Question about wallwart power adapters

[Beo]

What are the key things to look for when determining if a power adapter might be suitable for pedal use? I find lots of them as surplus at my work, or at surplus stores in town.

Obviously voltage (9v, 12v, ac, dc, etc.), current limit and pos/neg center. Plug type, but that can be re-wired.

What does the Class mean? Does that indicate if it's a switching supply, filtering/regulating, etc?

Or is it best to check it on a scope, perhaps under load or with a test circuit?

[R.G.]

What does the Class mean? Does that indicate if it's a switching supply, filtering/regulating, etc?

I'd have to see an example to be sure, but --IF-- it's a switching type power supply, "Class" can mean the limits of EMI radiated and conducted noise it generates. US FCC limits have a couple of different classes of allowable noise limits depending on whether it's intended for consumer/casual use anywhere, or only for business use where the unit may make more noise, but the supposedly-more-capable users can solve noise issues if they come up, and are in fact required to solve those issues if the unit creates interference with other devices.

Not sure without looking.

You can usually tell if a unit is a switching type by using your on-board differential mass comparator - your hand. Non-switching types have iron transformers in them, and are correspondingly denser. Switching types are lighter per unit volume, which was one of the points of doing them.

Or is it best to check it on a scope, perhaps under load or with a test circuit?

Again, it depends on what you mean by "best".  Non-switching and non-regulated units will have near-zero ripple under no load, but will both have DC outputs that sag noticeably with loading, especially at full load. You can see this on a scope, but your voltmeter set to AC volts and through a DC blocking capacitor will read the ripple on a line-frequency type. Anything more than millivolts means you need a follow-up regulator to wipe that ripple off.

There's all kinds of fancy stuff that could be done, but simply loading it down with resistors to near it's specified load current and seeing what DC voltage and AC ripple you get is probably good enough to start with. If there are no red flags with that test, you're probably OK to hook it up to a pedal and listen for funny noises.

[EBK]

I'm going to guess any wall wart you pick up says "Class 2"
https://standardscatalog.ul.com/standards/en/standard_1310_6

[Mark Hammer]

In a perfect world, wallwarts would be transparent, or at least have removable/replaceable backs, such that one could see the complexity of the regulation included.

Unfortunately, they are generally sealed (for your safety!), and the requirements for being "DC" are not always sufficient for powering audio circuits.  Indeed, a transformer, 10uf cap and single 1N4001 could qualify a wallwart as "9VDC @300ma".  All it has to do to be in the club is never go negative from 0V.  The ripple and hum would be intolerable, but it would still count as 9VDC.

As such, it is often helpful to make outboard power-smoothing boxes, to insert between wallwart and pedals.  For example, something that either straps a big cap across the V+ and ground, or takes in 12VDC of unspecified smoothness, throws a 9V regulator at it, and puts out an identifiably smoother 9VDC.  Either that, or check the PSU's ripple with a scope or suitable meter, prior to use/purchase.

[anotherjim]

Some switchers do say something like "Office/IT equipment only" or "For I.T.E. power supply". The ones supplied with DSL/broadband routers (nearly always 12v DC it seems to me) are usually that sort - and they certainly can be NOISY.

IF you make a multi out box to run several pedals from one supply, you do have the opportunity to fit some DC smoothing & anti RF filtering in there so it's pretty much going to work with any DC PSU with the right amps/volts.

[Beo]

A box would be easy for adding some heavy duty filtering before junctioning out to pedals. But I'm curious that I haven't seen anyone sell an inline filter adapter with our 2.1mm standard plugs. Seems like it would be pretty easy to do, and an easy to use add-on for pedals that are poorly filtered or particularly sensitive to ripply supplies.

[Mark Hammer]

It would be easy but then it would be easiest when you know what the input voltage is and what the expected output voltage needs to be.  making a general purpose box that doesn't really care what comes in or needs to go out would be a much more difficult design objective.

[bool]

A simple circuit called "capacitance multiplier" can reasonably clean up a smps psu. (Try google.)