Safe bipolar power from a wallwart?

[D Wagner]

Hey Pedalheads (and amp gurus),

Can I get safe bipolar power from a wallwart?  I know that a wallwart like the Boss power supply is full wave rectified.  This gives me V+ and ground.

After looking at the power supply article at Geofex, it appears that by using two diodes on the V+ supply (half wave rectified), it gives me V+ and V-.  I connect the ground to the junction of two capacitors between the V+ and V-, and I am set.  Will this work as I think?  Can I half wave rectify the supply that is full wave rectified?  I am trying to avoid using a transformer.

I arrived at this conclusion after looking at the power supply article at Geofex.  

http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

It is the bottom schematic titled, “There's an alternate but cruder way to make bipolar raw DC supplies for regulators”.  Excellent stuff from R.G. Keen…as usual.

Also, can I have a FF type effect connected to just the V+ and ground in a bipolar powered setup?

Thanks in advance,

Derek

[Nasse]

I did some time ago just what you mean, bipolar supply with one secondary voltage transformer. And I did it twice, two different applications. I have not tested it with pedals, but it was in continious use for several months without problems in audio monitoring and recording purposes in emergency response centre, totally illegal and experimental project of course. I powered some simple op amp stuff like balanced to unbalanced converters for audio recoding and other was in a "noise gate" or echo killer thingie. Well, if I can make echo and reverb so I can kill it, too . I did it on stripboard very quickly and cheap.

Supply ripple is higher than full wave rectified but it works. I bought one ac adapter from flea market and other was my wife´s old hand-held vacuum cleaner battery charger. They are now in my junk box, I have planned to put them in use with my small breadboard. Or somethin else

[Nasse]

:oops: !!! I DID NOT READ YOUR POST ENOUGH ACCURATELY !!!

I´m very sure that you must remove the bridge rectifier, and replace it with half wave two-diode one!!! Oops :oops:  :oops:

[Mark Hammer]

There are two ways to use wallwarts to provide bipolar supplies.  One is to build the entire bipolar supply in an outboard unit and find some foolproof (er, musician-proof) way to feed a bipolar DC voltage to the pedal.  This can be tricky since it will involve either stereo phone plugs that will require constant attentiveness to plugging them in before you turn them on, or something nonstandard like a 4-pin DIN connector, similar to a MIDI jack.  That latter approach is much safer but the machinng demands are a little more demanding.

The alternative is to use a wallwart that delivers an AC voltage to the pedal itself, and you derive the + and - DC voltages within the pedal.  PAiA uses this approach for their tube-based stuff all the time.  Look at the lower right hand corner of this schematic, and you'll see an example of what to do: http://www.paia.com/siabsch.pdf  You can regulate the derived +/- voltages down to whatever you want with the usual 3-pin regulators.

The advantage of this latter method is that you can use common barrel connectors, which are relatively foolproof.  The nice thing about it is that if the wallwart disappears on you or craps out, you can easily score another functionally equivalent one with a barrel plug, knowing that the onboard circuitry will take care of things.  Since the barrel plug/socket carries AC, instead of DC, there is none of this "which pin is ground?" crap to contend with.

[D Wagner]

The alternative is to use a wallwart that delivers an AC voltage to the pedal itself, and you derive the + and - DC voltages within the pedal.  PAiA uses this approach for their tube-based stuff all the time.  Look at the lower right hand corner of this schematic, and you'll see an example of what to do: http://www.paia.com/siabsch.pdf  You can regulate the derived +/- voltages down to whatever you want with the usual 3-pin regulators.

Mark,

Thanks!  That is exactly what I wanted to do!  It is close to what is shown on R.G.'s site, but all in one schematic, which helps me a lot.

Derek