Switch mode or Linear Supply for a 5 V PI and a 12 V Amplifier in a box?

I'd like to put a Raspberry PI in a box, along with a 15-25 W amplifier and a speaker.

The PI needs 5 V, the amp will run on 12 V to 18 V, so 12 V would do.

Simple idea was to run 12v into the box, take power directly off for the amp (via a big fat capacitor) and then take off a spur for the PI via either one of those cheap high efficiency £2 LM2596 switching buck converters or a normal voltage regulator such as the L7805.

I know there are some blog posts questioning the reliability and even authenticity of these cheap ebay buck converters, but the only times they seem to fail badly to the high input voltage is when pulling the full 2.5amps limit, and the PI won't even be drawing 1A. Any thoughts there?

But my main question is in regard to the actual power supply (or Walwart if you must call it that!). I seem to have 2 options - a "linear" 12v 1A regulator which seems to come in at a ridiculous £20 ($30+!!) from places like Farnell.

Or, 12v 2A switched mode supply for £5 from eBay, Amazon etc. Double the output for quarter the cost.

Should be a no brainer, right? But I've read a couple of people saying that you should use a linear supply when feeding an amp, although they don't expand on why. But surely, a coupling capacitor should take care of that?

What am I missing? What are the perils and pitfalls of my plans as laid out? Anyone got a better idea?

BTW, I'm happy to post links to all or any of the components I'm mentioning above on request. I just didn't want to be accused of link spam!

Why don’t you use a Darlington array arrangement with ordinary power transistors and simple zenor diodes

Ok I know more components but very reliable with some short circuit protection circuitry That’s the easiest way Grab an old transformer out of a thrown away disused radio / mini hifi which is what I do and low and behold you have one decent power supply Or ever seen how many laptop chargers / power supplies they throw away Take one of them apart 15 to 20 volts available on those rebuild it if you have too That’s what I do honestly when you see what this lot throw away you will do exactly what I do these days strip them apart see what I can use and rebuild I even make money out of it

What else can you use aaah DVD players . CD players VCR's All have power supplies in them soem of them switch mode and fully regulated with exactly what you need 12 volts and Five volts with more than 3 amp available

Even have the old housing for these , bit of paint stripper , clean of the old covering Pair of tins snips bit of wood and know how to draw an explode view of a case and low behold you can even make your own casing Done this many a time before when strapped for cash even fed myself when no money about you'd be surprised what you can do when needs must.

I agree with Mark whole heartedly, I love when you can salvage good components. I think if you look at some of the old desktop computer power supplies they had 12v and 5v outputs and most deliver high quality power. If you don't have an old PC laying around ask some friends and I'll bet you come up with a few quickly,

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Come to that nine times out of ten guess what the power supply from dilapidated computer or whatever even if its has a fault can be serviced right down to component level more often than not its just electronic caps or resistors opens circuit or occasionally its a chopper transistor that’s gone / fet

As an old analog guy, the 7805 seems like a simple solution; not many parts, cheap, quiet. I don't expect that you're too concerned with overall efficiency, so a switcher (and they are lovely for many applications) seems unneccessary. Be sure to heat sink the 7805 appropriately, though.

I've no idea why "people say" not to use a switcher for an audio amp, except that some switchers are electrically noisy, and some amps have a poor power supply [noise] rejection ratio (PSRR). Also, audio amps fall into the domain of Audiophiles (aka audiofools) who claim to be able to hear amazingly tiny, if not actually unmeasurable, distortions. Most of us aren't in that rarified company, and so won't mind a bit of low-level hum or buzz (though we all have our own thresholds of annoyance).

Cheap power supplies are cheap for a reason. Usually they've skimped on something like input or output filtering, or heat sinking, or quality components. A lot of imported switching supplies (especially "no name" ones) are terribly effective noise generators, and can make radio reception in their vicinity very difficult, and worse. The suggestion of repurposing a computer supply is probably not a bad one, particularly if it's a recent name brand.

All the info you want here on some of this regarding use of pc power supplies and how too's including reconstruction what chips modifications etc


http://www.harrington.force9.co.uk/Projects/Powersupplies/ATXPowerSupply.htm

Hi Mark,

Thanks for answering the actual question - have a point

The idea of visiting the local tip and "skip raiding" for a transformer to save £5 sounds like a legal nightmare waiting to happen if there was a fire or someone got a shock. (This is being installed in a public place for a week). At least with a sealed, standards-approved and kitemarked supply I've removed some of that chance.

You're right, the lower efficiency of the sturdy and simple 7805 - compared to a multi-component switching buck converter - is probably worth it for reliability considerations.

You say

bq). "I’ve no idea why “people say” not to use a switcher for an audio amp, except that some switchers are electrically noisy, and some amps have a poor power supply [noise] rejection ratio (PSRR)"

Well, that might indeed be it. But about 2 hours after posting my message here, I found this:

http://electronics.stackexchange.com/question...it-be-powered-by-a-switched-mode-power-supply

Which seems to give a good variety of opinions. I'll let you all know what I chose and how I get on :)

Thanks.

John,

A 25 watt audio amp requires at least 4 amps if it's class-AB and 3 amps or more if it's class-D. If you keep the peak audio power below 5-7 watts a 12 V, 2A power supply will power the audio amp and the Raspberry Pi.

The Raspberry Pi requires about 500 mA and a simple series-pass voltage regulator - such as a LM7805 or LM317 - can be used to derive 5 volts from 12 volts. The power dissipated will be (12-5)(500mA) = 3.5 watts. A flat heatsink having a surface area of 3 square inches will be adequate.

The RPI requires a bit more than that Dave Under full operational conditions usb , serial port, Ethernet port this is just over 900ma As good measure reckon on an 1 amp this is one of the main reason's why the small surface mount fuse blows under full load / operational conditions

This has been noticed by other people (professionals who have been programming working with hardware designing for years ) Just in case they might think this could be a serious health safety problem

Hi David,

Thanks for confirming both that the 12V 2A supply will work, and that the LM7805 would do.
In fact, the plan is for the heatsink to be thermal-glued to the back panel of the box, which is conveniently aluminium.

Parts ordered - will let you know how it turns out :)

Now, talking of dissipation... *glances around at, ahem... _some other replies_* how do you dissipate a massive amount of hot air?

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