any noise? If you had this design requirement, how would you solve it?

• Low noise = linear. – Ignacio Vazquez-Abrams Feb 11 '14 at 1:18
• The typical sound level in a bedroom is 30 dBA which is near the noise limit of even the best sound level meters. Your requirement of < 3 dBA is thus unrealistic. As pointed out in the above comment, a linear supply without a fan is probably the quietest you can get. However it may be difficult to find the actual level on a data sheet as this is not a common requirement. Also the nature of the noise is probably more important: wideband noise that is similar to white noise would be less annoying than noise containing significant spectral components. – Barry Feb 11 '14 at 1:57
• I agree that wideband noise is less annoying than typical flyback coil whine. But, I've had to replace a number of computer and cell phone transformers because coil whine makes them impossible to live with, so I believe 30 dB is off by orders of magnitude. Just because the peak noise may be 30 dBA, doesn't mean that softer sounds won't be noticeable. I can build a linear PSU with a $12 transformer and$20 worth of OSH park PCB and parts bin stuff. Maybe that's what I'll do. But I still have the question: How do I source a quiet PSU if I have that need? – Jon Watte Feb 11 '14 at 3:51
• Assuming that you mean acoustic noise, the most silent transformer in any case is toroidal.. However there is wide variation depends of the manufacturer. The reference background noise of a room is around 35~45 dBA (at 60Hz). A good quality torroidal transformer emits 60Hz noise at a level of 45~50 dBA, under normal conditions (load, mains noise etc). So in 3m away the noise becomes around 35 dB – GR Tech Feb 11 '14 at 5:30
• I have less than 20dB indoors where I live. I'm sure there are people with really quiet bedrooms! I can't see why people think the author of the question isn't allowed to attempt to design something really really quiet :-) – avl_sweden Jul 16 '15 at 16:24

Firstly, why <3 dB? This is a ridiculously strict requirement for general use, so do you have a specific need for zero noise?

This aside, the lowest-noise power supplies are linear. This is a basic circuit that pretty much looks like:

transformer -> rectifier -> smoothing capacitors -> regulator -> more smoothing capacitors

You can buy these off the shelf. However, they are declining in popularity as switchmode supplies are more compact and efficient. I would just build one yourself, as this way you can find your own specific requirements and parts. For part selection, keep in mind that:

• Toroidal transformers tend to have the lowest noise
• The regulator is specific to your voltage, power requirements and features (e.g. variable)
• These are the only components you really need to choose based on noise, as capacitors and diodes are silent!
• As @AndrejaKo quite rightly pointed out some capacitors (e.g. ceramics) are indeed not silent, so you should probably steer clear of them. Thanks AndrejaKo!

Look online for example circuits, calculators, and tutorials.

•  as capacitors and diodes are silent Ceramic capacitors are definitely NOT silent. See for example this – AndrejaKo Feb 11 '14 at 7:31
• @AndrejaKo As reference for measure electronic components/systems noise, is to connect a tungsten lamp to the mains with a diode in series. Using this you judge the floor of mains noise (sparks etc) – GR Tech Feb 11 '14 at 7:51
• @AndrejaKo True, but as a general rule... – felixphew Feb 11 '14 at 8:13
• @felixphew Well I, in general, have no problems hearing large SMD ceramic capacitors in switching power supplies and charge pumps, so I know the pain the OP is going through. :) – AndrejaKo Feb 11 '14 at 8:26
• I really appreciate this answer! It doesn't quite answer the question (it says "just specify a linear power supply," which isn't guaranteed to work, and goes against California energy conservation requirements.) As I said above, I know how to build a linear PS if needed, but saying that the quietest transformer has a hum of 50 dBA isn't true. For example, sticking the ear right next to my stereo amplifier, there is no audible hum. And finally, yes, I mean audible noise -- hence the specification of A-weighting. And I'm OK with 3 dB at 1.0 meters from the enclosure :-) – Jon Watte Feb 12 '14 at 3:10