Power supply find.

[cakeworks]

Hey!

I've just moved rooms and I'm now in a rather large one with room for deskwork, electronic work, and music (woot!). In the process of moving i stumbled across a power supply for some unused item powered by 12V... the bit which intrigued me was that it had a 4Amp current output... anyone getting the same vibe as me here?

Is there a way to pull down the voltage to manage 9V?

Also I'm assuming that 4Amps is plenty for five or six standard pedals?

Thanks!

[gez]

Is it AC or DC? (big difference).  You can check with a meter.  If DC, then is it switchmode or just a step-down and regulator?  If switchmode, I wouldn't be in too much of a hurry to open it up, but if its the latter then you could chance the regulator to a 9V.  Sounds like its unregulated or AC, though, as the highest rated regulators I can buy in the UK are 2A devices.

If it's AC, then there's no reason why you couldn't use it for a power-hungry build (incorporate the additional circuitry needed to turn it into DC into your pedal).

[gez]

PS 4 Amps would power a hell of a lot more pedals than 5 or 6!

[cakeworks]

To be completely honest I have no idea of the difference between switchmode and step-down/regulator... It's a 12Vdc output though and definitely 4A..

What might tell me the difference between the two?

And kinda relevant... I've always wonder how people work out how much current any given circuit draws... anyone shed any light?

thanks heaps gez

[the_random_hero]

To be completely honest I have no idea of the difference between switchmode and step-down/regulator... It's a 12Vdc output though and definitely 4A..

What might tell me the difference between the two?

And kinda relevant... I've always wonder how people work out how much current any given circuit draws... anyone shed any light?

thanks heaps gez

I'd put my money on it being switchmode, simply for the fact of the output power. Does it weight very heavy? Approximately how big is it? I was looking at 5A 12VDC transformers the other day and they were relatively heavy. These were unregulated though.
The easiest way to work out how much current a circuit draws is to measure it. Put your DMM in between the positive of the battery and the battery snap and just mark down your reading.

[gez]

Yeah, it sounds as though its might be switchmode: although you can get 4 terminal regulators that provide higher currents than 2A (I think those type source up to 4A), it's more common to see switchmode delivering that sort of current these days.

Edit:  PS, if that's the case (and it's quiet), then it would make a good supply for a low-power amp.

[cakeworks]

Thanks!

It weighs about twice what my 30gig ipod does and it is slightly larger too.

I have no idea if it's quiet yet haha but if it is "switchmode" (whatever that means ) then what sort of amp were you talking about... i'm guessing you aren't talking about valves... I've heard of starved plate valves but not in the amplifying section of the amp

I'd most definitely liking the idea of a power supply for pedals since i'm getting sick of replacing 9V batteries all the freaking time! haha

Anyone care to shed some light just briefly on the difference between step-down and switchmode?

thanks again

[Paul Perry (Frostwave)]

Yeah, that's switchmode, probably was running a laptop.
You can use it to drive one or more three terminal 9v regulators.

[MKB]

Anyone care to shed some light just briefly on the difference between step-down and switchmode?

A step-down supply uses an internal transformer to step down the 120V line voltage to a voltage near what you'd like as an output, then it is rectified and filtered.  A switchmode will basically convert the 60Hz line voltage to a much higher frequency, which is then rectified and filtered.  The upside of the switching supply is that they are much more efficient, which means a smaller footprint and less wasted heat.  The higher operating frequency allows for smaller caps for the same amount of filtering, further reducing size.  Also the regulation is done in a different way which does not generate heat, linear regulators will generate a lot of heat due to their basic design, which is not efficient.  The down side is that they are more complex and expensive, and instead of 120Hz noise in the regulated output you have the higher frequency noise to deal with. 

Switchmode supplies have come a long way in the last few years, they are now often used in audio (My ADA hybrid power amp has one, some of the pedalboard supplies are switching, even Z. Vex uses a switcher in his Nano guitar amp).  I'd drop some linear supplies on the output of your 12V supply and try it out, 4A is a LOT of current to play with, and 3V should be plenty to drive a LM317.  But be aware if your supply is designed for computer use, the switching frequency might have harmonics that could interfere with your audio path.  It shouldn't be a problem if you use a linear regulator, but is something to keep in mind.

[gez]

what sort of amp were you talking about... i'm guessing you aren't talking about valves... I've heard of starved plate valves but not in the amplifying section of the amp

If noise isn't a problem, then a simple low-power amp along the lines of what MKB suggested.  12V would get you a little 1W practice amp...possibly 2W (especially if you 'bridge').  A number of chips are on the market for this sort of thing.

A starved-plate pre-amp is perfectly feasible, and there's plenty of current available for the heaters.  There are also techniches to get decent plate voltages from DC supplies, but they involve a fair bit of circuity and take up space: square-wave oscillator, switching transistors and step-up transformer all take their toll.