Unregulated + regulator?

[wildebeest]

Hi, I'm a newbie here obviously, but starting my first Pimeta. Let me first say this forum is really awesome and Tangent is an amazing person for providing me/others with such detailed tutorials and info.

I'm still compiling my parts list (quickly growing past $100) and now I'm looking for ways to cut costs somewhat. My amp is going to be exclusively wall-powered, and I thought maybe instead of an expensive elpac supply, I could just buy a junky 24v wall wart. Then I could build a regulation circuit to clean it up.

A supply like this one

mouser

And then build this simple circuit

regulator

wall>wallwart>2.5mmjack>regulator>pimeta

would that work well?

Thanks, and I'm sure I'll be back and forth with more questions!

 

[Dreamslacker]

If you're willing to build a regulation circuit, you can just buy a cheap 24v transformer and build your own 24v linear regulated power supply.

On the other hand, it's possible for you to build a dual-voltage power supply using a 12-0-12 tranformer and bypass using the TLE2426 altogether.

 

[Kenny12]

thats what i done, i coudln't be bothered going through mains wireing and the hazards of it, maybie i'll build a 'real' power supply for my next project

 

[tangent]

Quote:

A supply like this one

Yes. It should manage 28V or so at 50mA, a typical load level for a PIMETA, as long as you're not doing something insane like stacking output buffers 4x and shorting R11.

Don't guess on this...read the BUF634 datasheet and plan your amp's current draw. If you push your amp's draw much past 50mA, your regulator will run into drop-out, and you'll get bad power quality.

EDIT: I'm assuming here that you're going to set the regulator's output for 24V, so you'll need 26V or so to ensure dropout-free operation. If you set the regulator for 22V, you should be safe clear up to 100mA draw, but you won't want to go that high because transformers don't like to be loaded to 100% of their rated current. Call 80mA your absolute maximum.

Quote:

And then build this simple circuit

Close, but a little too simple. You should add a 1N4001 from the O to the I terminal of the 317, as shown in the datasheet, because you've got output caps.

You might also consider splitting R2 into a fixed resistor plus a 500 ohm or so trim pot, as is done in the STEPS. This will only add about $2 to the cost of the power supply, and it will give you adjustability over a several volt range. You might as well read that whole article, because it gives good info on designing and building LM317-based power supplies. You don't have to build a STEPS to benefit from it.

Oh, and personally I'd make R1 120 ohms. I know the datasheet says you can get away with 5mA here, but it also says the regulator can require 10mA in the worst case situation. Best not to tempt fate.

Quote:

you can just buy a cheap 24v transformer

For $4.25 he's getting that transformer plus the bridge, main filter cap, case, AC plug, and DC wiring. Not bad at all. This is a perfectly reasonable way to build a cheap linear.

 

[wildebeest]

After reading a little bit more it looks like the velleman kit would make this a lot easier. That way I have a premade board to work with.

Would the kit and the $4 wall wart be all I need or do I need a fuse too?

 

[tangent]

The Velleman kit is designed to take AC in, not DC. Either you could get an AC-AC transformer in a wall wart package (Mouser does offer them) or you could connect the AC-DC wall wart past the diode bridge, and leave that bridge out of the circuit.

You would need a fuse, and you should add a heat sink for the regulator, too.

 

[wildebeest]

So would this work with the velleman?

budget ac-ac

Then I add a 1-amp fuse and I've got a PS superior to the elpac for about $10 cheaper?

Thanks for being so patient. Little by little this project is coming together

 

[tangent]

Quote:

So would this work with the velleman?

Yes, it's just an encased transformer.

Quote:

Then I add a 1-amp fuse

Oh, 0.25A ought to be sufficient. Buy an assortment, they're cheap. You might even be able to get away with 0.125A. The lower you can go, the more helpful the fuse will be in the event that it is needed.

Quote:

I've got a PS superior to the elpac

No worse, at any rate.

Quote:

for about $10 cheaper?

If your time is free, yes.