Power supply question

[deaconque]

This may sound dumb but here goes-
Is it possible to use an AC adapter from, say, a laptop on a pedal?  Laptop power cords usually have an output of 19v that seems like it could be usable for tube circuits.  Is it because of the amps that these won't work (usually b/t 2A and 4A)?  I'm pretty sure I'm an idiot but I thought I'd throw it out there 


Be gentle 

[.Mike]

There are tons of topics about this if you do a search on "laptop power supply."

The general consensus seems to be that:

- They will work, but might not be the best option.
- They are switching power supplies, and may introduce noise.
- They may have excessive ripple, and may require extra filtering.
- They may or may not be regulated, and may benefit from using a regulator.



Mike

[Ripthorn]

It's usually because they are unfiltered and noisy as all get out.  Sure they could be used, though I would filter the heck out of them, possibly regulate as well.  Also, you have to be aware that the DC connection is usually center positive and a larger size than is standard for stompboxes.  If you wanted to make a box that is tailor-made to your power supply, it shouldn't be a problem.

[slacker]

I use an old 18 volt laptop power supply to power my ADA flanger, and that works fine. I'm just in the middle of building a little box with some 9 volt regulators in it so I can run other pedals of it as well, it will basically end being like a Dunlop power brick.
Like has been said though they can be noisy, worth trying though you could power a lot of pedals with 2 Amps

[Cliff Schecht]

There are tons of topics about this if you do a search on "laptop power supply."

The general consensus seems to be that:

- They will work, but might not be the best option.
- They are switching power supplies, and may introduce noise.
- They may have excessive ripple, and may require extra filtering.
- They may or may not be regulated, and may benefit from using a regulator.



Mike

A typical laptop supply may LOOK noisier on a scope than it actually is. This has to do with the loading and how the output impedance gets smaller with a heavier load. With that being said, it should be noted that running a high current power supply way below it's rated limit will most likely give you more noise than finding a smaller unit that you can load properly.

Switching frequency is especially important as well, if it's too low then it will be audible and if it's too high, well you probably can't go too high for our applications. The further away from audio frequencies the better, and an added bonus of higher swithcing frequencies is smaller transformers/coupled inductors. But past a point, you're just not going to benefit from the higher frequency (aside from paying more).

Ripple is least at max load so again, choose the right supply for the job. It's like using a 1000V capacitor in a 9V circuit - yeah it will work but it's frickin' huge and probably really expensive.

I doubt that you will find any "unregulated" laptop SMPSs, they're quite accurate and will have a good transient response. You can't get either of these consistently without some form of feedback to regulate the switch PWM.

[.Mike]

A typical laptop supply may LOOK noisier on a scope than it actually is. This has to do with the loading and how the output impedance gets smaller with a heavier load. With that being said, it should be noted that running a high current power supply way below it's rated limit will most likely give you more noise than finding a smaller unit that you can load properly.

Switching frequency is especially important as well, if it's too low then it will be audible and if it's too high, well you probably can't go too high for our applications. The further away from audio frequencies the better, and an added bonus of higher swithcing frequencies is smaller transformers/coupled inductors. But past a point, you're just not going to benefit from the higher frequency (aside from paying more).

Ripple is least at max load so again, choose the right supply for the job. It's like using a 1000V capacitor in a 9V circuit - yeah it will work but it's frickin' huge and probably really expensive.

I doubt that you will find any "unregulated" laptop SMPSs, they're quite accurate and will have a good transient response. You can't get either of these consistently without some form of feedback to regulate the switch PWM.

Good info, thanks. I was just posting what I remember reading when I was considering using an old laptop power supply for my pedals.

Mike

[deaconque]

great info guys.  thanks.  I guess I wasn't completely wrong in thinking that it could work.

[Cliff Schecht]

great info guys.  thanks.  I guess I wasn't completely wrong in thinking that it could work.

Hey, it's worth a shot. Take that laptop supply and run it into a 9V linear regulator with caps on the input and output and hook it up to some pedals. If you don't hear any noise then you'll be powering a good 20-30 analog pedals before that power supply even winces.

[kristoffereide]

I'm not trying to overtake the thread, but what does isolated ground mean? That every output has its own transformer? And if I build one, should I put the rectifiers before or after the transformer?

[petemoore]

  Isolated ground is easy with a battery, it's not connected to ground, called a 'floating' or 'isolated'.
 Neither is a transformer secondary AC output, unless it's [DC'd] output gets assigned to ground...or any other reference], you get a nice 'floating' 9v [after the AC is turned to 9vDC, of course].
 A floating supply could then be assigned to a reference, ground-0.0v, or -82v or +300v, any voltage.
  Any reference voltage, from either 'side' of the 9vdc [+ or -], don't forget the plus's and minus's, they 'add'.
  Say the + pole of a 9v battery was connected to 300vdc, the voltage [referenced from ground = 0.0vdc] would be 291vdc on the - side of the battery.
 

     
 

[amptramp]

One thing to be aware of is that under light loads, many laptop supplies will go out of regulation.  I have an old Apple laptop supply rated at 7.5 VDC that goes over 11 VDC with no load.  In its normal application, it would never run without load so this is not an issue.  Many pedals only take a few milliamps, so this is an issue.

[petemoore]

  There have been tries to make computer power supplies hack it to provide nice DC voltages, IIRC it became evident that it is easier to build a 'conventional' power supply.

[kristoffereide]

 Isolated ground is easy with a battery, it's not connected to ground, called a 'floating' or 'isolated'.
 Neither is a transformer secondary AC output, unless it's [DC'd] output gets assigned to ground...or any other reference], you get a nice 'floating' 9v [after the AC is turned to 9vDC, of course].
 A floating supply could then be assigned to a reference, ground-0.0v, or -82v or +300v, any voltage.
  Any reference voltage, from either 'side' of the 9vdc [+ or -], don't forget the plus's and minus's, they 'add'.
  Say the + pole of a 9v battery was connected to 300vdc, the voltage [referenced from ground = 0.0vdc] would be 291vdc on the - side of the battery.

So... what are you saying? I know that rectified AC has a (+) and a (-), and that the (-) isn't really ground, but will the (-) side of the 9V-output-transofmer rectified be -9V or 0V?

[MarcoMike]

I used to have my laptop supply as pedalboard supply.... it worked nicely, but sometimes there was some noise, kind of windy noise, and high pitch whining now and then... even after regulation... now I changed my laptop to a dell, which has an almost impossible to find plug, so I went back to transformer... also because I had the need of a split supply which I couldn't get form the laptop psu (higher than 12V, so no max1044 or its friends)

but I always thought the noise from a switching PSU as laptop's would be reduced very effectively by an inductor-cap filter... high frequencies should be very easy to kill that way...