Quick Tranformer/Bridge Question

[Ian MacGregor]

Hi, I've been searching for some info on this, but I'm having a hard time finding anything.

If I have a Transformer that is suited to supply a +/-24V Bipolar supply (~45VAC CT), can I also use this same transformer to power a +48V rail? The two bridge rectifiers would be in parallel. The 48V bridge would have the negative DC side grounded to the same ground that the transformer center tap is grounded to. For some reason, it seems like one of the diodes in the 48V bridge would be shorting out one of the transformer windings.

Any ideas?

 

[JohnRoberts]

How much current does the +/- 24 draw...? If you ground one end of the winding, the center tap could give you +/- 24V half wave and +48V from the far end of the winding.

Or center tapped +/-24 and use a cap doubler...

If you wanted to be very clever you might do some synchronous switching to alternately use the windings between two modes but that wouldn't buy you anything over just half wave approach.

JR

 

[CJ]

Usually people do not do this due to crosstalk.

The current draw is so different between a line amp and a microphone, use this to your advantage.
The miniscule draw from a mic means filtering is a snap.

 

[Guest]

Ground center tap of transformer, get +/- 24 from a bridge rectifier, and get +48 using doubling rectifier because you will need only 5 mA maximum per channel for phantom power, while mic/line amps most probably will need more.

 

[JohnRoberts]

[quote author="Wavebourn"]Ground center tap of transformer, get +/- 24 from a bridge rectifier, and get +48 using doubling rectifier because you will need only 5 mA maximum per channel for phantom power, while mic/line amps most probably will need more.[/quote]

Agreed a cap doubler is effective and commonly used to generate phantom supply, however current draw is 14 ma with both pins (2+3) shorted to ground. Also some phantom mics load the inputs down more than 1/2 the phantom voltage available (nominally 7 ma), so conservative design IMO suggests a budget of more than 5 ma per ch. Across a larger console you can assume you won't encounter a lot of shorted inputs but ideally you want to prevent one or two shorted inputs from interfering with other channels.

JR