Wall-warts for powering Haible PCBs

[Resynth]

Hi

I'm building a Haible string filter
http://jhaible.com/legacy/string_filter/jh_string_filter.html

and varislope filter/phaser.
http://jhaible.com/legacy/varislope_filter_phaser/varislope_filter_phaser.html


I'm a little confused about the power supplies.

They have an onboard rectifier and regulators so require an ac voltage for which there is an AC1 and AC2 input. As the onboard power supply produces a bipolar +-15v supply I would guess I need a 16-0-16v transformer or, to put it another way, a 32v centre tapped transformer?

However everything I can find has Jurgen talking about using cheap and available wallwarts and, in the photos of the varislope, you can see he's only used one of the AC inputs which suggest a monopolar transformer/wallwart.

I'd love to use a wallwart as it would be way more convenient than separately housing a transformer or buying an expensive toroidal. I'm also worried that if the circuit only requires 18v and I give it +-16v (ie 32v) I'll blow something. A bit lost here and don't have the full expertise to work it out from the schematic.

I can't find 18-0-18 wall warts anywhere, how have other people been powering their PCBs?


Cheers
Ben

[JovianPyx]

Looking at the schematic from the first link, I see what you write about. I see the 2 AC inputs which are connected together to form the ground and the other ends feed a fullwave rectifier. That's a pretty standard thing. A center-tapped transformer would work, or a transformer that has 2 equal size secondary windings and the schematic suggests the latter. I also see what appears to be a large amount of devices drawing power from the PSU, so I'd want to know how much current it needs from each of the rails.

Finding center tapped wallwarts is probably possible, but they aren't to be found everywhere and with the amount of circuitry there, I have a feeling a wallwart at that voltage level might not be able to push the current required. I've no idea what Juergen used when he built it. You are correct that the two AC input voltages should be somewhere around 16 volts each or 16-0-16. Watts are important here, if the transformer needs to push one ampere into each of the PSU regulator circuits, at 15 volts DC, that would potentially be 30 watts. 30 watts is enough power to make a soldering iron, so there can be some substantial heat generated. Wallwarts aren't great at shedding heat since they are encased in a plastic box. Open air transformers are used when more heat dissipation is needed - and my guess is that Juergen may have used a bolt-on open air dual secondary transformer.

Hope that helps a bit.

[Resynth]

Thanks for your reply Jovian.

In the end I decided to go for DC-DC converters. I have plenty of power available in 12v DC so am gonna try 12v DC to +-15v converters and see how it goes