Can I Use Dual Polarity Supply on Single Supply Load?

[penmarker]

I am planning to do a minor mod on my phono preamp. It comes with a walwart 12V power supply and I want to use a linear regulated power supply.


Looking through eBay there are quite a few LM317/337/LT1083/84 based circuits.
For LM317/337 there are mostly dual rail circuits with adjustable +V/0/-V. Is it possible for me to use it for my phono preamp by tapping the +12/-12? Should the ground be connected too? What happens if its not connected?



http://www.ebay.com/itm/262339244872

Most of the searches I've done returns results for how to split single rail to dual rail with caps and resistors, but not how to use a dual rail supply with a single rail load.

[IanMacdonald]

You can use the positive output on its own, connecting between  0v and +V.

I think you'll find this type of board requires a transformer with a center-tapped secondary though. It would probably work with a single winding connected between the input center and one 'hot' terminal but in that case you would only have half wave rectification. Which might not be ideal.

[penmarker]

Center tapped transformers might be a little hard to find cheaply, so my previous amp project that required a CT 28V transformer was achieved by connecting the + and - of two transformer outputs to achieve a center tap like this.



If I'm only going to be using the +V/0 and not the -V/0, would it would be preferable to use a single LM317 instead of the dual polarity 317/337?

One more question: If I want to get 12V DC, should the transformer output be 12V or a little higher like 15V?

[IanMacdonald]

"If I want to get 12V DC, should the transformer output be 12V or a little higher like 15V?"

There is no simple answer to this as it depends on the transformer's internal resistance, the reservoir cap value and the load current in quite a complex way. 12Vac will give about 1.4X12 - 0.7 Vdc offlload -about 16Vdc - but that will drop to nearer 12V under full load. In other words probably enough for a featherweight load but regulation will be lost under substantial loads.

15Vac will give about 20V at the regulator input and thus be a safer bet, as it will always be enough to allow for the regulator drop.

The main disadvantage of upping the supply to a linear regulator is that the heat dissipation goes up by leaps and bounds. Unlikely to be a problem with the current drawn by a preamp though.