48V Mic Preamp PSU - or Magic Smoke® Machine?

[jim_griff]

Hello.

I have been working on a design for a microphone preamp in my studio but would also like to make a small linear power supply for it too. It needs dual +/-15 rails as well as a 48V output for phantom power. It also needs power filtering to prevent any unwanted RF junk in the circuit.

I use the waterfall analogy for understanding how electrons fall out of jugs of water (supply) before they splat to the floor (ground), unless diverted by special pipes (copper wire) to do work for us. But I'm having trouble understanding how it works if I have two transformers that are "floating" and using a common ground (chassis) as a ground reference. Also, I can't get it round my head that I can have the 48V power supply connected to the same chassis ground as the +15/-15 rails, if ground is half way inbetween. I know it is isolated with a transformer, so it should be able to 'float' 48V there, but won't other equipment connected to the chassis ground see some voltage difference? Or is it fine, since they are all connected to the houses 'Earth' ground anyway?

Questions:
(1) Some circuits with toroidal transformers connect the middle of the secondary windings both to chassis ground. Would this not inject 50Hz hum and other noise into the circuit ground? As I understand it, noise in a ground is actually not that bad, as long as all circuits have the same reference with no ground loops inbetween. (attachment 2)
(2) Is it possible to have two bridge rectifiers both connected to ground but used to provide a dual power supply? This is how I have it set up in my circuit (in attachment 1.)

Thanks in advance for any help.

EDIT: P.S. Will it release magic smoke? That was the first question I wanted to ask. I've built a cheap and crappy 13.8V power supply before but never one with two transformers or a dual supply.

[Richard Crowley]

Some circuits with toroidal transformers

For the purposes of this discussion, it makes no difference whether the transformer is toroidal or conventional E/I core.

connect the middle of the secondary windings both to chassis ground. Would this not inject 50Hz hum and other noise into the circuit ground? 

No. Unless there was some other gross problem with the circuit design elsewhere. There are multi-millions of power supplies designed like this.

As I understand it, noise in a ground is actually not that bad, as long as all circuits have the same reference with no ground loops inbetween.

No, actually a "noisy ground" is a problem. If the ground/reference is noisy, then what is your reference to zero/quiet/stable?  It would be like trying to use a plumb-line on the deck of a ship being tossed around in a storm.

Is it possible to have two bridge rectifiers both connected to ground but used to provide a dual power supply? This is how I have it set up in my circuit

Either of those circuits will work. The circuit with two bridge rectifiers will provide "full-wave" rectification, while the circuit with one bridge rectifier will provide only "half-wave" rectification. However, in the real-world, the difference is negligible assuming you provide adequate filtering and regulation.

Will it release magic smoke? That was the first question I wanted to ask. I've built a cheap and crappy 13.8V power supply before but never one with two transformers or a dual supply.

If you wire your circuits properly, no it will not release any magic smoke.

[Zero999]

Either of those circuits will work. The circuit with two bridge rectifiers will provide "full-wave" rectification, while the circuit with one bridge rectifier will provide only "half-wave" rectification. However, in the real-world, the difference is negligible assuming you provide adequate filtering and regulation.

Correction: both circuits provide full wave rectification.

[jim_griff]

Thanks for the replies guys. I had a feeling the dual bridge rectifier (Example 2) circuit would work. I'm going to stay with my current circuit for simplicity so I can have it all on the board, then ground it to the chassis at the Earth point. But I'll test it first on the variac to make sure I've wired everything correctly!

P.S. I love this forum. Glad to be back.