Power supplies

[Chris S]

Can any one reccomend a cheap any not noisy 240v power adapter (Australia) for both 9v and 18v power supplies for guitar pedals?

I know it isn't quite D.I.Y to buy these things but I'm an amature at this stuff and playing with 240volts scares me.

Thanks!!

Chris

[Nasse]

I have had luck with adapters that say "regulated" or something in their package.

[Rob Strand]

You can add a regulator to the output of a standard wall-wart to clean it up.  The wall-wart out voltage must be higher than the input by at least 2V in order to compensate for the regulator drop-out.   A wall wart specified/rated at 12VDC would be suitable for a 9VDC regulator (some 9VDC wall-warts will work but it's hit and miss).  For 18V you will need to hunt down a higher voltage wall-wart which arent easy to find, a cheap solution is to by a 12V AC wall-wart and use a voltage doubler, then regulate that.

While regulators give a nice clean output with a well defined voltages, it's often possible to filter out the humm  with a simple RC filter on the output. 47ohm + 1000uF cap will do a good job but the idea is only good for low currents (like effects) say < 20mA.

[Chris S]

Hi Rob,

If I had a 12VDC 500ma wall wart lying around. If I put that into a PCB curcuit that has 3 voltage regulators two 9v and one 18v, am I going to get hum as if I had one adapater powering all 8 of my pedals?

Also the RC filter you described. (excuse my ignorance) is that on the 9v output or the signal out put and were do you put the 47ohm + 1000uF cap? ie: in series or in parallel or to earth?

Thanks!!!

[Rob Strand]

If I had a 12VDC 500ma wall wart lying around

You should first work out if it's filtered or not, it's likely to be filtered but some aren't.  What you do is power it up, then switch-off the power, and shortly after that you short the output terminals with a wire.  If you see a small spark then it's filtered.

If I put that into a PCB curcuit that has 3 voltage regulators two 9v and one 18v

Why 3?   What configuration of supply rails do you want?  For example you can have a common 0V then a +9V rail and a +18V rail, that only needs two regulators.  You can make this with a 9V regulator and an 18V regulator.

(You can also build a +9V 0V -9V system using a +9V regulator and a -9V regulator, you hen treat the -9V terminal as 0V, treat the 0V terminal as +9V and the +9V terminal as +18V.    There's isn't much difference between this method and the previous method. There are some technical differences though.)

I going to get hum as if I had one adapater powering all 8 of my pedals?

In rare cases it can but for most equipment these days you are pretty safe - I wouldn't worry too much about it.

What I would worry about is you can only use a common supply if the supply's 0V connects to a common ground in all the effects.  If you can't guarantee this then you are in big trouble, you will get intentional short circuits of the supply through the external common ground.

the RC filter you described

+9V -> series Resistor -> Cap +  (this becomes the new + output terminal)
and,
0V  ->   Cap -  (this remails the supply 0V)